土壤英文文献
土壤英文文献
Plant and Soil 260: 69–77, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.69Root-induced acidification and excess cation uptake by N2 -fixing Lupinus albus grown in phosphorus-deficient soilJ. Shen1,2,4 , C. Tang3 , Z. Rengel2 & F. Zhang11 Department of Plant Nutrition, China Agricultural University,Key Laboratory of Plant-Soil Interactions, Ministry of Education, Beijing 100094, P. R. China. 2 Soil Science and Plant Nutrition, School of Earth and Geographical Sciences, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Hwy, Crawley WA6009, Australia. 3 Department of Agricultural Sciences, La Trobe University, Bundoora, Vic. 3086, Australia. 4 Corresponding author∗Received 7 October 2002. Accepted in revised form 26 July 2003Key words: proton release, balance of cation-anion uptake, P deficiency, Lupinus albusAbstract White lupin plants (Lupinus albus L. cv. Kiev) were grown in soil columns under controlled conditions at 20/12 ◦ C (12/12 h) for 76 d to investigate the effect of phosphorus (P) deficiency on root-induced acidification and excess cation uptake by N2 -fixing plants. Phosphorus was added in each column as FePO4 at a level of 10 (limited P) or 200 µg P g−1 (adequate P). Supply of 10 µg P g−1 restricted plant growth from 58 d after sowing (DAS) and decreased P concentrations significantly in shoots from 49 DAS and in roots from 40 DAS compared with plants supplied with 200 µg P g−1 . Phosphorus concentrations in shoots of plants receiving 10 µg P g−1 decreased steadily from 2.1 to 1.1 mg P g−1 dry weight from 40 to 76 DAS, but P concentrations in roots were constant with time. Total P uptake increased with time irrespective of P supply, and the P uptake by plants at 10 µg P g−1 was only 35–75% of that at 200 µg P g−1 . Plants fed with 10 µg P g−1 had higher Ca and Mg concentrations but lower S concentration in shoots than the plants fed with 200 µg P g−1 . The concentrations of excess cations in plants were higher at 10 µg P g−1 than 200 µg P g−1 after 49 DAS. Phosphorus deficiency decreased the pH of root exudate solution due to the enhanced release of protons (H+ ) from roots. The pH of root exudate solution decreased rapidly with time and dropped to the lowest (4.28) at 58 DAS in the 10 µg P g−1 treatment. The decreased pH of root exudate solution was correlated with the increased concentrations of excess cations in plants. The pH of root exudate solution showed a different pattern of change with time compared with citrate exudation, suggesting that exudation of citrate anions contributes only a part of total acidification, but excess cation uptake dominantly contributes net proton release from roots of plants grown in P-deficient soil. Plant tissue had a significant accumulation of citrate in the treatment of 10 µg P g−1 compared with 200 µg P g−1 after 67 DAS. The results suggest that P deficiency enhances the excess cation uptake and concomitant proton release, and non-synchronous processes are involved in tissue accumulation and root exudation of organic anions under P deficiency.Introduction Phosphorus deficiency is one of the major yieldlimiting nutrition problems for plants in both acidic and calcareous soils due to low bio-availability of P (Barber, 1984; Marschner, 1995). Some plant species∗ FAX No: +86 10 62891016. E-mail: jbshen@have developed various morphological and physiological strategies to acquire sparingly soluble phosphorus from soil. The mechanisms based on morphological responses include enhanced root elongation (Barber, 1984; Raghothama, 1999), root-to-shoot ratio, root hair formation (Ma et al., 2001; Marschner, 1995), mycorrhizal associations (Smith et al., 1992),70 and decreased root radius (Föhse et al., 1991). In addition, physiological strategies are characterized by enhanced rhizosphere acidification (Hedley et al., 1982; Hinsinger et al., 2003; Marschner, 1995; Tang et al., 2001), and increased root exudation of organic chelates (Dinkelaker et al., 1989; Gardner et al., 1983; Jones, 1998; Ryan and Delhaize, 2001), reducing agents and phosphatase (Dracup et al., 1984; Neumann et al., 1999, 2000). White lupin (Lupinus albus L.) is an important grain and forage crop widely grown in infertile soils. Under P deficiency, white lupin increases the formation of cluster roots, release of H+ and exudation of citrate (Dinkelaker et al., 1989; Gardner et al., 1983; Lamont, 2003; Neumann et al., 1999). For example, the enhanced net release of H+ from cluster roots acidified rhizosphere from pH 7.5 to 4.8 (Dinkelaker et al., 1989, 1995) and even as low as pH 3.6 in some cases (Li et al., 1997). Citrate concentration in root cluster rhizosphere can reach as high as 100 µmol g−1 soil, and the amount of citrate exudation from roots represented up to 23% of the total plant dry weight (Dinkelaker et al., 1989, 1995; Gerke et al., 1994). Large amounts of proton release and citrate exudation may facilitate P acquisition by white lupin from sparingly soluble P in soils (Gerke et al., 1994; Hinsinger, 1998; Marschner, 1995; Raghothama, 1999). Rhizosphere acidification and citrate exudation induced by P deficiency have been studied mostly in white lupin grown in nitrate-containing nutrition solutions (Gardner et al., 1983; Johnson et al., 1994, 1996a, b; Keerthisinghe et al., 1998; Neumann et al., 1999; Neumann and Römheld, 1999; Shane et al., 2003; Watt and Evans, 1999a, b), or in soil culture with nitrogen supply as nitrate (Dinkelaker et al., 1989; Kamh et al., 1999) or ammonium nitrate (Li et al., 1997). Nitrogen forms have a marked influence on rhizosphere acidification due to influencing the balance of anion and cation uptake by plants (Hinsinger, 2001; Hinsinger et al., 2003; Marschner, 1995). Only a few studies used N2 -fixing P-deficient white lupin plants to elucidate the pH changes caused by H+ release and root exudation (Sas et al., 2001; Shen et al., 2003; Tang et al., 1997, 2001). The legume plants dependent on N2 -fixation could take up excess cations over anions, resulting in enhanced H+ release with an increasing amount of N2 fixed (Tang et al., 1997, 2001). Moreover, localized rhizosphere acidification and citrate exudation was also reported previously (Dinkelaker et al., 1989; Keerthisinghe et al., 1998; Neumann et al., 1999, 2000). However, littleTable 1. Basic nutrients applied to soil as solution form prior to planting. The nutrients added were thoroughly mixed with soil and then the soil mixture was carefully packed into plastic soil-column pots (diameter×height = 86×380 mm) for planting. There was 3-kg dry soil per pot Chemicals K2 SO4 CaCl2 .2H2 O MgSO4 .7H2 O MnSO4 .H2 O ZnSO4 .7H2 O CuSO4 .5H2 O H3 BO3 Na2 MoO4 .5H2 O CaCl2 .2H2 O MgSO4 .7H2 O Co2 SO4 .7H2 O Application rates (mg pot−1 ) 400 500 130 20 30 6 2 0.5 500 130 1information was available about patterns of pH change caused by H+ release by the whole root system and excess cation uptake of N2 -fixing Lupinus albus grown in P-deficient soil during various growth stages. The objective of the present study was to investigate the effect of P deficiency on root-induced acidification and excess cation uptake by N2 -fixing white lupin at different growth stages, and assess the relationship between balance of anion-cation uptake and H+ release, and organic acid anion exudation under P deficiency.Materials and methods Plant cultivation Seeds of white lupin (Lupinus albus L. cv. Kiev) were germinated and grown in column pots containing 3 kg of air-dried soil. Virgin brown sand (Uc4.22, Northcote, 1971) was collected from a bushland site 15 km south-east of Lancelin, WA (31.56 S, 115.20 E). The soil fertility was characterized as follows: organic carbon of 8.4 g kg−1, available P (Colwell P) of 5 µg g−1 , NO3 − -N of 2 µg g−1 and NH4 + -N of 1 µg g−1 . The soil conductivity was 0.0022 S m−1 , and the soil pH was 5 (CaCl2 ) or 5.7 (H2 O) (Shen et al., 2003). The soil texture is sand with 96% sand, 2% slit and 2% clay. The contents of oxy-hydroxides are very low with 0.54% Fe2 O3 and 0.11% Al2 O3 (Brennan et al., 1980). Phosphorus was added into the soil as FePO4 at71 a rate of 10 (limited P) or 200 µg P g−1 (adequate P). Nitrogen was supplied only via biological N2 fixation through inoculating the seeds with Bradyrhizobium sp. (Lupinus) WU 425 by adding suspension solution (Sas et al., 2001; Tang et al., 2001). Other basic nutrients were added according to Table 1. The experiment was conducted in a controlled glass-house with temperature of 20/12 ◦ C (12-h day/12-h night) and relative humidity of 75–85%. The soil was watered up to 90% of field capacity by weighing pots with plants once or twice daily. Three replicate pots of each treatment were used for root exudate collection, plant harvest, and nutrient uptake and carboxylate analysis at each of five harvests. The pots were completely randomized and repositioned weekly to minimize any effect of uneven environmental factors. for determination of mineral composition including total K, Na, Ca, Mg, P, S and Cl. Measurements The pH of the root exudate solution A 30-mL aliquot of collected solution was subsampled from the collected solution, and the pH was determined using an Orion 940E pH meter with a combined glass electrode. Organic anions Retention times and absorption spectra of 12 organic acid standards (oxalic, tartaric, formic, malic, malonic, lactic, acetic, maleic, citric, succinic, fumaric and aconitic acids) were used to identify the composition of organic anions in tissue extracts or root exudates. The 10-mL samples of plant extracts or root exudates were filtered through sterile Millex GS Millipore 0.22-µm filters and directly analyzed for organic anions on a reversed phase column (Alltima C18 5 Micron, length 250 mm, i.d. 4.6 mm) using Waters HPLC (Shen et al., 2003). Mineral composition of plant tissues Concentrations of total K, Na, Ca, Mg, P and S were determined in roots and shoots after digesting plant materials in a mixture of concentrated nitric and perchloric acids (Johnson and Ulrich, 1959). Concentrations of total K, Na, Ca, Mg and S were determined using inductively coupled plasma emission spectrometry. Phosphorus was assayed using the vanadomolybdate method (Westerman, 1990). Chloride was analysed colourimetrically in the water extract. The concentrations of excess cations [cmol (+) kg−1 ] in plants were calculated as the sum of charge concentration of K+ , Ca2+ , mg2+ and Na+ minus the sum of H2 PO4 − , SO4 2− and Cl− according to the method described by Tang et al. (1997). Statistics Analysis of variance for comparisons among means was conducted using the SPSS statistical software (SPSS, 1998).Sampling of root exudates and plant materials Root exudates were collected by percolating the soil column containing intact plants with deionized water for 2 h (from 10:00 to 12:00 in the morning) at 40, 49, 58, 67 and 76 DAS according to the method described elsewhere (Shen et al., 2003). An aliquot of 600-mL deionized water was percolated through the soil column three times. The gravimetric analysis showed that 300 mL of solution was retained within the soil matrix, and thus the total volume for calculating carboxylate concentrations was 900 mL. The leachate contained water-soluble root exudates and was referred to as the ‘root exudate’. The pH of the leachate was determined immediately after collecting root exudate solution. Afterwards, Micropur (Sicheres Trinkwasser, Germany) at 0.01 g L−1 and three drops of concentrated H3 PO4 were added to the collected root exudates to inhibit the activity of microorganisms. A sub-sample of 10 mL from the collected solution was kept at −20 ◦ C for later analysis of carboxylates. After each root exudate collection, the plants were harvested, and roots and shoots were separated. Extraction of organic acids from plant materials was done according to the method reported by Neumann et al. (1999). Plant tissue samples were homogenized using ceramic mortar with 5% (v/v) H3 PO4 at the ratio of 1 mL H3 PO4 solution to 0.1 g fresh weight of plants. After centrifugation for 10 min at 10 000 g, the supernatant was diluted 10-fold with HPLC-elution buffer and analyzed by reversed-phase HPLC. Shoot and root weights were recorded after drying in an oven at 70 ◦ C for 3 d. After grinding, the plant materials were used72Figure 1. Fresh weights of shoots and roots (g plant−1 ) of white lupin with 10 (limited P) and 200 (adequate P) µg P g−1 soil at various growth stages. Vertical bars on symbols represent means±SE (n = 3 replicate pots). The separate bar represents LSD (P = 0.05) for any two means according to an analysis of variance for different treatments.Results Plant growth A significant difference was observed in fresh weight between 10 and 200 µg P g−1 at 58 DAS for shoots, which occurred earlier than shoot dry weight changes (Shen et al., 2003), and at 67 DAS for roots (Figure 1). From day 58, plants receiving 10 µg P g−1 showed reduced growth as compared with those at 200 µg P g−1 . Fresh weights of shoots of white lupin at 10 µg P g−1 were 83%, 72% and 53%, respectively, of those at 200 µg P g−1 at 58, 67 and 76 DAS. The plants in two P treatments started to flower at 55 DAS, and the number of flowers in plants fed with 200 µg P g−1 was much greater than that at 10 µg P g−1 at 58 DAS. From day 58, the plants at 200 µg P g−1 also grew taller than those at 10 µg P g−1 (data not shown).Figure 2. P concentrations and contents in plants of white lupin grown at 10 or 200 µg P g−1 soil for 76 d. Vertical bars on symbols represent means±SE (n = 3 replicate pots). The separate bar represents LSD (P = 0.05) for any two means according to an analysis of variance for different treatments.Chemical composition of plant tissues The concentrations of P in shoots of plants fed with 10 µg P g−1 were significantly lower than those at 200 µg P g−1 from 49 DAS, and there were the higher concentrations of P in roots of plants receiving 20073 than 10 µg P g−1 from 40 to 76 DAS (Figure 2). Phosphorus concentrations in shoots of plants receiving 10 µg P g−1 decreased steadily from 2.1 to 1.1 mg P g−1 shoot dry weight from 40 to 76 DAS. In contrast, P concentrations in roots supplied with 10 µg P g−1 had no significant changes with time. At 200 µg P g−1 , there was a trend for decreasing P concentrations in both shoots and roots with time from 40 to 76 DAS. The P concentrations in shoots and roots of plants receiving 200 µg P g−1 were significant lower at 76 than at 40 DAS. The higher P concentrations in shoots were observed at 58 compared with those at 49 and 76 DAS. The total P uptake increased with time irrespective of P supply, but the P uptake by the plants at 10 µg P g−1 was only 35–75% of that at 200 µg P g−1 (Figure 2). Plants fed with 10 µg P g−1 soil had higher Ca concentrations in shoots at 49 and 67 DAS, and had higher Mg concentrations in shoots at 49, 67 and 76 DAS (Figure 3). The concentrations of Ca in roots were higher at 67 DAS in the treatment of 10 than 200 µg P g−1 . Plants supplied with 10 µg P g−1 showed higher K concentrations in shoots at 49 DAS, and in roots at 67 and 76 DAS in comparison to plants receiving 200 µg P g−1 . However, Na concentrations in shoots were lower at 40 DAS in the treatment of 10 than 200 µg P g−1 . There was a lower S concentration at 58 DAS and higher Cl concentration at 67 DAS in the treatment of 10 than 200 µg P g−1 . However, there was no significant difference in the concentrations of Na, Mg, S and Cl in roots between P treatments. Ca and Cl concentrations were higher, and Na, Mg and S concentrations were lower in shoots than in roots.Figure 3. Concentrations of K, Na, Ca, Mg, S and Cl in shoots and roots of white lupin grown at 10 or 200 µg P g−1 soil for 76 d. Vertical bars on symbols represent means ±SE (n = 3 replicate pots). The separate bar represents LSD (P = 0.05) for any two means according to an analysis of variance for different treatments.Root-induced acidification Excess cation uptake Growing plants decreased the pH of soil leachate (root exudates) compared with the control without any plants irrespective of P treatments (Figure 4). The pH of root exudate solution in the treatment of 10 µg P g−1 was significantly lower at 58 and 67 than at 40 or 49 DAS. In contrast, for the treatment of 200 µg P g−1 , the pH of root exudate solution remained constant with time. At 58 and 67 DAS, the pH of collected solutions from the plants fed with 10 µg P g−1 was significantly lower than that from plants receiving 200 µg P g−1 . There was no significant difference in the pH of root exudate solution between the treatments of 10 and 200 µg P g−1 at 40, 49 and 76 DAS. The concentrations of excess cations in plants were significantly higher in the treatments of 10 than 200 µg P g−1 at 49 and 76 DAS in shoots, and only at 76 DAS in roots (Figure 5). The concentrations of excess cations in plants tended to be lower at 10 than 200 µg P g−1 soil at 40 DAS despite no significant difference. However, after 49 DAS, the 10 µg P g−1 treatment surpassed the adequate P treatment in the concentrations of excess cations in plants. The highest concentrations of excess cations for the plants grown at 10 µg P g−1 were observed at 49 DAS for shoots and at 58 DAS for roots, and then the concentrations decreased with time.74Figure 4. The pH in root exudate solution of white lupin grown at 10 or 200 µg P g−1 soil for 76 d. Vertical bars on symbols represent means±SE (n = 3 replicate pots). The separate bar represents LSD (P = 0.05) for any two means according to an analysis of variance for different treatments. There was one replicate for the treatments without plants.The plants grown at 200 µg P g−1 showed a continuous decrease in excess cation concentrations in shoots with time, while in roots an increased concentration from 40 to 58 DAS and then a decrease till 76 DAS were observed. There were higher concentrations of excess cations in the whole plants grown at 10 µg P g−1 compared with the plants receiving 200 µg P g−1 after 49 DAS, and the difference in concentrations of excess cations between P treatments was significant at 49 and 76 DAS. Concentrations of carboxylates in roots and shoots of plants and citrate exudation The concentrations of citrate in shoots or roots of the plants fed with 10 µg P g−1 significantly increased with time from 40 to 76 DAS, but remained unchanged for the plants fed with 200 µg P g−1 over the experimental period (Figure 6). The citrate concentrations in shoots and roots were significantly higher in the plants receiving 10 than 200 µg P g−1 after 67 DAS. The increased exudation of citrate from whole root system of white lupin in the limited P treatment was observed compared with the adequate P treatment over the period monitored (Figure 6, calculated from Shen et al., 2003). There was a higher exudation of citrate at 49 and 67 DAS in the treatment of 10 µg P g−1 , but the citrate exudation rates remained unchanged with time for the plants fed with 200 µg P g−1 from 40 to 76 DAS.Figure 5. Concentrations of excess cations in shoots and roots of white lupin grown at 10 or 200 µg P g−1 soil during 76 d. Vertical bars on symbols represent means ±SE (n = 3 replicate pots). The separate bar represents LSD (P = 0.05) for any two means according to an analysis of variance for different treatments.Discussion Effect of P limitation on plant growth and P uptake In the present study, P deficiency led to a significant decrease in biomass of both shoots and roots, but the shoot growth appeared to be more sensitive to P deficiency than root growth. The decrease in plant growth due to P deficiency was also observed in the reduced75 laas et al., 2003). From the first to final harvests, the ratios between P uptake and P adding are 25–50% for 10 µg P g−1 , and 1.7–7.2% for 200 µg P g−1 . However, because P has a low diffusion in soil, particularly FePO4 , only the P close to root surface has a chance to be absorbed. According to the ratio of 10% [generally, 1–5% under field condition (Marschner, 1995)] between root volume and total soil volume, only 3 mg and 60 mg FePO4 can be readily accessed by plants, and then the ratios between P uptake and the accessed FePO4 become 250–500% and 17–72%, suggesting that much P could be mobilized from soil. The significant difference in P concentrations in plant tissues between P treatments occurred prior to plant fresh weights. The reduced growth of white lupin receiving 10 µg P g−1 was correlated with the decrease in P concentrations in shoots, but had no significant correlation with P concentrations in roots. The results showed that the P concentrations in shoots were not completely dependent on the P concentrations in roots, being consistent with the previous report (Marschner, 1995). Root-induced acidification and excess cation uptake The decreased pH of root exudate solution showed that the low P supply increased H+ release from roots. In symbiotically-grown legumes, rhizosphere acidification may be caused by cation-anion balance, the excretion of organic anions and symbiotic nitrogen fixation (Marschner, 1995; Tang et al., 1997). Under the specific experimental condition, it is not possible to separate the quantitative contribution of each of these processes to total net-release of protons. Moreover, in the present study, all process occurring in soil could have influenced the pH of the leachate and in fact we can not separate the net H+ release from other soil processes. However, the difference in pH between P treatments showed a clear effect of P limitation on H+ release from roots because P limitation caused excess uptake of cations, resulting in much H+ release from roots. Plant roots extrude a net amount of H+ to maintain charge balance when cation uptake exceeds anion uptake (Dinkelaker et al., 1989; Hedley et al., 1982; Marschner, 1995; Tang et al., 2001). In particular, excess cation uptake by plants reflected H+ release when N was supplied through N2 -fixation (Tang et al., 1997). In the present experiment with N2 -fixing Lupinus albus grown in P-deficient soil, net H+ release was most likely caused by excess cation uptake. Phosphorus deficiency increased Ca concen-Figure 6. The concentrations of citrate [µmol (g FW)−−1 ] in shoot and root tissues and citrate exudation [µmol h−1 (g FW)−−1 ] from roots of white lupin grown with 10 and 200 µg P g−1 soil at different growth stages. Vertical bars on symbols represent means ±SE (n = 3 replicate pots). The separate bar represents LSD (P = 0.05) for any two means according to an analysis of variance for different treatments.plant height and number of flowers per plant after 58 DAS. The reduced fresh weight and plant height were the first visible symptoms of P deficiency. The increased fresh weights and P contents in the adequate P plants in comparison to the limited P plants showed that white lupin plants could take up large amounts of P from FePO4 added in soil, and that the amount of P taken up increased with the amounts of FePO4 added in soil despite FePO4 being a form of sparingly soluble P. The results showed a support for the viewpoint that white lupin plants have a great ability of mobilizing the sparingly soluble P through changing rhizosphere processes, particularly, citrate exudation (Hinsinger, 1998; Marschner, 1995; Venek-76 trations in shoots and roots, and Mg concentrations in shoots, but decreased S accumulation in shoots, showing that concentrations of excess cations was higher in the P-deficient than P-sufficient N2 -fixing plants. Furthermore, a clear negative correlation was observed between the number of cluster roots (Shen et al., 2003) and the pH of root exudate solution, suggesting that cluster roots of white lupin are the main site for release of H+ under P deficiency, which is consistent with other studies (Dinkelaker et al., 1989, 1995; Marschner, 1995; Neumann and Römheld, 1999; Watt and Evans, 1999a; Yan et al., 2002). Relationship between proton release, accumulation and exudation of carboxylates Accumulation of citrate was found in shoot and root tissues of P-deficient plants, which is in agreement with the observation of citrate accumulation in mature cluster roots under P deficiency (Neumann et al., 1999). However, in the present study, concentrations of citrate in shoots and roots of the plants fed with 10 µg P g−1 soil increased continuously with time (Figure 6), while a higher exudation of citrate was observed at 49 and 67 DAS in the treatment of 10 µg P g−1 (Figure 6 calculated from Shen et al., 2003). These results showed that the high concentrations of citrate in plants could be a prerequisite for citrate exudation, but citrate exudation did not show the consistent change pattern with time in comparison to citrate accumulation in plant tissue, being consistent with previous reports (Keerthisinghe et al., 1998; Neumann et al., 1999). Moreover, exudation of citrate showed a clearly different pattern of change with time compared with the pH of root exudate solution, showing a lack of dependence of proton release on exudation of citrate anions. The results suggest that exudation of citrate contributes only a part of total acidification and excess cation uptake is the dominant factor affecting net proton release from roots of white lupin plants grown in P-deficient soil. In conclusion, white lupin plants can take up large amounts of P from FePO4 as a sparingly soluble P form through changing rhizosphere processes. Phosphorus deficiency of N2 -fixing plants grown in soil resulted in excess cation uptake and enhanced concomitant proton release, causing a decreased pH of root exudate solution. Citrate exudation did not show the consistent change pattern with time in comparison to citrate accumulation in plant tissue. Exudation of citrate anions contributes only a part of total acidification, but excess cation uptake dominantly contributes net proton release from roots of white lupin plants grown in P-deficient soil. Acknowledgements This study was supported by grants from the MSBRDP (Project No: G1999011709), by the National Natural Science Foundation of China (No. 30000102) and by AUSAid. ReferencesBarber S A 1984 Soil Nutrient Bioavailability: A mechanistic approach. John Wiley & Sons, Inc. New York. 398 p. Brennan R F, Gartrell J W and Robson A D 1980 Reactions of copper with soil affecting its availability to plants. I. Effect of soil type and time. Aust. J. Soil Res. 18, 447–459. Dinkelaker B, Romheld V and Marschner H 1989 Citric acid excretion and precipitation of calcium in the rhizosphere of white lupin (Lupinus albus L.). Plant Cell Environ. 12, 285–292. Dinkelaker B, Hengeler C and Marschner H 1995 Distribution and function of proteoid roots and other root clusters. Bot. Acta. 108, 183–200. Dracup M N H, Barrett-Lennard E G, Greenway H and Robson A D 1984 Effect of phosphorus deficiency on phosphatase activity of cell walls from roots of subterranean clover. J. Exp. Bot. 35, 466–480. Föhse D, Classen N and Jungk A 1991 Phosphorus efficiency of plants. II. Significance of root radius, root hairs and cation-anion balance for phosphorus influx in seven plant species. Plant Soil 132, 261–272. Gardner W K, Barber D A and Parbery K G 1983 The acquisition of phosphorus by Lupinus albus L. III. The probable mechanism by which phosphorus movement in the soil/root interface is enhanced. Plant Soil 70, 107–124. Gerke J, Roemer W and Jungk A 1994 The excretion of citric and malic acid by proteoid roots of Lupinus albus L. Effect on soil solution concentrations of phosphate, iron and aluminium in the proteoid rhizosphere in samples of an oxisol and luvisol. Z. Pflanzenernähr. Bodenkd. 157, 289–294. Hedley M J, White R E and Nye P H 1982 Plant-induced changes in the rhizosphere of rape (Brassica napus var. Emerald) seedlings. III. Changes in L value, soil phosphate fractions and phosphatase activity. New Phytol. 91, 45–56. Hinsinger P 1998 How do plant roots acquire mineral nutrients? Chemical properties involved in the rhizosphere. Adv. Agron. 64, 225–265. Hinsinger P 2001 Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: A review. Plant Soil 237, 173–195. Hinsinger P, Plassard C, Tang C and Jaillard B 2003 Origins of rootmediated pH changes in the rhizosphere and their responses to environmental constraints: A review. Plant Soil 248, 43–59. Johnson C M and Ulrich A 1959 Analytical methods for use in plant analysis. Cali. Agric. Exp. Stat. Bull. No.766. Johnson J F, Allan D L and Vance C P 1994 Phosphorus stressinduced proteiod roots show altered metabolism in Lupinus albus. Plant Physiol. 104, 657–665.。
土地污染的作文英文
土地污染的作文英文英文：Land pollution is a serious problem that affects not only the environment but also human health. There are many causes of land pollution, such as industrial activities, agricultural practices, and improper waste disposal. These activities can lead to the contamination of soil, water, and air, which can have harmful effects on plants, animals, and humans.One of the main causes of land pollution is industrial activities. Many industries release harmful chemicals and pollutants into the environment, which can contaminate the soil and water. For example, factories that produce chemicals and plastics can release toxic substances into the air, which can then settle on the ground and enter the soil and water. This can lead to the contamination of crops and groundwater, which can then be consumed by humans and animals.Another cause of land pollution is agricultural practices. The use of pesticides and fertilizers can lead to the contamination of soil and water. These chemicals can seep into the ground and enter the groundwater, which can then be consumed by humans and animals. In addition, the use of heavy machinery in agriculture can lead to soil erosion, which can further degrade the quality of the soil.Improper waste disposal is also a major cause of land pollution. Many people dispose of their waste in landfills, which can lead to the contamination of the soil and groundwater. Landfills can also emit harmful gases into the air, which can contribute to air pollution. In addition, improper disposal of hazardous waste can lead to serious health problems for humans and animals.In order to address the problem of land pollution, itis important to take action at both the individual and governmental levels. Individuals can reduce their impact on the environment by properly disposing of their waste, using environmentally-friendly products, and reducing their useof harmful chemicals. Governments can implement regulations and policies to reduce pollution from industrial activities and agriculture, as well as improve waste management practices.中文：土地污染是一个严重的问题，它不仅影响环境，还影响人类的健康。
土壤污染英语作文初三
土壤污染英语作文初三英文回答：Soil pollution is a major environmental issue that affects the health of our planet and its inhabitants. It occurs when harmful substances contaminate the soil, degrading its quality and affecting its ability to support life.Soil pollution can result from various sources, including industrial activities, agricultural practices, and improper waste disposal. Industrial activities, such as mining, manufacturing, and oil and gas extraction, can release toxic chemicals and heavy metals into the environment, contaminating the soil and groundwater. Agricultural practices, such as the excessive use of pesticides and fertilizers, can also contribute to soil pollution by accumulating harmful chemicals in the soil. Additionally, improper waste disposal, including landfills and illegal dumping, can leach contaminants into the soil,polluting it with organic matter, heavy metals, and other hazardous substances.Soil pollution has severe consequences for human health and the environment. It can lead to a range of health issues, including cancer, reproductive disorders, and neurological damage. Contaminants in the soil can be absorbed by plants and animals, entering the food chain and potentially affecting human health. Soil pollution also damages ecosystems by reducing biodiversity, disruptingsoil processes, and impacting the ability of plants to grow and thrive.Addressing soil pollution requires a multifaceted approach involving government regulations, technological advancements, and individual actions. Governments can implement regulations to control industrial emissions and agricultural practices, promoting sustainable waste management and remediation efforts. Technological advancements can provide innovative solutions for soil remediation, such as bioremediation and electrokinetic remediation. Individuals can contribute by reducing theirconsumption of chemicals and fertilizers, composting organic waste, and properly disposing of hazardous materials.中文回答：土壤污染。
农学毕业论文参考文献(中英文范例)
农学毕业论文参考文献（中英文范例）参考文献是写作农学毕业论文不可或缺的一部分，导师可在审核学生论文时，可根据参考文献的引用、数量、时间及其权威性，来了解作者对本课题研究的程度，进而评价论文的水平和结论的可信度，由此可见文献的重要性，本文精选了50个"农学毕业论文参考文献";，含中英文文献，供广大学子参考。
农学毕业论文参考文献范例一：汤文光, 肖小平, 唐海明, 等. 不同种植模式对南方丘陵旱地土壤水分利用与作物周年生产力的影响. 中国农业科学, 2014, 47(18): 3606-3617.【2】刘星, 张书乐, 刘国锋, 等. 连作对甘肃中部沿黄灌区马铃薯干物质积累和分配的影响. 作物学报, 2014, 40(7): 1274-1285.Qiu S J, He P, Zhao S C, et al. Impact of nitrogen rate on maize yield and nitrogen use efficiencies Northeast China. Agronomy Journal, 2015, 107(1): 305.Chuan L M, He P, Zhao T K, et al. Agronomic characteristics rrelated to grain yield and nutrient use efficiency for wheat production in China. Plos One, 2016, 11(9): e0162802.李书田, 金继运. 中国不同区域农田养分输入、输出与平衡. 中国农业科学, 2011, 44(20): 4207-4229.陈庆瑞, 赵秉强, 等. 四川省作物专用复混肥料农艺配方. 北京: 中国农业出版社, 2014.陈印军, 肖碧林, 方琳娜, 等. 中国耕地质量状况分析. 中国农业科学, 2011, 44(17): 3557-3564.西奥多-舒尔茨．改造传统农业．北京：商务印书馆，1987陈春霞．农业现代化的内涵及其拓展．生产力研究，2010（01）：54-56+267程绍铂，杨桂山，李大伟．长三角典型农业区农业现代化水平分区研究以江苏省兴化市为例．地域研究与开发，2011，30（04）：149-152+157迟清涛．中国农业现代化发展研究．吉林农业大学，2015崔凯. 粮食主产区农业现代化评价指标体系的构建与测算研究.中国农业科学院,2011.丁志伟，张改素，康江江，翟伟萍．中原经济区农业现代化的状态评价与定位推进．农业现代化研究，2015，36（05）：760-766傅晨．广东省农业现代化发展水平评价．农业经济问题，2010（5）：26-33+110高芸，蒋和平．我国农业现代化发展水平评价研究综述．农业现代化研究，2016，37（03）：409-415郭强，李荣喜．农业现代化发展水平评价体系研究．西南交通大学学报报，2003（01）：97-101Shibayama M Akiyama T. 1986.Aspectroradiometer for field use.Radiometric estimation of nitrogen levels in field rice canopies. 55(4): 439-445.柯炳生．对推进我国基本实现农业现代化的几点认识．中国农村经济，2000（09）：4-8何传启．农业现代化的事实原理和选择中国科学院中国现代化研究中心．科学与现代化）．北京：2012：11李黎明，袁兰．我国的农业现代化评价指标体系．华南农业大学学报（社会科学版），2004（02）：20-24李进平．河南省农业现代化评价与发展对策研究．华中师范大学，2015李林杰，郭彦锋．对完善我国农业现代化评价指标体系的思考．统计与决，2005（13）：34-36Geary B, Clark J, Hopkins B G, et al. Deficient, adequate and excess nitrogen levels established inhydroponics for biotic and abiotic stress-interaction studies in potato. Journal of Plant Nutrition, 2014,38(1): 41-50.Alva A, Fan M S, Qing C, et al. Improving nutrient-use efficiency in Chinese potato production experiences from the United States. Journal of Crop Improvement, 2011, 25(1): 46-85.吕文广．甘肃农业现代化进程测度及特色农业发展路径选择研究．兰州大学，2010.农学毕业论文参考文献范例二：蒋和平．中国农业现代化发展水平的定量综合评价（世界银行、联合国环境计划署．"全球农业科技与发展评估";国际会议论文集）．北京．世界银行、联合国环境计划署：中国农业技术经济研究会，2005：10蒋和平．蒋和平：发展中国现代农业要稳定小农与发展大农并举．江苏农村经济，2012（01）：21孔祥智，毛飞．农业现代化的内涵、主体及推进策略分析．农业经济与管理，2013（02）：9-15李芳远．新型城镇化引领下的河南省农业现代化发展研究．郑州大学，2015李燕凌，汤庆熹．我国现代农业发展现状及其战略对策研究．农业现代化研究，2009，30（06）：641-645李周，蔡昉，金和辉，张元红，杜志雄．论我国农业由传统方式向现代方式的转化．经济研究，1990（06）：39-50刘巽浩．论中国农业现代化与持续化．农业现代化研究，1998（5）：17-21刘晓越．农业现代化评价指标体系．中国统计，2004（2）：11-13+10Broge N H, Leblanc E.2003paring prediction power and stability of broadband and hype rspectral vegetation indices for estimation of green leaf area index and canopy chlorophyll density.Remote Sensing Of Enviro nment, 76(2):156-172.潘世磊．粮食主产区农业现代化发展研究．重庆工商大学，2016沈琦，胡资骏．我国农业现代化评价指标体系的优化模型基于聚类和因子分析法．农业经济，2012（05）：3-5.孙纲．黑龙江县域农业现代化路径选择研究．东北林业大学，2016王宝义．中国农业生态化发展的评价分析与对策选择．山东农业大学，2018Everitt J H, Pettit R D Alaniz M A.1987. Remote sensing of room snake weed(Gutierrezia sarothrae)and spiny aster(Aster spinosus).Weed Sei, 35(2):295-302.许志发．福建省农业现代化发展水平评价研究．福建农林大学，2017宣杏云．国外农业现代化的模式及其借鉴．江苏农村经济，2006（05）：16-17朱剑峰，朱媛媛．安徽省农业现代化水平区域差异与发展模式研究．中国农业资源与区划，2013，34（04）：120-124辛岭，蒋和平．我国农业现代化发展水平评价指标体系的构建和测算．农业现代化研究，2010，31（06）：646-650杨秀艳．农业现代化指标体系与评价方法研究．西北农林科技大学，2004伊霞．山东省农业现代化发展水平评价．辽宁大学，2017Pinter P J, Jackson R D, Idso S B. 1983.Diurnal Patterns of Wheat Spectral Reflectances. Jackso Remote Sensing, 21(2): 156-163.赵文英，付仁玲，何佳琪，李瑞敏．我国各省农业现代化发展水平综合评价．中国农机化学报，2018，39（12）：94-100张宝丹．山东省农业现代化发展研究．山东大学，2018张成龙．广西农业现代化发展水平研究．广西大学，2014张航，李标．中国省域农业现代化水平的综合评价研究．农村经济2016，（04）：53-57。
农业学术文献英语土壤改良
农业学术文献英语土壤改良In the realm of agricultural research, soil improvement remains a crucial aspect that demands constant attentionand innovation. The significance of soil health in enhancing crop yield and overall productivity is well-documented. However, the approach to soil improvement often varies across different geographical regions and cultural contexts. This article aims to explore soil improvement techniques from an agricultural academic perspective, highlighting the importance of soil management practices in sustainable agriculture.**Soil Fertility and its Enhancement**Soil fertility is the foundation of agricultural productivity. It refers to the soil's capacity to support plant growth by providing essential nutrients, water, and air. To enhance soil fertility, several techniques can be employed, including the addition of organic matter, lime, and fertilizers. Organic matter, such as compost and manure, not only adds nutrients but also improves soil structureand water holding capacity. Lime is used to neutralize soil acidity, which is essential for the availability of certainnutrients. On the other hand, fertilizers provide a quick source of nutrients to the soil, promoting plant growth.**Soil Conservation and Erosion Control**Soil erosion is a major threat to soil health and fertility. Wind and water erosion can lead to the loss of topsoil, reducing soil fertility and productivity. To conserve soil and control erosion, agricultural practices such as contour farming, strip cropping, and terracing are employed. These practices help in reducing water runoff and wind erosion, thus maintaining soil health and fertility. **Irrigation and Drainage Management**Effective irrigation and drainage systems are crucial for soil improvement. Adequate water supply is essentialfor plant growth, while excessive water can lead to waterlogging and soil degradation. Irrigation systems, such as drip irrigation and sprinkler systems, ensure uniform water distribution to the soil, promoting healthy plant growth. On the other hand, drainage systems help in removing excess water from the soil, preventing waterlogging and maintaining soil aeration.**Crop Rotation and Intercropping**Crop rotation and intercropping are agricultural practices that contribute significantly to soil improvement. Crop rotation involves planting different crops in the same field in successive seasons, which helps in breaking the disease cycle and preventing soil depletion. Intercropping, on the other hand, involves planting two or more crops together, which improves soil fertility by adding different nutrients and organic matter.**Soil Tillage and Mixing**Soil tillage and mixing are essential for soil improvement. Tillage helps in loosening the soil, improving aeration and water infiltration. It also mixes the soil layers, ensuring uniform distribution of nutrients and organic matter. However, excessive tillage can lead to soil erosion and degradation, thus careful consideration oftillage intensity and frequency is crucial.**Soil Monitoring and Evaluation**Soil monitoring and evaluation are integral to soil improvement. Regular soil testing helps in assessing soilfertility, pH, and nutrient status, guiding farmers in making informed decisions on soil management practices. Soil monitoring also alerts farmers to potential soil degradation issues, enabling timely corrective measures. In conclusion, soil improvement is a multifaceted process that requires a holistic approach. It involves enhancing soil fertility, conserving soil, managing irrigation and drainage, practicing crop rotation and intercropping, and monitoring soil health. As agricultural practices evolve, it is crucial to adapt soil improvement techniques to suit specific geographical and cultural contexts, promoting sustainable agriculture and environmental stewardship.**农业实践中的土壤改良：跨文化视角**在农业研究领域，土壤改良始终是一个需要持续关注和创新的关键方面。
介绍土壤类型英文作文
介绍土壤类型英文作文英文：As a soil scientist, I have studied various types of soil and their characteristics. There are several different soil types, each with its own unique properties and uses.One common type of soil is sandy soil. Sandy soil has a gritty texture and is made up of larger particles. It is well-draining and does not hold water well, which can be both a benefit and a drawback. Sandy soil is good for growing plants that prefer dry conditions, such as cacti and succulents. However, it may require more frequent watering for other types of plants.Another type of soil is clay soil. Clay soil is made up of very fine particles and has a sticky, dense texture. It holds water well and is often rich in nutrients, making it good for growing a variety of plants. However, it can also become waterlogged and compacted, which can make itdifficult for plant roots to grow. 。
土壤中硒的研究报告文献
土壤中硒的研究报告文献以下是一些关于土壤中硒的研究报告文献的例子：1. Zhang, Y., et al. "Spatial variation of soil selenium in a selenium-rich area of China." Environmental Geochemistry and Health 41.5 (2019): 2303-2317.该研究通过采集和分析位于中国一个富硒地区的土壤样品，探讨了土壤中硒的空间变化规律。
研究结果表明，该地区土壤中的硒含量存在明显的空间差异，并且受土壤特征、地貌和地理位置等因素的影响。
2. Huang, Y., et al. "Effects of selenium supplementation on soil selenium, crop selenium content and human selenium intake in a seleniferous region of China." Food Chemistry 190 (2016): 243-250.该研究在中国一个富硒地区进行了一项试验，研究了硒添加对土壤硒含量、农作物硒含量和人体硒摄入量的影响。
研究结果表明，硒添加能够显著提高土壤中的硒含量，进而提高农作物的硒含量，并且对人体硒摄入量有积极影响。
3. Dinh, Q.T., et al. "Spatial distribution of selenium in paddy soils and rice grains in a seleniferous region, Northern Vietnam." Science of the Total Environment 450 (2013): 365-372.该研究调查了越南北部一个富硒地区稻田土壤和稻谷中硒的空间分布情况。
基于Web_of_Science_对土壤介质中抗生素抗性基因的文献计量分析
生态毒理学报Asian Journal of Ecotoxicology第18卷第6期2023年12月V ol.18,No.6Dec.2023㊀㊀基金项目:国家自然科学基金资助项目(41701360);河南省科技攻关项目(212102110109)㊀㊀第一作者:冯衍(1997 ),女,硕士研究生,研究方向为污染生态与生态修复,E -mail:********************㊀㊀*通信作者(Corresponding author ),E -mail:**********************DOI:10.7524/AJE.1673-5897.20230409001冯衍,王章凯,余雪巍,等.基于Web of Science 对土壤介质中抗生素抗性基因的文献计量分析[J].生态毒理学报,2023,18(6):302-313Feng Y ,Wang Z K,Yu X W,et al.A bibliometric analysis of antibiotic resistance genes in soil media based on Web of Science [J].Asian Journal of Eco -toxicology,2023,18(6):302-313(in Chinese)基于Web of Science 对土壤介质中抗生素抗性基因的文献计量分析冯衍,王章凯,余雪巍,刘天初,顾海萍*河南农业大学林学院,郑州450000收稿日期:2023-04-09㊀㊀录用日期:2023-06-22摘要:由于经济社会建设快速推进,土壤污染问题日益严峻,有关土壤治理的研究逐渐增多,但针对土壤抗生素抗性基因(anti -biotic resistance genes,ARGs)的论文仍缺乏系统整理㊂为了解ARGs 研究的发展趋势及演变特征,本文基于CiteSpace 软件和Web of Science 核心合集数据库检索出的英文文献,对2002 2022年间关于土壤ARGs 的研究进行可视化分析㊂结果显示:(1)20年间发文数量呈整体上升态势,目前正进入快速增长阶段㊂(2)朱永官㊁朱冬等形成土壤抗生素抗性基因研究的核心作者群,为新型污染物的研究提供了前沿探索,是该领域内的领导力量;中国科学院大学㊁中国科学技术大学㊁南京农业大学以及浙江大学等机构合作相对密切,已初步形成一定规模㊂(3)研究热点集中在抗生素㊁四环素㊁肥料等领域,涉及土壤ARGs 的来源㊁传播,以及主要涉及携带ARGs 的微生物等方面㊂(4)研究领域内的前沿关键词随时间不断变化,呈现出阶段性演变特征;研究领域逐渐拓宽,热点内容从宏观向微观发展,表现出持续深度划分的趋势㊂(5)高被引文献均聚焦于土壤ARGs 的来源和扩散方面,但对新型污染物与ARGs 的关系以及ARGs 的阻控研究相对较少㊂关键词:抗生素抗性基因;土壤;文献计量学;Web of Science ;CiteSpace文章编号:1673-5897(2023)6-302-12㊀㊀中图分类号:X171.5㊀㊀文献标识码:AA Bibliometric Analysis of Antibiotic Resistance Genes in Soil Media Based on Web of ScienceFeng Yan,Wang Zhangkai,Yu Xuewei,Liu Tianchu,Gu Haiping *College of Forestry,Henan Agricultural University,Zhengzhou 450000,ChinaReceived 9April 2023㊀㊀accepted 22June 2023Abstract :Due to the rapid advancement of economic and social development,soil pollution has increasingly be -come a concern,with gradually increasing soil treatment research;however,a systematic review on soil antibiotic resistance genes (ARGs)research is still lacking.In order to understand the evolution characteristics and develop -ment trend of ARGs research,this study visually analyzes the research on soil ARGs from 2002to 2022based on the English literature retrieved from the database of the Web of Science using the CiteSpace software.The findings reveal that:(1)The number of publications has generally increased in the past 20years,and it is currently entering a stage of rapid growth.(2)Yongguan Zhu,Dong Zhu,et al.formed the core author research group on soil antibi -第6期冯衍等:基于Web of Science对土壤介质中抗生素抗性基因的文献计量分析303㊀otic resistance genes,providing frontier investigation for the study of new pollutants.University of Chinese Acade-my of Sciences,University of Science and Technology of China,Nanjing Agricultural University,Zhejiang Univer-sity,and other institutions cooperate relatively closely.(3)The research hotspots mainly focus on antibiotics,tetra-cyclines,and fertilizers,dealing with the sources and transmission of soil ARGs as well as the microorganisms in-volved.(4)Frontier keywords continue to change with time,showing the characteristics of phased evolution.The research field has gradually expanded,with content shifting from macro to micro,demonstrating a trend of continu-ous in-depth division.(5)The highly cited literature focuses on the source and diffusion of soil ARGs,with rela-tively few studies on the relationship between new pollutants and ARGs,as well as the prevention and control of ARGs.Keywords:antibiotic resistance gene;soil;bibliometrics;Web of Science;CiteSpace㊀㊀抗生素在预防和控制人类细菌感染㊁畜牧业和农业生产中发挥了重要的作用[1]㊂由于抗生素的长期广泛使用和滥用,抗生素耐药性问题日益严重,已经成为世界上最严重的公共卫生问题之一[2]㊂抗生素抗性基因(antibiotic resistance genes,ARGs)是一类新型环境污染物,其可以通过水平基因转移(hori-zontal gene transfer,HGT)在环境中扩散,导致耐药性致病菌甚至超级细菌的产生,进而使传统抗生素疗法无效[3]㊂土壤是环境ARGs的重要储存库,土壤中一部分ARGs通过土壤-植物系统向人类传播扩散,另一部分则长期存在于土壤环境中,由于其具有难消除㊁易转移扩散等特点对环境产生深远影响[4]㊂目前,土壤ARGs相关研究已受到研究人员的广泛关注㊂科学知识图谱(Mapping Knowledge Domains)于2005年引入中国,其以知识域(Knowledge Domain)为对象,展示科学知识发展进程和结构关系的图像[5-6]㊂随着信息可视化的普及,绘制科学知识图谱的各种工具纷至沓来㊂其中,CiteSpace知识可视化软件为目前最流行的知识图谱绘制工具之一[7]㊂该软件应用Java语言开发,基于共引分析理论(Co-Citation)和寻径网络算法(Path Finder)等来计算特定领域的文献[8],探索出科学领域演化的主要路径及其知识拐点,并通过一系列可视化图谱的绘制来分析学科演化潜在动力机制和探测学科发展前沿[6]㊂ 文献计量学 由英国情报学家阿伦㊃普里查德于1969年提出,因其在宏观研究的客观性㊁量化和模型化的显著优势而受到许多学科的认可[9]㊂本文基于Web of Science(WOS)数据库,结合CiteSpace 工具对土壤ARGs领域的相关文献进行定量分析,了解该领域的研究现状和趋势,帮助研究人员掌握该领域的总体发展状况,并决定未来的研究方向㊂1㊀研究方法(Research method)1.1㊀数据来源数据来源于WOS核心合集数据库,设置检索的主题词为soil arg OR soil args OR soil antibiotic resistan*gene*OR arg in soil OR args in soil OR antibiotic resistan*gene*in soil,检索时间跨度为2002年1月1日至2022年11月19日,检索主题词中星号(*)代表任何字符组,或空字符,如本文resitan*可表示resistant和resistance,gene*可表示gene和genes㊂通过检索得到4274条检索结果,对检索结果进行逐条筛选,去除重复文章㊁专利和书评等,共得到1022条有效文献㊂将筛选后的文章以 摘要㊁全记录(包含引用的参考文献) 的格式下载保存为纯文本文件,作为分析数据样本㊂1.2㊀分析方法运行版本为5.7.R2的CiteSpace软件,将样本文献进行规范化处理后的数据导入CiteSpace,然后运用软件内置的作者㊁关键词以及机构等运算分析模型,绘制出土壤ARGs的知识图谱,对其研究动态㊁发展进程等进行可视化分析,以此确定土壤ARGs 相关研究的学术热点,有助于对土壤ARGs的相关深入研究提供借鉴和参考㊂2㊀结果与讨论(Results and discussion)2.1㊀土壤抗生素抗性基因相关研究时间分布特征分析发文量反映了研究领域论文数量与时间变化关系,进而使读者了解该领域的发展速度和研究现状,并预测来发展走势[10]㊂从收集到的数据来看,从2002年到2022年,发表的关于土壤ARGs SCI论文共计1022篇,通过对各年数量进行统计得到发文趋势图(图1)㊂从图1可见,自2006年后发文量逐步上升,是304㊀生态毒理学报第18卷因为有研究者将ARGs 列为一种新型污染物[11]㊂2015年世界卫生组织(World Health Organization,WHO)将ARGs 列为 21世纪人类面临的最大挑战之一 ,并宣布将在全球范围内部署防控ARGs [12]㊂此后,发文量基本呈上升态势,尤其是2020年后关于土壤ARGs 相关研究激增,这与COVID -19的大流行促使人们更加关注健康问题有关,ARGs 就是一种全球性的健康威胁㊂而从2021年到2022年呈现的下降态势,可能是2022年数据未完全统计(截至2022年11月19日)而致㊂土壤ARGs 相关研究的发文量反映了ARGs 作为一种潜在的全球健康威胁受到愈加广泛的关注,需要学者深入研究以制定有效的战略来预防和控制ARGs 在环境中的传播㊂预测2022年后相关领域的研究会呈继续增长的趋势㊂2.2㊀土壤抗生素抗性基因相关研究空间分布特征分析2.2.1㊀作者及合作共现图谱有效分析文献作者及其合作关系,有助于从整体上掌握土壤ARGs 研究领域的核心学术群体及高产作者[13]㊂土壤ARGs 研究相关作者的共现图谱如图2所示,图中的每个节点表示一个作者,节点间连线反映不同作者之间的合作关系㊂运行结果显示节点数N =199,连线数E =267,网络密度Density =0.0136,即在土壤ARGs 领域研究作者共现知识图谱中,共有199个作者及267条作者间连线㊂连线高于节点,密度相对较高,这表明对该领域有重要贡献的研究者之间存在紧密合作关系,且交流程度相对较高[6],尤其是以 朱永官(YONGGUAN ZHU)团队为中心的几位作者合作较为密切,已经形成一定规模㊂其中, 朱永官(YONGGUAN ZHU) ㊁ 朱冬(DONG ZHU) 为发文数最多的作者,分别为63篇和34篇, 陈青林(QINGLIN CHEN) 和 苏建强(JIANQIANG SU) 紧随其后,分别发表28篇和27篇㊂此外,如图2所示,共有10位作者的发文量在13篇及以上,构成土壤ARGs 的核心作者群,占总发文量的22.8%,对土壤抗性基因领域相关研究贡献较大,其他作者以独立发文或者小规模群体交流为主㊂2.2.2㊀机构及合作关系分析有效分析发文机构及其合作关系,有助于从整体上掌握土壤ARGs 研究领域的核心研究机构[14]㊂图3为土壤ARGs 研究相关机构的共现图谱,图中图1㊀2002 2022年土壤抗生素抗性基因发文趋势图Fig.1㊀Trends of publication on soil antibiotic resistancegenes from 2002to2022图2㊀2002 2022年土壤抗生素抗性基因领域作者合作共现图谱Fig.2㊀Author collaboration co -emergence map of soil antibiotic resistance genes from 2002to 2022第6期冯衍等:基于Web of Science 对土壤介质中抗生素抗性基因的文献计量分析305㊀图3㊀2002—2022土壤抗生素抗性基因领域机构合作共现图谱Fig.3㊀Institutional cooperation in the field of soil antibiotic resistance genes from 2002to 2022的每个节点表示一个研究机构,节点间连线反映着不同机构之间的合作关系㊂运行结果为节点数N =178,连线数E =152,网络密度Density =0.0096,即在土壤ARGs 领域研究机构共现知识图谱中,共有178个机构及152条机构间连线㊂连线少于节点,密度相对较高,这表明该领域有重要贡献的机构之间存在紧密合作关系,且交流程度相对较高[6]㊂其中, 中国科学院(Chinese Acad Sci) 中国科学技术大学(Univ Chinese Acad Sci) 南京农业大学(Nanjing Agr Univ) 浙江大学(Zhejiang Univ) 墨尔本大学(Univ Melbourne) 中国农业大学(Chinese Acad Agr Univ) 华南农业大学(SouthChina Agr Univ) 几个机构间合作较为密切,已经形成一定规模㊂在发文量前十的机构中,中国科学院(213篇)以绝对优势居于首位,是土壤ARGs 领域最具引领性的机构,其次是中国科学技术大学(97篇),其余机构发文量相差较小㊂2.3㊀土壤抗生素抗性基因相关研究热点分析2.3.1㊀关键词共现分析关键词是作者对于某一具体研究的学术思想㊁研究主题和研究内容的高度总结和提炼[15]㊂通过研究特定领域关键词及其出现频率可以了解掌握该领域的研究热点,评估该领域研究内容的更新速度及学科研究活力[16]㊂通过CiteSpace 软件的 Keyword 功能对2002 2022年间WOS 核心合集数据库所得到的关键词共现构建了可视化图谱(图4),图4中,每一个节点代表着一个关键词,节点大小代表关键词出现频率的高低㊂依据核心关键词,可将该领域研究分为2个主体方向㊂(1)土壤ARGs 的来源㊂ 四环素(tetracycline) 兽用抗生素(veterinary antibiotics) 大肠杆菌(Escherichia coli ) 施肥(fertilization) 粪肥(manure) 动物粪便(animal manure) 污泥(sewage sludge) 废水(waste water) 等,这些关键词表明土壤ARGs 来源于抗生素㊁粪肥㊁污泥以及废水等㊂窦春玲等[17]的研究表明,由于我国长期大量使用四环素㊁万古霉素㊁青霉素等抗生素,污水处理厂水体中不断检测出各类抗性基因㊂此外,农田土壤施用畜禽粪肥或者污水灌溉在提升土壤肥力㊁促进作物生长[18]的同时也显著提高了ARGs 的丰度和多样性,可能导致耐药性致病菌(如大肠杆菌)和ARGs 在农田生态系统中的扩散与传播[19-21]㊂(2)ARGs 对环境的影响㊂ 微生物群落(microbial com -munity) 细菌群落(bacterial community) 多样性(diversity) 恶化(degradation) 归趋(fate) 等关键词表明ARGs 能够通过微生物或者水平基因的转移而扩散,从而影响土壤微生物群落的多样性,导致土壤环境恶化,甚至可能随着食物链转移到人体,影响人类健康㊂已有研究表明,微生物为ARGs 的宿主,ARGs 的变化势必影响微生物的群落结构[22]㊂同样的,改变细菌的群落结构也会影响ARGs 的组成[23]㊂HGT 促进了ARGs 在不同种类的微生物之间进行传播,导致更多微生物(尤其是致病菌)获得抗性,并引起多重耐药菌出现[24-27]㊂依据WOS 核心合集数据库中的文献数据对排名前十的关键词进行汇总,并剔除该研究领域共有306㊀生态毒理学报第18卷图4㊀2002—2022土壤抗生素抗性基因领域关键词共现图谱Fig.4㊀Keyword co-emergence map in the field of soil antibiotic resistance genes from2002to2022关键词 抗生素抗性基因(antibiotic resistance gene) 土壤(soil) ,所得结果如表1所示㊂中心性用来量化关键词在网络中地位的重要性[28]㊂在这里把中心性ȡ0.1的关键词定义为土壤ARGs研究的核心关键词[15]㊂结合图4和表1可以看出,该领域除共有关键词外,出现频次较高的关键词有 抗生素耐药性(antibiotic resistance) 细菌(bacteria) 粪肥(ma-nure) 丰度(abundance) 等㊂体现了该领域研究鲜明的中心性,也表明这些关键词是统领土壤ARGs 领域所有研究成果的核心关键词㊂2.3.2㊀关键词聚类分析根据关键词聚类分析的数据处理结果,生成2002 2022年土壤ARGs高频关键词聚类知识图谱(图5),Q值为0.7783(>0.3),该结果表明类团结构具有显著性;S值为0.9299(>0.7),表明所得聚类具有可信性[29]㊂由图5可知,2002 2022年土壤ARGs研究高频关键词共形成了9个聚类,分别为 抗生素抗性基因(#0args) 革兰氏阴性菌(#1 Gram negative bacteria) 四环素抗性基因(#2tetra-cycline resistance genes) 抗菌素耐药性(#3antimi-crobial resistance) 溶纤维丁酸弧菌(#4Butyrivibrio fibrisolven) 肥料应用(#5manure application) 敏感性(#6susceptibility) 聚合酶链式反应(#7 qPCR) 超广谱β内酰胺酶(#8extended-spectrum beta-lactamase) ㊂聚类编号按聚类大小从0开始,即最大的聚类为 #0args ,这一聚类占有的类群最多㊂根据聚类位置分析可以将9个研究类群分为2种类型,第一类型是聚集主体研究,且研究数量较多,相互交叉重叠,是继承和衍生关系密切的类群,是土壤ARGs研究的主流领域㊂该类型主要包括以革兰氏阴性菌(#1Gram negative bacteria) 四环素抗性基因(#2tetracycline resistance genes) 抗菌表1㊀关键词重要性Table1㊀The importance of keywords频次Frequency中心性Centrality关键词Keywords2340.2Antibiotic resistance2090.13Bacteria2050.42Manure2000.25Abundance1620.14Veterinary antibiotics1480.19Tetracycline1430.42Escherichia coli1350.33Diversity1300.11Agricultural soil1290.35Gene1030.26Heavy metal750.63Environment450.12Vegetable第6期冯衍等:基于Web of Science 对土壤介质中抗生素抗性基因的文献计量分析307㊀素耐药性(#3antimicrobial resistance) 肥料应用(#5manure application) 聚合酶链式反应(#7qPCR) 为聚类关键词的类群,此类群聚集了 大肠杆菌(Escherichia coli ) 四环素抗性(tetracycline resistance) 等关键词(图4),说明这一类群主要研究土壤ARGs 的来源㊁传播及其检测手段等问题㊂第二类型和主体研究有一定距离,相对较为独立,但具有一定研究量,这一类群包括 溶纤维丁酸弧菌(#4Butyrivibrio fibrisolven ) 敏感性(#6susceptibility) 超广谱β内酰胺酶(#8extended -spectrum beta -lacta -mase) ,此类群聚集了 肥料(manure) 微生物群落(microbial community) 等关键词(图4)㊂如Barbo -sa 等[30]发现溶纤维丁酸弧菌携带tet (W)㊂生物肥料中的ARGs 可能会传播给致病微生物,从而降低致病微生物对抗生素的敏感性,对公共卫生构成高风险[31]㊂Raseala 等[32]的研究表明农业环境污染可能与沙门氏菌有直接关系,该菌株能产生超广谱β内酰胺酶,从而对多种常用抗生素具有抗性㊂说明这一类群研究内容主要涉及携带ARGs 的微生物㊂图5㊀2002—2022土壤抗生素抗性基因领域关键词聚类图谱Fig.5㊀Keywords clustering map in the field of soilantibiotic resistance genes from 2002to 20222.4㊀土壤抗生素抗性基因相关研究前沿分析研究前沿作为研究领域中最活跃的部分,有助于把握学科领域未来的研究方向和发展趋势,促进学术研究的理论升华[33]㊂CiteSpace 时区图可以直观反映不同时间段某一研究领域的研究前沿及其衍生关系,进而对未来的发展方向做出合理预测[34]㊂关键词突现是指某研究领域在某一特定时期内对某种主题关注程度的变化情况,关键词突现率反映的是某一时期内关键词使用次数增加情况的比率㊂因此,有必要在关键词突现的基础上结合相应文献综合分析研究前沿㊂本研究使用CiteSpace 将1022篇土壤ARGs 研究文献的关键词突现情况绘制成关键词突现图谱,进而结合相关文献分析土壤ARGs 研究的前沿主题,预测该领域未来的发展趋势㊂2.4.1㊀突现词统计分析通过对2002 2022年土壤ARGs 领域的核心关键词共现图谱进一步处理,点击可视化界面快捷功能键 Burstness 进入突发性探测功能区,修改参数为 The Number of States =2,γ=0.8,Minimum Du -ration =3 ,点击 View 查看突发性探测结果,最终得到2002 2022年土壤ARGs 相关研究的25个高突现值关键词,可以准确地分析出该领域的研究热点及时空演变规律[35]㊂将这些关键词按照突现开始年代由远到近顺序进行排列,得到图6㊂根据图6可知,突现强度较高的关键词有 抗生素耐药性(antibiotic resistance)(16.18) 大肠杆菌(Escherichiacoli )(12.34) 中国(China)(9.96) 四环素抗性(tetra -cycline resistance)(9.57) 抗菌素耐药性(antimicrobial resistance)(8.55) 等,即表明这些关键词在其对应时间段里均为研究者关注的前沿主题㊂从突现持续的时间跨度来看, 抗生素耐药性(antibiotic resistance) 持续时间为13年㊁ 大肠杆菌(Escherichia coli ) 持续时间为12年㊁ 质粒(plasmid) 持续时间为11年㊁ 动物(animal) 持续时间为10年㊁ 细菌(bacteria) 持续时间为9年㊁ 进展/演变(evolution) 持续时间为9年,这表明上述关键词在较长时间内均为研究的焦点㊂时间排序表明前沿关键词是随着时间推移不断变化的,整体呈现出阶段性的演变㊂因此,本研究将按照时间阶段对土壤ARGs 领域的研究前沿进行划分,并选取该时段内突现强度较高的关键词进行分析㊂由图6可知,2002 2007年为早期,突现强度较高的前沿关键词有 抗生素耐药性(antibiotic resistance)(16.18) 大肠杆菌(Escherichia coli )(12.34) ㊂徐小艳等[36]的研究表明,20世纪90年代后期兽医临床上抗生素的广泛㊁长期使用造成大肠杆菌耐药性增强㊂2008 2015年为中期,突现强度308㊀生态毒理学报第18卷较高的前沿关键词有 四环素抗药性(tetracycline resistance)(9.57) 和 Ⅰ型整合子(class1integron) (8.49) (图6)㊂冀秀玲等[37]对上海市某养殖场污水及附近农田灌溉水进行分析,发现ARGs相对表达量总体呈现出磺胺类ARGs高于四环素类ARGs的趋势㊂另外,苏志国等[38]的研究证明,Ⅰ型整合子(IntI)介导的ARGs水平转移是环境中微生物产生耐药性的重要途径,Ⅰ型整合子整合酶基因(intI1)与ARGs丰度在环境中表现出了较高的正相关性㊂2016 2022年为近期,突现强度较高的前沿关键词有 中国(China)(9.96) 污水(waste water)(8.46) ㊂土壤ARGs能在土壤甚至土壤-植物体系中扩散传播,通过食物链对动物和人体产生毒害风险,威胁人体健康和公共卫生安全,使得科研工作者对环境中ARGs的生态风险和公共卫生风险的研究愈加广泛和深入㊂2015年WHO宣布在全球范围内部署防控ARGs[12],2015年10月的十八届五中全会上 加强生态文明建设 首次被写进了 五年计划 ㊂至此,我国对包括ARGs在内的土壤污染问题重视程度显著提升㊂污水通过处理,作为再生水浇灌土壤,使土壤中ARGs大量富集,污泥作为污水处理系统的副产物,是环境中ARGs的重要存储库,直接施用污泥或污泥堆肥有可能在土壤中引入新的ARGs[39]㊂由此可见,从早期到近期土壤ARGs领域关注的焦点虽主要集中在ARGs的来源㊁传播等方面,但其研究过于单一㊂最新的研究表明,有机污染物多环芳烃能够刺激ARGs的产生[40],新型污染物微塑料是环境中ARGs传播的重要载体[41],土壤ARGs 丰度对土壤相关因子如土壤pH值具有显著依赖性[42]㊂此外,ARGs的阻控还未引起足够的重视,相关研究较为匮乏㊂因此,未来研究的热点应聚焦复合污染对ARGs的影响以及ARGs的阻控等方面㊂Top 25 keywords with the strongest citation burstsKeywords Year Strength Begin End2002 — 2022antibiotic resistance200216.1820022015▃▃▃▃▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂bacteria2002 3.720022011▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂animal2002 3.5420022012▃▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂Escherichia coli200212.3420042016▂▂▃▃▃▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂antimicrobial resistance20028.5520052012▂▂▂▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂plasmid2002 5.8420052016▂▂▂▃▃▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂diversity20027.5920072015▂▂▂▂▂▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂gene2002 6.920072015▂▂▂▂▂▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂prevalence2002 5.6720072015▂▂▂▂▂▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂evolution2002 4.6320082015▂▂▂▂▂▂▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂integron2002 4.2520092018▂▂▂▂▂▂▂▃▃▃▃▃▃▃▃▃▃▂▂▂▂environment20027.0820102018▂▂▂▂▂▂▂▂▃▃▃▃▃▃▃▃▃▂▂▂▂activated sludge2002 5.9420102018▂▂▂▂▂▂▂▂▃▃▃▃▃▃▃▃▃▂▂▂▂lagoon2002 5.2520112013▂▂▂▂▂▂▂▂▂▃▃▃▂▂▂▂▂▂▂▂▂residue2002 5.2520112013▂▂▂▂▂▂▂▂▂▃▃▃▂▂▂▂▂▂▂▂▂tetracycline resistance20029.5720132017▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃▂▂▂▂▂class 1 integron20028.4920132017▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃▂▂▂▂▂agricultural soil2002 4.5220132015▂▂▂▂▂▂▂▂▂▂▂▃▃▃▂▂▂▂▂▂▂beta lactamase2002 5.7920142018▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃▂▂▂▂land application2002 5.220142018▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃▂▂▂▂Chin a20029.9620162018▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▂▂▂▂waste2002 5.0520162019▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▂▂▂waste water20028.4620172019▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▂▂▂increase2002 3.7520192022▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃biodegradation2002 3.5520192022▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃图6㊀2002—2022土壤抗生素抗性基因领域突现词注: Strength 表示突现强度, Begin 和 End 表示开始和结束的时间,红色线段表示突现出现的起止时间段㊂Fig.6㊀Emerging words in the field of soil antibiotic resistance genes from2002to2022 Note: Strength means the burst strength; Begin and End mean the start and end time,respectively;the red linesegment means the start and end time period of the burst.第6期冯衍等:基于Web of Science 对土壤介质中抗生素抗性基因的文献计量分析309㊀2.4.2㊀时区视图分析研究热点是动态变化的,每个时间段内的研究热点都不尽相同㊂CiteSpace 软件提供了Timezone View 文献共被引网络的呈现方式,本研究中2002 2022年土壤ARGs 的研究热点时区图如图7所示㊂图7中节点代表关键词,所在年份是收集数据中该关键词首次出现的年份,此后年份关键词再出现,会在首次出现的位置进行频次累加,圆圈相应变大,所以 抗生素耐药性(antibiotic resistance) 节点圆圈大并不代表当年出现频次高,而是收集数据中该关键词出现的总频次高㊂线条代表着关键词之间的联系,若关键词同时出现在一篇论文中,那么2个关键词之间就会出现一条连线,2个年份之间也就产生联系,如果2个关键词同时出现在多篇论文中,则连线加粗㊂总体来看,土壤ARGs 领域已经形成一定规模,土壤ARGs 的研究领域将逐渐拓宽,研究的热点内容也将进一步从宏观走向微观,如 细菌群落(bacterial community) 重金属(heavy metal) 耐药基因组(resistome) 以及 微生物群落(microbial community) 等,可能随着时间推移进一步深度划分㊂2.4.3㊀文献共被引总被引频次是衡量文章质量和重要性的关键指标,期刊影响因子和被引频次都高的文章则是该领域的代表性文章,对该领域的发展具有重要影响[6]㊂CiteSpace 软件提供的2002 2022年土壤ARGs 领域高被引文献如图8所示㊂引用率高的文献体现了该领域的重要研究成果,中心性高(ȡ0.1)的节点通常是在不同领域进行连接的关键枢纽[43]㊂本文把被引频次ȡ100,且中心性ȡ0.1的文献进行整理,结果如表2所示㊂这里对表2中的高被引文献进行总结如下㊂朱永官团队[43]研究表明,中国养猪场使用抗生素和重金属作为饲料添加剂导致猪粪中二者的含量升高,进而刺激了ARGs 产生并释放到环境中,且ARGs 的丰度与二者浓度相关㊂Forsberg 等[44]对18种农业和草地土壤中的抗生素的耐药性进行功能宏基因组测序,结果表明不同土壤类型具有不同ARGs 类型,细菌群落组成是土壤ARGs 丰度的主要决定因素㊂Su 等[45]发现污水污泥堆肥过程中ARGs 的丰度和多样性显著增加,且在堆肥过程中观察到ARGs 与细菌群落结构显著相关,即直接在田间施用污泥堆肥可能会导致土壤中大量ARGs 的扩散㊂Heuer 等[46]研究发现,在农业土壤中施用肥料可以显著增加土壤中ARGs 和耐药细菌的丰度,ARGs 可能通过可移动遗传元件转移㊂Wang 等[47]的研究指出,种植对施用粪肥土壤中ARGs 的分布有一定影响,从种植在施用粪肥土壤中的蔬菜中能图7㊀2002—2022抗生素抗性基因领域热点关键词时区图Fig.7㊀Time zone map of hot keywords in the field of antibiotic resistance genes from 2002to 2022310㊀生态毒理学报第18卷图8㊀2002 2022土壤抗生素抗性基因领域高被引文献Fig.8㊀Highly cited papers in the field of soil antibiotic resistance genes from2002to2022表2㊀2002 2022土壤抗生素抗性基因高被引文献Table2㊀Highly cited papers on soil antibiotic resistance genes from2002to2022被引次数Citation中心性Centrality第一作者First author发表年份Year of publication论文题目Title of article2920.25Zhu Y G2013Diverse and abundant antibiotic resistance genes in Chinese swine farms 1750.37Forsberg K J2014Bacterial phylogeny structures soil resistomes across habits1260.22Su J Q2015Antibiotic resistome and its association with bacterial communities during sewage sludge composting1150.25Heuer H2011Antibiotic resistance gene spread due to manure application on agricultural fields 1000.12Wang F H2015Antibiotic resistance genes in manure-amended soil and vegetables at harvest够检测到ARGs,这对人体健康有潜在威胁㊂由此可见,上述5篇高被引文献均聚焦于土壤ARGs的来源和扩散方面,与2.3.1节中核心关键词研究结果一致,这再次表明,目前关于ARGs从产生到传播的过程研究较为成熟,但对复合污染物与ARGs的关系以及ARGs的阻控相关研究还有待进一步完善㊂3㊀总结与展望(Summary and prospect)本文利用CiteSpace软件,对2002 2022年间WOS核心合集数据库中有关土壤ARGs的1022篇英文文献进行归纳总结,根据作者㊁发文机构㊁关键词㊁时区图㊁高被引文献等方面趋势特点绘制图谱,研究得出如下结论㊂(1)20年间土壤ARGs的相关研究数量逐年增多,尤其是近年来进入快速增长阶段㊂人们对于土壤污染现状的关注日渐提升,针对ARGs这类新型污染物的研究正走向高峰㊂(2)朱永官㊁朱冬等形成土壤ARGs领域研究的核心作者群,为新型污染物的研究提供了前沿探索,是该领域内的领导力量㊂其余作者群体研究相对独立,缺乏跨领域合作㊂中国科学院大学㊁中国科学技术大学㊁南京农业大学㊁浙江大学等机构合作相对密切,以高校和研究机构为主体,已初步形成一定规模㊂(3) 抗生素耐药性 细菌 肥料 四环素 等是目前土壤ARGs研究的核心关键词㊂关键词聚类主要分为两大方面,一是研究土壤ARGs的来。
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Until recently, methods used for residue analysis for soil were largely those established for food analysis with minor modification, and no specific methods were available for soil (i). Although some progress has b e e n made recently in extraction methods (2, 3) there are many undeveloped areas in pesticide methodology with soil. In this study, the suitability and limitations of three analytical techniques, thin-layer chromatography (TLC), gas chromatography (GLC) and insect bioassay are compared for the determination of aldrin, dieldrin and DDT residues in different soils w i t h o u t cleanup. Analysis without cleanup has certain advantages because cleanup procedures are usually time consuming and may cause a loss of extracted pesticides (4). Determination without cleanup was thought to be possible with soil because its organic content is usually low. Three soil types (sandy-loam, clay and muck) of known pesticide spray history were extracted with five solvent systems. Extracts were evaluated using the above 3 analytical techniques in parallel in a similar manner to that used for foods by Phillips et al. (5)~ i_/ Present address: Research Station, Dept. of Agriculture, Vineland Vineland Station, Onto Canada
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from a sandy-loam soil (2) but since other soil types were b e i n g used, other p e s t i c i d e s w e r e b e i n g measured, and other analytical techniques b e i n g evaluated a c r o s s - s p e c t r u m of solvents was evaluated~ Post e x t r a c t i o n procedures, such as partitioning, w a s h i n g and c o n c e n t r a t i o n are those described by Chiba and Morley, since they have b e e n shown to give quantitative recoveries of extracted pesticides (4). A n a l y t i c a l Techniclues Thin-layer chromato@raphy. Thin-layer chromatography was carried out b y using 5~ acetone in n-hexane as a d e v e l o p i n g solvent (6). The a m o u n t and position of b a c k g r o u n d on TLC (related to the amount of coextractives) was o b s e r v e d under u l t r a v i o l e t (UV) light, then the plate was sprayed with ammonical silver nitrate as the chromogenic reagent (6). Gas chromatography. An A e r o g r a p h Hi-Fi Model 600 with an e l e c t r o n - c a p t u r e detector was used. Experimental parameters were as follows: Column temp. 173~ injection temp. 164~ detector tempo 173~ N 2 gas 15 p.s.i. (20 ml. per minute); column a m i x t u r e of DC-II and QF-I (3 to 2) 4% on chromosorb W Regular (60-80 mesh), 2 feet x 1/8 inches aluminum; injection volume 2 ~i. Bioassay. Solvent extracts of soils w e r e assayed with D r o s o p h i l a using a dry-film technique (7), and direct b i o a s s a y (8). Results and D i s c u s s i o n Thin-layer C h r o m a t o g r a p h y The use of this technique in parallel screening showed that b a c k g r o u n d interference varied m a r k e d l y for different e x t r a c t i o n systems and soil types and, as expected, was almost directly related to the amount of co-extractives, which was expressed in w e i g h t (mg. per g. of original soil) and shown in Table io When the equivalent of 2 g. of sandy loam or ቤተ መጻሕፍቲ ባይዱlay soil was spotted, the lower limits of d e t e c t a b i l i t y for p,p'DDE, p,p'-DDT, aldrin and dieldrin were 0.i mmg., 0.2 mmg., 0~ mmgo and 1.0 mmg. respectively. Thus, a convenient screening of these pesticides can b e made w i t h o u t cleanup. With m u c k soil, however, Rf values and lower limits of d e t e c t a b i l i t y varied m a r k e d l y owing to the b a c k g r o u n d interference from the coex-
2--/ Present address: Depto of Fisheries and Forestry, Chemical Control Research Institute, Ottawa, Can. Contribution Service~ No. 94 Analytical Chemistry Research
Vol. 5, No. 3, 1970, published by Springer-Verlag New York Ine.
Bulletinof Environmental Contamination Toxicology, &
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Only field-treated soils of known pesticide history were used for comparison purposes~ Although absolute amounts of residues present were not known for comparison purposes this was not important since the same extract was used to evaluate all 3 systems~ Cleanup was avoided except where partial cleanup was effected inadvertently by solvent transference. Thus, the simplified experimental design allowed a direct evaluation of the analytical techniques and of the effect of co-extractives. GLC was used as a "yardstick" to evaluate the other two techniques. Use of GLC with soil extracts that have not Deen cleaned-up appears to present few problems (2) but little has been done using TLC. It was hoped that TLC might provide a "fingerprint" as to soil type and also provide a rapid screening technique for residues in soil similar to that used previously for wheat extracts (6). Bioassa~ while nonspecific, measures the availability of all material toxic to the insect in contrast to GLC which responds to individual compounds~ Results obtained by these two techniques were compared to see if differences could be correlated with different soil types. Materials and Methods Soils Samples of top soil of three general types (Matilda sandy loam, muck and Rideau clay) each with a known history of organochlorine insecticide treatments, were collected from the Experimental Farm, Canada Department of Agriculture, Ottawa. All samples were air-dried at room temperature and screened through a 10-mesh sieve Defore extraction. Solvent and Extraction Solvents used were distilled in glass grade (Burdick and Jackson lab., Inc.) and all were checked for impurities before u s e . Acetonitrile, acetonitrile/acetone (i/i, v/v), acetone/n-hexane (1/9, v/v), acetone/n-hexane (i/i, v/v) and methanol/methylene chloride (i/i, v/v) were used as extraction solvents~ Many of these systems were known to be relatively inefficient for the extraction of aldrin and dieldrin