中亚热带四种森林土壤团聚体及其有机碳分布特征

选择中亚热带地区4种典型森林类型:杉木林、湿地松林、毛竹林和次生林4种森林土壤为研究对象,研究了森林类型对土壤不同发生层水稳性团聚体及其有机碳分布特征的影响。结果表明:不同森林类型对>5 mm和2~5 mm土壤团聚体含量影响显著(p<0.05),表现为次生林>杉木林>毛竹林>湿地松林,而在其他粒径无显著差异。0~30 cm土层内团聚体R0.25和MWD次生林显著高于其他人工林,杉木林次之,湿地松林和毛竹林最低,其他土层无显著差异。各森林类型同土层不同粒径团聚体中有机碳含量随粒径大小变化,团聚体粒径越小,有机碳含量越高。0~10 cm土层同粒径土壤团聚体有机碳含量从大到小依次是次生林、杉木林、湿地松林和毛竹林,而在其他土层各森林类型之间差异不显著。     

黄土区大型露天煤矿复垦24a土壤碳氮组分特征

研究大型露天煤矿典型植被恢复模式下土壤碳、氮组分特征,为深入理解露天煤矿高压实复垦土壤碳、氮固持过程及复垦土壤成土过程提供依据。以黄土区典型露天煤矿-平朔露天煤矿的复垦24a的刺槐-油松混交林（RP）、刺槐纯林（RM）、白杆-青杆-沙棘混交林（PPH）、沙棘纯林（HM）4种典型植被恢复模式样地及未复垦样地（UR）、原地貌样地（UD）为研究对象,采集0～20 cm表层土壤样品,分析土壤碳、氮组分特征。结果表明：1）经过24a演替,复垦土壤理化性质得到明显提高,但各样地间土壤理化性质明显显著。2）同UR样地相比,RP、RM和PPH样地的土壤有机碳含量分别提高594.03%、158.66%、99.25%;RP和PPH样地的微生物生物量碳则分别提高174.68%、209.34%;RP和HM样地的水溶性碳分别提高110.13%、82.99%;RP、RM样地的易氧化碳含量则分别为UR样地的35.55倍和7倍。3) 与UR样地相比,RP、RM、PPH、HM样地的土壤全氮含量分别提高413.24%、250.00%、100%和82.35%;RP、PPH、HM样地的水溶性氮含量分别提高206.33%、153.02%、99.95%;PPH样地的微生物生物量氮含量提高368.07%。4）复垦土壤碳、氮组分之间呈现正相关关系,但与全钾呈现显著负相关（P<0.05）。上述结果表明,土地复垦显著提高了土壤碳、氮组分含量,尤以刺槐-油松混交林效果较好,但复垦24a后,矿区土壤可能存在钾限制的现象。     

Land use effects on soil carbon fractions in the southeastern United States. II. changes in soil carbon fractions along a forest to pasture chronosequence

Since land use change can have significant impacts on regional biogeochemistry, we investigated how conversion of forest and cultivation to pasture impact soil C and N cycling. In addition to examining total soil C, we isolated soil physiochemical C fractions in order to understand the mechanisms by which soil C is sequestered or lost. Total soil C did not change significantly over time following conversion from forest, though coarse (250–2,000 m) particulate organic matter C increased by a factor of 6 immediately after conversion. Aggregate mean weight diameter was reduced by about 50% after conversion, but values were like those under forest after 8 years under pasture. Samples collected from a long-term pasture that was converted from annual cultivation more than 50 years ago revealed that some soil physical properties negatively impacted by cultivation were very slow to recover. Finally, our results indicate that soil macroaggregates turn over more rapidly under pasture than under forest and are less efficient at stabilizing soil C, whereas microaggregates from pasture soils stabilize a larger concentration of C than forest microaggregates. Since conversion from forest to pasture has a minimal impact on total soil C content in the Piedmont region of Virginia, United States, a simple C stock accounting system could use the same base soil C stock value for either type of land use. However, since the effects of forest to pasture conversion are a function of grassland management following conversion, assessments of C sequestration rates require activity data on the extent of various grassland management practices.     

不同植被类型对滨海盐碱土壤有机碳库的影响

对江苏滨海盐碱地5种不同植被类型土壤（0 ~ 40 cm）有机碳（SOC）含量、密度和表层（0 ~ 20 cm）土壤微生物量碳（SMBC）、可溶性有机碳（DOC）含量及其占总有机碳（TOC）的比例进行了分析。结果显示,随土层深度的增加,SOC含量降低,表层SOC密度占整个剖面的54.6% ~ 75.8%。表层SOC含量和密度分别介于2.02 ~ 9.61 g/kg和5.87 ~ 21.54 t/hm2,平均值分别为4.77 g/kg和12.56 t/hm2。随着原生植被群落的演替（光滩→盐蒿→茅草）,SOC、SMBC和DOC含量均依次增加。茅草荒地围垦后,稻-油轮作地和菊芋地表层SOC密度分别比茅草地的增加了55%（5.77 t/hm2）和107%（11.15 t/hm2）;稻-油轮作地的SMBC含量及SMBC/TOC比值下降,而菊芋地的上升;围垦后土壤DOC含量及DOC/TOC比值都明显下降。结果表明,滨海盐碱地SOC主要分布在表层,原生植被群落的顺行演替使SOC库容增加且活性增强,在盐荒地围垦初期（3年）,SOC库容增加但活性有所减弱。经估算,滨海盐碱非耕地具有较大的固碳潜力,但需要合理的耕作管理措施来保证农业生产的可持续发展并实现增汇减排的目标。     

Physical protection of soil organic matter following mechanized forest operations in Pinus radiata D.Don plantations

Mechanized forest operations have a large impact on soil systems, and may cause disruption of aggregates and exposure of previously protected soil organic matter to microbial attack. In this study, we investigated how C storage, soil structure and unprotected, physically protected and resistant C pools recover 0, 7 and 20 years after mechanical harvesting and site preparation in second rotation Pinus radiata D.Don plantations. We detected differences in dry mean weight diameter (DMWD) of aggregates, with higher DMWD in 0- and 7-year-old pine stands (8.7 and 7.5 mm, respectively) than in 20-year-old stands (4.1 mm). This was attributed to the compaction induced by heavy machinery, as reflected by the positive relationship between DMWD and bulk density (r = 0.85, P < 0.05). Organic C contents in the top 5 cm were similar 0 and 7 years after disturbance, but were twice as high after 20 years, with mean values of 25, 28 and 52 Mg C ha⁻¹, respectively. In addition, the resistant C pool was also reduced by up to 7% after clearance. In contrast, unprotected C and physically-protected C were greatest in the youngest stands, indicating that stand establishment and harvesting with heavy machinery may have broken soil aggregates and exposed the previously protected SOM to microbial decomposition and that forest operations may create new mega-aggregates able to protect organic residues. However, the lowest physically-protected C values in 20-year-old pine stands may also be attributed to differences in SOM quality. The alkyl C/O-alkyl C ratios were highest in the oldest stands, indicating that SOM in these stands was more resistant to further decomposition. Mechanized forest operations in radiata pine stands released at least 30 t C ha⁻¹ from the first 5 cm of the soil profile immediately after clearance, caused significant alterations in the soil structure, which lasted for a minimum of seven years, and also reduced the resistant C pool. As the Kyoto Protocol encourages forest management practices that potentially increase carbon sequestration, mechanical harvesting and site preparation in these steep slopes should be reconsidered.     

Contribution of lignin and polysaccharides to the refractory carbon pool in C-depleted arable soils

We assessed the contribution of polysaccharides and lignin, major components of plant residues, to the refractory pool of soil organic carbon (SOC) in arable soils. Soil samples from two contrasting treatment types of European long-term agroecosystem experiments, i.e. conventionally managed (fertilized) and C-depleted plots, enriched in refractory compounds, were compared. Bulk samples from eight experimental sites and particle-size fractions of two of the sites were investigated. The CuO oxidation technique was used as a relative measure of lignin and its degree of structural alteration. The contents and composition of polysaccharides were determined following hydrolysis with trifluoroacetic acid (TFA). For the bulk samples, the amount of lignin phenols declined more than the total OC in the course of C-depletion. The contribution of lignin phenols to total OC was thus lower in the C-depleted versus the fertilized plots. A greater lignin biodegradation was found in the bulk samples of the depleted plots compared with the fertilized plots. The analysis of size fractions revealed lower OC-normalized contents of lignin phenols and a higher degree of lignin alteration in fractions <63 μm of the depleted versus the fertilized plots. These findings indicate that lignin does not accumulate within the refractory C pool of arable soils. The refractory SOC pool shows a lower contribution of lignin as compared with more labile fractions of SOC. If lignin-derived carbon is present in the stable pool it has been extensively modified so that it can no longer be identified as phenolic CuO oxidation products. OC-normalized contents of polysaccharides (neutral sugars and galacturonic acid) were similar in bulk samples of the C-depleted and fertilized plots. The contrasting treatments showed similar polysaccharide contents especially in separates <6 μm. The separates <6 μm in the C-depleted plots retained between 50 and 100% of the polysaccharide amounts in the fertilized plots. The mass ratio of (galactose+mannose)-to-(arabinose+xylose) (GM/AX) was higher in bulk samples of the C-depleted versus the fertilized plots, indicating a higher relative contribution of microbial sugars. Within a particular soil, the fine separates were those with the highest GM/AX ratio. These results indicate that the refractory C pool has a similar proportion of polysaccharides as the labile C pool, but refractory polysaccharides are mainly associated with fine separates and show a dominant contribution of microbial sugars. Our results provide evidence that polysaccharides, mainly those of microbial origin, are stabilized over the long-term within fine separates of arable soils. In contrast, CuO lignin is associated mainly with the coarse fractions and does not contribute to the refractory C pool.     

有机肥对棕壤不同粒级有机碳和氮的影响

采集棕壤长期肥料定位试验站不施肥和施用不同用量有机肥的土壤,通过超声波分散—离心分离得到细黏粒(<0.2μm)、粗黏粒(0.2～2μm)、粉粒(2～53μm)、细砂粒(53～250μm)和粗砂粒(250～2000μm)5个颗粒级别后,分析全土及不同粒级中土壤有机碳和氮并进行含量与分布的比较。结果表明,有机质主要分布于黏粒级中,其含量占全土有机碳的42.8%、全氮的58.3%,碳氮比随着粒级的增加而逐渐增大,表明氮易于在小粒级中富集。长期施用有机肥后,全土及各粒级有机碳和氮含量均有显著增加;砂粒级中有机碳和氮的富集系数升高,黏粒级中富集系数降低,粉粒级和砂粒级中的碳氮比降低。增加有机肥的用量加强了全土和各粒级对有机碳和氮的积累,同时加强了粉粒级和砂粒级碳氮比降低的程度。     

适宜超高压处理条件脱除大蒜臭味保持抗氧化和抑菌能力

为了提升大蒜头产品的品质,该研究将超高压技术应用于大蒜头产品处理中,探究了在200、300、400、500 MPa压力条件下处理10 min,大蒜风味物质,尤其是含硫挥发性化合物的变化,同时考察超高压对大蒜主要活性成分大蒜素含量、抗氧化和抑菌能力的影响.试验结果表明,超高压处理较于在95℃下60 s的蒸汽漂烫处理,不仅具有良好的杀菌作用,同时还可以去除大蒜中的刺激性风味,起到脱臭作用.大蒜经500 MPa处理后,主要蒜臭味嗅感物质二烯丙基二硫化物含量降低至30.69%,经过热处理的大蒜,二烯丙基二硫醚化合物则降低至54.68%,与超高压处理后的大蒜具有显著性差异(P<0.05).500 MPa处理后的大蒜中大蒜素浓度上升至0.079 mmol/L,高出热处理组具有显著性差异(P<0.05);铁离子还原能力较热处理组高出64.24%,具有显著性差异(P<0.05),1,1-二苯基-2-三硝基苯肼清除率高出热处理组28.68%,具有显著性差异(P<0.05);经热处理后的大蒜均丧失全部抑菌能力,而超高压处理后的大蒜对不同种的细菌仍具有一定的抑菌能力,对黑曲霉的抑菌能力与无处理组无显著差异.相关性分析结果显示,大蒜的抑菌能力与硫醚类化合物显著相关(r>0.884),与二烯丙基二硫醚、总酚含量未呈现显著相关,抗氧化能力未与硫醚类化合物含量、二烯丙基二硫醚、总酚呈显著相关趋势.研究结果为大蒜头产品的品质改良提供参考.     

Changes in soil organic carbon fractions and bacterial community composition under different tillage and organic fertiliser application in a maize−wheat rotation system

ABSTRACT

The objective of this study was to assess the impact of different tillage and organic fertiliser regimes on soil carbon fractions and bacterial community composition within a maize–wheat cropping system. We conducted a six-year experiment on the Huang-Huai-Hai Plain of China. Six treatments were established: deep tillage (DT), shallow tillage (ST), no-tillage (NT), deep tillage with organic fertiliser (DTF), shallow tillage with organic fertiliser (STF), and no-tillage with organic fertiliser (NTF). Results indicated that during the winter wheat growing season, the highest contents of soil organic carbon (SOC) and easily-oxidised organic carbon (EOC) were in the STF treatment. During the summer maizegrowing season, the DTF treatment had the highest SOC and EOC contents. Compared with the other treatments, the NTF treatment had higher Chao1 and Shannon indices for bacteria; however, the relative abundance of Proteobacteria is highest in all treatments. A redundancy analysis (RDA) revealed that bacterial community composition was correlated with variation of the SOC, DOC, EOC, and microbial biomass carbon (MBC). Our results showed that combining the two components of the SOC fractions and bacterial community composition, STF practice in a maize–wheat rotation was a sustainable approach to optimising soil structure and improving soil quality.     

Use of organic carbon and loss-on-ignition to estimate soil organic matter in different soil types and horizons

Summary Loss-on-ignition (LOI) and the organic C content have been used to estimate soil organic matter. Organic matter is often estimated from organic C by applying a factor of 1.724. Several authors have examined the relationship between LOI, used as an estimate of organic matter, and C by simple linear regressions. In the present study, this approach was examined in relation to two sets of data. LOI overestimates organic matter in soils with significant proportions of clay minerals because of bound water, and correcting for bound water gives some LOI: C ratios of less than 1. It is concluded that differences in the nature of the organic matter in different soils and horizons make the simple regression approach unsuitable. More attention needs to be paid to studies of the nature of the organic matter.     

Spatial and seasonal variations in soil respiration in a temperate deciduous forest with fluctuating water table

Our objectives were to determine both spatial and temporal variations in soil respiration of a mixed deciduous forest, with soils exhibiting contrasting levels of hydromorphy. Soil respiration (R_S) showed a clear seasonal trend that reflected those of soil temperature (T_S) and soil water content (W_S), especially during summer drought. Using a bivariate model (RMSE=1.03), both optimal soil water content for soil respiration (W_SO) and soil respiration at both 10 °C and optimal soil water content (R_S10) varied among plots, ranging, respectively, from 0.25 to 0.40 and from 2.30 to 3.60 μmol m⁻² s⁻¹. Spatial variation in W_SO was related to bulk density and to topsoil N content, while spatial variation in R_S10 was related to basal area and the difference in pH measured in water or KCl suspensions. These results offer promising perspectives for spatializing ecosystem carbon budget at the regional scale.     

Residue cover and rainfall intensity effects on runoff soil organic carbon losses

Soil cover and rainfall intensity (RI) are recognized to have severe impacts on soil erosion and an interaction exists between them. This study investigates the effect of rainfall intensity (RI) and soil surface cover on losses of sediment and the selective enrichment of soil organic carbon (SOC) in the sediment by surface runoff. A field rainfall simulator was used in the laboratory to produce 90 min rainfall events of three rainfall intensities (65, 85 and 105 mm h^− 1) and four cover percentages (0%, 25%, 50% and 75%) on soil material at 9% slope. A strong negative exponential relation was observed between cover percentage and RI on sediment loss under 85 and 105 mm h^− 1 of rain, while under RI of 65 mm h^− 1, the highest sediment loss was observed under 25% cover. Overall, higher RI and lower cover produced higher sediment and consequently higher nutrient loss, but resulted in a lower SOC enrichment ratio (ER_SOC) in the sediment. The amount of runoff sediment rather than the ER_SOC in the sediment was the determinant factor for the amount of nutrients lost. The values of ER_SOC were high and positively correlated with the ER values of particles smaller than 20 µm (p < 0.01). Although the sediment contained substantially more fine fractions (fine silt and clay, < 20 µm), the original soil and runoff sediment were still of the same texture class, i.e. silt clay loam.     

浙江南部亚热带森林土壤植硅体碳的研究

植硅体封存有机碳(Phytolith-occluded organic carbon,Phyt OC)是一种稳定的有机碳形态。它由植物自身硅化作用产生,在植物死亡或凋落后归还于土壤,从而影响森林生态系统稳定性碳库的储量。本文以浙江庆元县5种不同亚热带典型森林立地土壤为研究对象,利用不同土层深度(0~10 cm、10~30 cm、30~60 cm和60~100 cm)土壤样品,分析土壤植硅体含量和植硅体碳含量,并估算土壤中植硅体碳储量。结果表明,毛竹林、杉木林、针阔混交林、阔叶林和马尾松林土壤植硅体含量(土壤剖面平均值)变化范围在8.14~19.74 g kg-1,其中毛竹林土壤植硅体含量最高。而植硅体中Phyt OC平均含量最高的为马尾松林(24.31 g kg-1),最低的为针阔混交林(13.06 g kg-1)。土壤Phyt OC/TOC比值随土层深度增加而急剧增加。统计分析表明,不同林分下土壤硅含量与土壤植硅体含量呈极显著相关关系(p0.01),与土壤Phyt OC含量之间呈显著的正相关关系(p0.05)。我国亚热带毛竹林、杉木林、马尾松林、阔叶林和针阔混交林1 m土体Phyt OC总储量分别为1.988×107、4.025×107、2.575×107、2.542×107和0.340×107 t。     

The influence of different fertilization practices on concentrations of organic carbon and total nitrogen in particle-size fractions during 34 years of a soil formation experiment in loamy marl

Summary The concentrations of organic C and total N in five different particle-size fractions were studied under different mineral and organic fertilizer regimens by examining soil samples from the 34-year-old soil-formation pot experiment Hu 3 in Rostock. The C and N concentrations were generally highest in the clay fraction and decreased in the order medium silt >fine silt >coarse silt and sand. For nearly all years and size fractions the following order was obtained for C and N concentrations under the various fertilizer regimens: Compost >farmyard manure >straw + mineral fertilizer >mineral fertilizer. The various particle-size fractions and fertilizer regimens differed in the development of soil organic matter levels. Consequently, characteristic redistributions were found in the proportions of C and N in the various particle-size fractions, particularly after organic fertilizer was no longer applied (years 20–34). This experimental phase was characterized by decreased organic C and increased total N concentrations, and increased proportions of C and N in the clay-size at the expense of the sand fractions.     

Variability in soil properties at different spatial scales (1 m-1 km) in a deciduous forest ecosystem

The purpose of this research was to test the hypothesis that variability in 11 soil properties, related to soil texture and soil C and N, would increase from small (1 m) to large (1 km) spatial scales in a temperate, mixed-hardwood forest ecosystem in east Tennessee, USA. The results were somewhat surprising and indicated that a fundamental assumption in geospatial analysis, namely that variability increases with increasing spatial scale, did not apply for at least five of the 11 soil properties measured over a 0.5-km² area. Composite mineral soil samples (15 cm deep) were collected at 1, 5, 10, 50, 250, and 500 m distances from a center point along transects in a north, south, east, and westerly direction. A null hypothesis of equal variance at different spatial scales was rejected (P?0.05) for mineral soil C concentration, silt content, and the C-to-N ratios in particulate organic matter (POM), mineral-associated organic matter (MOM), and whole surface soil. Results from different tests of spatial variation, based on coefficients of variation or a Mantel test, led to similar conclusions about measurement variability and geographic distance for eight of the 11 variables examined. Measurements of mineral soil C and N concentrations, C concentrations in MOM, extractable soil NH₄-N, and clay contents were just as variable at smaller scales (1-10 m) as they were at larger scales (50-500 m). On the other hand, measurement variation in mineral soil C-to-N ratios, MOM C-to-N ratios, and the fraction of soil C in POM clearly increased from smaller to larger spatial scales. With the exception of extractable soil NH₄-N, measured soil properties in the forest ecosystem could be estimated (with 95% confidence) to within 15% of their true mean with a relatively modest number of sampling points (n?25). For some variables, scaling up variation from smaller to larger spatial domains within the ecosystem could be relatively easy because small-scale variation may be indicative of variation at larger scales.     

Carbon limitations to nitrous oxide emissions in a humid tropical forest of the Brazilian Amazon

The availability of labile organic C for microbial metabolic processes could be an important factor regulating N₂O emissions from tropical soils. We explored the effects of labile C on the emissions of N₂O from a forest soil in the State of Rondônia in the southwestern quadrant of the Brazilian Amazon. We measured emissions of N₂O from a forest soil after amendments with solutions containing glucose, water only or NO₃^–. Addition of glucose to the forest soil resulted in very large increases in N₂O emissions whereas the water only and NO₃^– additions did not. These results suggest a strong C limitation on N₂O production in this forest soil in the southwestern Amazon.     

Natural nitrogen-15 abundance and carbohydrate content and composition of organic matter particle-size fractions from a sandy soil

Sandy soil samples collected from under a woody/grass savanna in the Lamto experimental area (6°13N, 5°20W; Côte dIvoire, West Africa), were fractionated according to particle size with the aim of measuring the natural abundance of ¹⁵N and determining the contents and composition of hydrolysable carbohydrates of soil organo-mineral particles for a better understanding of the contribution of each individual fraction to the soil function. The contributions of the fractions <20 m to the total pool of organic matter were 77% for C and 84% for N. Larger amounts of carbohydrates were found in the clay and silt fractions (3,784–6,043 g g^–1 soil). The carbohydrate composition indicated that microbe-derived carbohydrates [e.g. galactose (Gal) and mannose (Man)] accumulated preferentially in the fine fractions while plant-derived sugars [e.g. arabinose (Ara) and xylose (Xyl)] were dominant in coarse fractions. A negative relationship was observed between C:N ratio and ¹⁵N natural abundance on the one hand, and on the other hand between C:N and (Gal+Man):(Ara+Xyl), Man:(Ara+Xyl) and Man:Xyl ratios, clearly indicating that the chemistry of the organic materials of the particle-size fractions reflects a change from soil chemistry dominated by plant materials to that dominated by microbial biomass and metabolites. The contribution of a given fraction to soil microbial activity is controlled by the quality or quantity of associated soil organic matter, its microbial biomass and also by the accumulation of microbial-derived carbohydrates which can be resynthesized or recycled.     

Landuse impacts on soil organic carbon fractions in different rainfall areas of a subtropical dryland

Landuse can alter soil organic carbon (SOC) fractions by affecting carbon inflows and outflows. This study evaluated changes in SOC fractions in response to different landuses under variable rainfalls. We compared cropland, grassland and forest soils in high rainfall (Islamabad ~1142 mm) and low rainfall (Chakwal ~667 mm) areas of Pothwar dryland, Pakistan. Forest soils in both rainfall areas had highest SOC (11.32 g kg^?1), particulate organic carbon (POC, 1.70 g kg^?1), mineral-associated organic carbon (MOC, 7.17 g kg^?1) and aggregate-associated organic carbon (AOC, 7.86 g kg^?1). However, in rangeland and cropland soils, these varied with rainfall. Under high rainfall, SOC and MOC were 12% and 17% higher in rangeland than in cropland while POC and AOC were equal. Under low rainfall, SOC and MOC were higher in rangeland than in cropland by 7.21 and 1.79 g kg^?1 at 0–15 cm and equal at 15–30 cm depth. POC and AOC were higher in rangeland than in cropland, in both depths. Averagely, SOC, POC, MOC and AOC were 26%, 68%, 76% and 30% higher in high rainfall than in low rainfall soils. Sensitivity of SOC fractions to landuses observed under different rainfalls could provide useful information for soil management in subtropical drylands.     

Plant species affect the concentration of free sugars and free amino acids in different types of soil

Substantial amounts of low molecular weight organic compounds (LMWs) such as sugars and amino acids are transferred from plant roots into soil. These substances are released due to decomposition processes or leaching (exudation). Afterwards they can be metabolized by soil microorganisms into different compounds, or they can be partially re‐absorbed by the plants. The aim of this study was to clarify the influence of five wild plant species on the composition and pool sizes of LMWs extractable from three different soils. Four of the five species caused significant changes in soil LMW pools. In Chernozem, the sugar concentrations of soil with plants were up to 60 % higher than those of the bulk reference soil, and amino acids increased by as much as 207 %. The relative abundance of free amino acids in roots did not correlate with the relative abundance of amino acids in soil after six weeks of plant growth. The relative abundance of soil amino acids, that increased after plant growth, was strongly dependent on the type of soil and on the plant species present. We suggest that rather than rhizodeposition being dependent on soil type, it reflects differential microbial metabolization of amino acids in the respective soils.