Microbial residues as indicators of soil restoration in South African secondary pastures

Prolonged intensive arable cropping of semiarid grassland soils in the South African Highveld resulted in a significant loss of C, N and associated living and dead microbial biomass. To regenerate their soils, farmers converted degraded arable sites back into secondary pastures. The objective of this study was to clarify the contribution of microorganisms to the sequestration of C and N in soil during this regeneration phase. Composite samples were taken from the topsoils of former arable land, namely Plinthustalfs, which had been converted to pastures 1-31 years ago. Amino sugars were determined as markers for microbial residues in the bulk soil and in selected particle-size fractions. The results showed that when C and N contents increased during the secondary pasture usage, the amino sugar concentration in the bulk soil (0-5 cm) recovered at similar magnitude and reached a new steady-state level after approximately 90 years, which corresponded only to 90% of the amino sugar level in the primary grassland. The amino sugar concentration in the clay-sized fraction recovered to a higher end level than in the bulk soil, and also at a faster annual rate. This confirms that especially the finer particles contained a high amount of amino sugars and were responsible, thus, for the restoration of microbially derived C and N. The incomplete recovery of amino sugars in bulk soil can only in parts be attributed to a slightly coarser texture of secondary grassland that had lost silt through wind erosion. The soils particularly had also lost the ability to restore microbial residues below 5 cm soil depth. Overall, the ratios of glucosamine to muramic acid also increased with increasing duration of pasture usage, suggesting that fungi dominated the microbial sequestration of C and N whereas the re-accumulation of bacterial cell wall residues was less pronounced. However, the glucosamine-to-muramic acid ratios finally even exceeded those of the primary grassland, indicating that there remained some irreversible changes of the soil microbial community by former intensive crop management.     

Fate of microbial residues in sandy soils of the South African Highveld as influenced by prolonged arable cropping

Long‐term cultivation of former grassland soils results in a significant decline of both living and dead microbial biomass. We evaluated the effect of duration of cropping on the preservation of fungal and bacterial residues in the coarse‐textured soils of the South African Highveld. Composite samples were taken from the top 20 cm of soils (Plinthustalfs) that have been cropped for periods varying from 0 to 98 years in each of three different agro‐ecosystems in the Free State Province. Amino sugars were determined as markers for the microbial residues in bulk soil and its particle‐size fractions. Long‐term cultivation reduced N in the soil by 55% and the contents of amino sugars by 60%. Loss rates of amino sugars followed bi‐exponential functions, suggesting that they comprised both labile and stable fractions. With increased duration of cropping the amino sugars attached to silt dissipated faster than those associated with the clay. This dissipation was in part because silt was preferentially lost through erosion, while clay particles (and their associated microbial residues) remained. Erosion was not solely responsible for the reduction in amino sugar concentrations, however. Bacterial amino sugars were lost in preference to fungal ones as a result of cultivation, and this effect was evident in both silt‐ and clay‐sized separates. This shift from fungal to bacterial residues was most pronounced within the first 20 years after converting the native grassland to arable cropland, but continued after 98 years of cultivation.     

Composition of organic matter in a subtropical Acrisol as influenced by land use, cropping and N fertilization, assessed by CPMAS 13C NMR spectroscopy

We know much about the influence of management on stocks of organic matter in subtropical soils, yet little about the influence on the chemical composition. We therefore studied by CPMAS ¹³C NMR spectroscopy the composition of the above-ground plant tissue, of the organic matter of the whole soil and of silt- and clay-size fractions of the topsoil and subsoil of a subtropical Acrisol under grass and arable crops. Soil samples were collected from three no-till cropping systems (bare soil; oats−maize; pigeon pea + maize), each receiving 0 and 180 kg N ha⁻¹ year⁻¹, in a long-term field experiment. Soil under the original native grass was also sampled. The kind of arable crops and grass affected the composition of the particulate organic matter. There were no differences in the composition of the organic matter in silt- and clay-size fractions, or of the whole soil, among the arable systems. Changes were observed between land use: the soil of the grassland had larger alkyl and smaller aromatic C contents than did the arable soil. The small size fractions contain microbial products, and we think that the compositional difference in silt- and clay-size fractions between grassland and the arable land was induced by changes in the soil's microbial community and therefore in the quality of its biochemical products. The application of N did not affect the composition of the above-ground plant tissue nor of the particulate organic matter and silt-size fractions, but it did increase the alkyl C content in the clay-size fraction. In the subsoil, the silt-size fraction of all treatments contained large contents of aromatic C. Microscopic investigation confirmed that this derived from particles of charred material. The composition of organic matter in this soil is affected by land use, but not by variations in the arable crops grown.     

Long-term impacts of manure, straw, and fertilizer on amino sugars in a silty clay loam soil under temperate conditions

There is increasing evidence that microorganisms participate in soil C sequestration and stabilization in the form of resistant microbial residues. The type of fertilizers influences microbial activity and community composition; however, little is known about its effect on the microbial residues and their relative contribution to soil C storage. The aim of this study was to investigate the long-term impact (21 years) of different fertilizer treatments (chemical fertilizer, crop straw, and organic manure) on microbial residues in a silty clay loam soil (Udolls, USDA Soil Taxonomy). Amino sugars were used to indicate the presence and origin of microbial residues. The five treatments were: CK, unfertilized control; NPK, chemical fertilizer NPK; NPKS₁, NPK plus crop straw; NPKS₂, NPK plus double amounts of straw; and NPKM, NPK plus pig manure. Long-term application of inorganic fertilizers and organic amendments increased the total amino sugar concentrations (4.4–8.4 %) as compared with the control; and this effect was more evident in the plots that continuously received pig manure (P?<?0.05). The increase in total amino sugar stock was less pronounced in the straw-treated plots than the NPKM. These results indicate that the accumulation of soil amino sugars is largely influenced by the type of organic fertilizers entering the soil. Individual amino sugar enrichment in soil organic carbon was differentially influenced by the various fertilizer treatments, with a preferential accumulation of bacterial-derived amino sugars compared with fungal-derived glucosamine in manured soil.     

Nitrogen biomarkers and their fate in soil

More than 90 % of the nitrogen (N) in soils can be organically bound, but the mechanisms and rates by which it is cycled have eluded researchers. The objective of this research was to contribute to a better understanding of the origin and transformation of soil organic N (SON) by using amino sugars and the enantiomers of amino acids as markers for microbial residues and/or aging processes. Studied samples presented here comprised (1) soil transects across different climates, (2) arable soils with different duration of cropping, and (3) radiocarbon‐dated soil profiles. The results suggested that increased microbial alteration of SON temporarily results in a sequestration of N in microbial residues, which are mineralized at later stages of SON decomposition. Microorganisms increasingly sequestered N within intact cell wall residues as frost periods shortened. At a mean annual temperature above 12–15 °C, these residues were mineralized, probably due to limitations in additional substrates. Breaking the grassland for cropping caused rapid SON losses. Microbial residues were decomposed in preference to total N, this effect being enhanced at higher temperatures. Hence, climate and cultivation interactively affected SON dynamics. Nevertheless, not all SON was available to soil microorganisms. In soil profiles, L‐aspartic acid and L‐lysine slowly converted into their D‐form, for lysine even at a similar rate in soils of different microbial activity. Formation of D‐aspartate with time was, therefore, induced by microorganisms while that of D‐lysine was not. The racemization of the two amino acids indicates that SON not available to microorganisms ages biotically and abiotically. In native soils, the latter is conserved for centuries, despite N deficiency frequently occurring in living terrestrial environments. Climate was not found to affect the fate of old protein constituents in surface soil. When native grassland was broken for cropping, however, old SON constituents had become available to microorganisms and were degraded.     

Quantitative assessment of the fungal contribution to microbial tissue in soil

The fungi-to-bacteria ratio in soil ecological concepts and its application to explain the effects of land use changes have gained increasing attention over the past decade. Four different main approaches for quantifying the fungal and bacterial contribution to microbial tissue can be distinguished: (1) microscopic methods, (2) selective inhibition, (3) specific cell membrane components and (4) specific cell wall components. In this review, the different methods were compared and we hypothesized that all these approaches result in similar values for the fungal and bacterial contribution to total microbial biomass, activity, and residues (dead microbial tissue) if these methods are evenly reliable for the estimation of fungal biomass. The fungal contribution to the microbial biomass or respiration varied widely between 2 and 95% in different data sets published over the past three decades. However, the majority of the literature data indicated that fungi dominated microbial biomass, respiration or non-biomass microbial residues, with mean percentages obtained by the different methodological approaches varying between 35 and 76% in different soil groups, i.e. arable, grassland, and forest soils and litter layers. Microscopic methods generally gave the lowest average values, especially in arable and grasslands soils. Very low ratios in fungal biomass C-to-ergosterol obtained by microscopic methods suggest a severe underestimation of fungal biomass by certain stains. Relatively consistent ratios of ergosterol to linoleic acid (18:2ω6,9) indicate that both cell membrane components are useful indicators for saprotrophic and ectomycorrhizal fungi. More quantitative information on the PLFA content of soil bacteria and the 16:1ω5 content of arbuscular mycorrhizal fungi is urgently required to fully exploit the great potential of PLFA measurements. The most consistent results have been obtained from the analysis of fungal glucosamine and bacterial muramic acid in microbial residues. Component-specific δ¹³C analyses of PLFA and amino sugars are a promising prospect for the near future.     

外源氮添加对土壤微生物残体积累动态的影响－基于Meta分析

微生物残体是土壤有机碳库的重要贡献者。为明确外源氮添加对土壤微生物残体积累动态的影响,本文收集整理了1980—2020年已发表的文献,共选取122组试验观测数据,利用整合分析方法（Meta-analysis）,以微生物残体标识物－氨基糖为目标组分,定量分析了不同种类和数量的外源氮添加对土壤中微生物来源细胞残体积累数量和组成比例的影响,并系统解析其主要影响因素。结果表明:外源氮添加（0 ~ 6000 kg hm?1）对微生物细胞残体的积累有显著的促进作用,并能引起土壤中真菌和细菌来源细胞残体相对比例发生明显变化。与不加氮对照相比,氮添加使土壤氨基糖总量增加27%,其中氨基葡萄糖、氨基半乳糖和胞壁酸含量分别增加22.5%、29.8%和19.0%。同时,不同种类外源氮素添加对氨基糖积累特征的影响也有所不同,表现为有机氮（如动物厩肥）比无机氮添加对氨基糖积累的促进作用更大。此外,氮添加对氨基糖的影响程度还与土壤自身的碳氮比、土地利用类型和自然降雨量等环境因子密切相关。其中是否添加碳源对微生物残体的响应有较大影响,表现为:无碳源添加会降低土壤氨基糖葡萄糖和胞壁酸对氮添加的响应,削弱了微生物残体对土壤有机质的贡献比例;而氮源同时配合碳源添加条件下,土壤氨基糖积累量显著高于单一氮源添加的处理,说明氮添加对微生物残体积累的影响存在着碳氮耦合效应。     

Land-use effects on amino sugars in particle size fractions of an Argiudoll

Identifying amino sugar pools from different land-use systems may advance our knowledge of land-use effects on the fate of microbially-derived substances. Surface soils (0–10 cm) from (1) native pasture, (2) a >80-years-arable site, and (3) a >80-years-afforested site were fractionated into clay, silt, fine-, and coarse-sand fractions. Then, soil organic carbon, N, glucosamine, galactosamine, mannosamine, and muramic acid were analyzed.Afforestation did not influence the amino sugar content in bulk soil, whereas cultivation reduced the content by 54%. The concentrations of amino sugars in g kg⁻¹ SOM declined after both long-term cropping and afforestation by 6% and 13%, respectively, relative to that in the grassland. The amino sugar depletion at the forest site occurred mainly from the silt fraction (by 25%), while that in the cultivated site was mainly due to preferential loss of amino sugars from clay (by 19% compared with the grassland). Both ratios of glucosamine to galactosamine and glucosamine to muramic acid increased when the prairie was converted to forest or cultivated land, suggesting that bacterial N especially is better preserved than fungal N under prairie conditions.     

Soil biological quality after 36 years of ley-arable cropping,permanent grassland and permanent arable cropping

Insight is needed into how management influences soil biota when sustainable grassland systems are developed. A crop rotation of grass and maize can be sustainable in terms of efficient nutrient use. However, there is lack of information on the effect of such a crop rotation on soil biological quality. Earthworms, nematodes, bacteria and fungi were sampled over three years in a 36 years old experiment. Permanent arable land was compared with permanent grassland and with a ley-arable crop rotation. In the rotation, a period of three years of grassland (temporary grassland) was followed by a period of three years of arable land (temporary arable land) and vice versa. In the first year of arable cropping in the rotation, the number of earthworms was already low and not different from continuous cropping. In the three-year grass ley, the abundance of earthworms returned to the level of permanent grassland in the second year. However, the restoration of earthworm biomass took a minimum of three years. Furthermore, the anecic species did not recover the dominance they had in the permanent grassland. The numbers of herbivorous and microbivorous nematodes in the ley-crop rotation reached similar levels to those in the permanent treatments within one to two years. Although the same holds for the nematode genera composition, the Maturity Index and the proportion of omnivorous nematodes in the temporary treatments remained significantly lower than in their permanent counterparts. Differences in recovery were also found among microbial parameters. In the temporary treatments, bacterial growth rate and the capacity to degrade a suite of substrates recovered in the second year. However, the Community-Level Physiological Profiles in the permanent grassland remained different from the other treatments. Our results suggest that many functions of soil biota that are well established in permanent grassland, are restored in a ley-arable crop rotation. However, due to a reduction in certain species, specific functions of these soil biota could be reduced or lost. The ley-arable crop rotations were intermediate to permanent grassland and continuous arable land in terms of functioning of soil biota (e.g., N-mineralization). In terms of the functional aspects of the soil biota, permanent grassland might be preferable wherever possible. For maize cultivation, a ley-arable crop rotation is preferable to continuous arable land. However, a ley-arable crop rotation is only preferable to continuous arable cropping if it is not practised at the expense of permanent grassland at farm level.     

无机氮素加入量对玉米秸秆分解过程中棕壤氨基糖含量的影响

作物秸秆还田作为一种调控土壤养分循环、减少氮肥损失、维持和提高土壤有机质水平的有力措施,越来越受到人们的重视[1-2]。还田的秸秆是微生物的碳源和能源,常导致土壤微生物量迅速增加,相应的微生物死亡率和微生物残体积累量也提高[3-4]。根据Appuhn等[5]对微生物细胞壁组分的     

Amino sugars as specific indices for fungal and bacterial residues in soil

Amino sugars are important indices for the contribution of soil microorganisms to soil organic matter. Consequently, the past decade has seen a great increase in the number of studies measuring amino sugars. However, some uncertainties remain in the interpretation of amino sugar data. The objective of the current opinion paper is to summarize current knowledge on amino sugars in soils, to give some advice for future research objectives, and to make a plea for the correct use of information. The study gives an overview on the origin of muramic acid (MurN), glucosamine (GlcN), galactosamine (GalN), and mannosamine (ManN). Information is also provided on measuring total amino sugars in soil but also on compound-specific δ¹³C and δ¹⁵N determination. Special attention is given to the turnover of microbial cell-wall residues, to the interpretation of the GlcN/GalN ratio, and to the reasons for converting fungal GlcN and MurN to microbial residue C. There is no evidence to suggest that the turnover of fungal residues generally differs from that of bacterial residues. On average, MurN contributes 7% to total amino sugars in soil, GlcN 60%, GalN 30%, and ManN 4%. MurN is highly specific for bacteria, GlcN for fungi if corrected for the contribution of bacterial GlcN, whereas GalN and ManN are unspecific microbial markers.     

无机氮素加入量对玉米秸秆分解过程中棕壤氨基糖含量的影响

在室内恒温(25℃)培养条件下,通过气相色谱法研究高C/N比玉米秸秆降解过程中微生物来源的氨基糖含量及其占有机质比例的变化及其对无机氮素添加水平(0, 60.3, 167.2, 701.9 mg•N•kg-1土,依次标记为N0, Nlow, Nmed, Nhigh)的响应情况。结果表明：在玉米秸秆分解过程中,土壤中的氨基糖含量及其对有机质贡献的比例随着无机氮素供应水平的增加而增加,即以微生物代谢物形式截获的有机碳/氮相应增多。Nmed和Nhigh处理中氨基糖积累量显著高于Nlow和N0处理。不同微生物来源的氨基糖受外源氮素的影响情况不同,胞壁酸比氨基葡萄糖更易于受到土壤中碳氮供给的影响,具有相对较快的转化速率;而在数量上氨基葡萄糖对土壤有机质的贡献比例显著高于前者;氨基半乳糖在土壤中的积累过程较为缓慢,受外源无机氮素添加水平的影响并不明显。可见,在高C/N比作物残体分解过程中,无机氮素的供应水平是影响土壤中氨基糖积累转化的重要因素之一。但是,过多的无机氮素施入并不能被微生物完全同化利用,因此秸秆还田的土壤中必须要考虑有效氮素的水平问题。     

Microbial residues were increased by film mulching with manure amendment in a semiarid agroecosystem

Amino sugars, as a kind of microbial residue, are strongly associated with cycling of microbial-derived soil organic matter. However, responses of amino sugars to agricultural practices on the Loess Plateau in North-western China are poorly known. The objective was to evaluate effects of film mulching (no film mulching + NPK fertilizers, CK; film mulching + NPK fertilizers, PF; film mulching + NPK fertilizers + cow manure, FM) on accumulations of amino sugars in this region. FM significantly increased total amino sugar by 190.46 mg kg^?1 in 0–10 cm layer and 214.66 mg kg^?1 in 10–20 cm layer relative to CK, but PF significantly decreased it by 139.28 mg kg^?1 in 0–10 cm layer. Ratios of glucosamine to muramic acid were markedly decreased by 2.50 in 0–10 cm layer and 2.28 in 10–20 cm layer in FM than CK, suggesting a tendency of microbial residues pool shift towards bacterial residues in this agroecosystem. These results indicated film mulching alone was not benefitial to accumulation of amino sugar while organic manure contributed to the build-up of amino sugar partly due to manure contained microbial residues. The different patterns of amino sugars suggested significant changes in the quality of microbial-derived organic matter.     

Soil microbial community composition as affected by restoration practices in California grassland

Agricultural practices have strong impacts on soil microbes including both the indices related to biomass and activity as well as those related to community composition. In a grassland restoration project in California, where native perennial bunchgrasses were introduced into non-native annual grassland after a period of intensive tillage, weeding, and herbicide use to reduce the annual seed bank, microbial community composition was investigated. Three treatments were compared: annual grassland, bare soil fallow, and restored perennial grassland. Soil profiles down to 80 cm in depth were investigated in four separate layers (0-15, 15-30, 30-60, and 60-80 cm) using both phospholipid ester-linked fatty acid (PLFAs) and ergosterol as biomarkers in addition to microbial biomass C by fumigation extraction. PLFA fingerprinting showed much stronger differences between the tilled bare fallow treatment vs. grasslands, compared to fewer differences between restored perennial grassland and annual grassland. The presence or absence of plants over several years clearly distinguished microbial communities. Microbial communities in lower soil layers were little affected by management practices. Regardless of treatment, soil depth caused a strong gradient of changing habitat conditions, which was reflected in Canonical Correspondence Analysis of PLFAs. Fungal organisms were associated with the presence of plants and/or litter since the total amount and the relative proportion of fungal markers were reduced in the tilled bare fallow and in lower layers of the grassland treatments. Total PLFA and soil microbial biomass were highly correlated, and fungal PLFA biomarkers showed strong correlations to ergosterol content. In conclusion, microbial communities are resilient to the grassland restoration process, but do not reflect the change in plant species composition that occurred after planting native bunchgrasses.     

Amino sugars in soils of the North American cultivated prairie

Characterizing amino sugar dynamics in cultivated soils helps to further understand the influence of cultivation on soil organic matter turnover. This study was designed to evaluate accumulations and patterns of four amino sugars in 17 surface (0–10 cm) soil samples along a climosequence in the North American long-term cultivated prairie from Saskatoon, Candada, to Texas, USA. Mean annual temperature (MAT) ranged from 0.9 to 22.2°C and mean annual precipitation (MAP) from 300 to 1308 mm. Samples were analyzed for glucosamine, mannosamine, galactosamine, and muramic acid. Amino sugar contents (mg kg^?1 soil) varied markedly among the 17 sites and were controlled by mean annual temperature (MAT) and clay and silt contents, mainly. The relationship between amino sugar-N proportions to total N (%) and MAT followed parabolic regression models. Compared with native sites, amino sugars were depleted by 53% and the amino sugar-N by 18% of the total, on average, after long-term cropping. The intensity of amino sugar-N depletion correlated positively with MAT (r = 0.77^***). Bacterially-derived galactosamine and muramic acid declined preferentially to mainly chitin-derived glucosamine after long-term cropping. The glucosamine-to-galactosamine and glucosamine-to-muramic acid ratios can be used, therefore, as indicators for the identification of land use effects on microbially-derived SOM residues.     

Losses of glomalin-related soil protein under prolonged arable cropping: A chronosequence study in sandy soils of the South African Highveld

Residues of arbuscular mycorrhizal fungi (AMF) may be important for agroecosystem functioning due to their ability to promote soil aggregation, especially in coarse textured soils with little biomass input and low capacity to conserve soil organic matter (SOM). Our aim was to assess the fate of AMF residues with prolonged arable cropping in coarse textured soils in a subtropical savannah assuming that glomalin-related soil protein (GRSP), especially the MAb32B11-immunoreactive fraction, mainly constitutes material of AMF origin. In three agroecosystems on the South African Highveld, surface soils were sampled. The former grassland soils had a history of up to 98 yr of cropping. We measured four GRSP fractions: Bradford-reactive soil protein (BRSP) and immunoreactive soil protein (IRSP), and easily extractable fractions of both. The primary grassland sites exhibited generally low contents of SOM and low GRSP contents. Prolonged arable land use of former grassland soils reduced the content of GRSP further. The decline could be described with a mono-exponential function with rate constants ranging from 0.04 to 0.41 yr⁻¹. Depending on the GRSP fraction, steady-state conditions were reached after 11-92 yr on a level of 39-69% of the initial contents. We conclude that even though GRSP fractions had the same hypothesized origin, they comprised pools with different stability or replacement rate. Easily extractable IRSP was lost most rapidly. In contrast to carbon, nitrogen and microbial residue dynamics, GRSP contents were not reduced below a certain steady-state level, despite potentially negative management effects on AMF, such as tillage, inclusion of fallows into crop rotation and fertilization with inorganic phosphorus. The steady-state GRSP contents coincided with low, but steady agroecosystem yields under the given cropping management.     

外源氮素和凋落物添加对温带森林土壤氨基糖转化特征的影响

【目的】温带森林土壤氨基糖的转化特征对外源氮素和凋落物加入的响应研究,对于温带森林土壤氮素管理和缓解氮沉降所带来的负面影响具有重要的意义。【方法】采用室内恒温恒湿模拟培养的方法,研究了外源氮素和凋落物添加条件下温带森林土壤有机层中3种微生物来源的氨基糖含量的变化特征,并利用真菌和细菌来源氨基糖的比值（氨基葡萄糖/胞壁酸）,分析了外源物质添加条件下真菌和细菌残留物对土壤氮素转化和积累的相对贡献。【结果】温带森林有机层土壤中不同微生物来源氨基糖对外源物质加入的响应不同。单施氮素以及氮素与凋落物同时添加均有利于细菌残留物胞壁酸的积累,但是单施氮素添加对真菌残留物氨基葡萄糖含量的积累没有影响,且氮素与凋落物同时添加不利于氨基葡萄糖含量的积累。氨基半乳糖对外源物质添加的响应较小。真菌残留物的稳定性高于细菌残留物,氮素与凋落同时加入时不利于土壤微生物残留物的稳定性。此外,土壤中真菌和细菌来源氨基糖的比值受到外源物质加入的影响,单施氮素以及氮素与凋落物添加降低了氨基葡萄糖/胞壁酸比值（分别降低28.3%和30.5%）,两种外源物质加入时细菌残留物对氮素转化的相对贡献大于真菌残留物。【结论】外源氮素和...     

Carbon sequestration in secondary pasture soils: a chronosequence study in the South African Highveld

Soil restoration is a means of combating desertification in semi‐arid and arid parts of the world. There, vast areas of the cropped soil degrade, particularly because of the loss of organic matter. One approach to reverse this loss is the conversion of cropland into permanent grassland for use as pasture. This study was designed to evaluate how fast and to what degree degraded cropland may re‐sequester soil organic carbon (SOC) when converted into permanent secondary pasture. Topsoil samples (0–5, 5–10 and 10–20 cm) were taken from chronosequences of secondary pastures (1 to 31 years old) at three agro‐ecosystems in the semi‐arid Highveld of South Africa. Long‐term croplands and primary grassland used as pastures served as the controls. In bulk soil samples (<2 mm) and their clay (<2 µm), silt (2–20 µm), fine sand (20–250 µm) and coarse sand (250–2000 µm) fractions, the contents of carbon (C) and nitrogen were determined. In all three agro‐ecosystems, using a mono‐exponential model, the SOC stocks increased exponentially until a maximum was reached 10–95 years after land conversion. This gain in SOC was clearly pronounced for the top 0–5 cm of soil, but hardly detectable at 10–20‐cm depth. The sand fractions recovered organic C more rapidly but less completely than did the finer size separates. Overall, between 9.0 and 15.3 t of SOC were sequestered in the 0–20 cm of surface soil by this land conversion. Thus, the SOC recovery in the secondary pastures resulted in SOC stocks that were 29.6–93.9% greater than those in the arable land. Yet, in no agro‐ecosystem, at any soil depth, nor in any soil fraction, did the measured SOC content reach that of the primary grassland. In part this can be attributed to a slightly finer texture of the primary grassland that had not lost silt through wind erosion or had never been used as arable land because of slightly elevated clay contents. Overall it appears, however, that previous losses of SOM cannot easily be rectified, suggesting that the native primary grassland soils are only partially resilient to land‐use change.     

不同土地利用方式对黑土有机无机复合体的影响

采用超声波分散、颗粒大小分组法研究不同土地利用方式下黑土有机无机复合体组成及有机碳的分布特征。不同土地利用方式下黑土均以细砂级复合体为主,有机碳含量随粒径增加而减少,C/N比随粒径增加而增加。NP和NPM处理比NF处理土壤<20μm粒级复合体含量减少,>20μm复合体含量增加;草地与裸地相比<20μm粒级复合体含量减少,>20μm复合体含量增加。土地利用方式不同,土壤固碳的机制是不同的,草地生态系统的土壤固碳潜力高于农田土壤。施肥和有机质的输入使土壤各粒级复合体中的有机碳含量增加;使>20μm粒级复合体固持的SOC量比例增加,表明在土壤有机质含量增加的情况下,有机质有向大粒级复合体的积累增加。     

Soil organic carbon,nitrogen, and phosphorus distribution in stable aggregates of an Ultisol under contrasting land use and management history

Different land‐use affects the organization of mineral soil particles and soil organic components into aggregates and the consequent arrangement of the aggregates will influence essential ecosystem functions. We investigated a continuous rubber plantation (forested), land fallowed for 10 y (fallow), 10‐y continuous arable cropping land and cropped land with top soil removed (TSR) for concentrations of C, N, and P in bulk soil and dry aggregates. Results showed that a high level of soil disturbance decreased the proportion of surface (0–15 cm) soil aggregate stability (low mean weight diameter) in TSR by 149% and arable cropping by 125% compared with the forested. Aggregate associated SOC was higher in aggregate‐size fractions of forested land‐use when compared with that in 10‐y fallow, continuous arable cropping, and TSR. For aggregate associated N, fallow and forested land‐use types concentrated higher proportion across aggregate sizes than the arable cropping and TSR. Macro aggregate fractions generally contained higher concentrations of C, N, and P compared with the micro‐aggregates. Water transmission indicators like total porosity and saturated hydraulic conductivity recorded higher values with forested and fallow land‐use than the others. We can thus conclude that long‐term soil disturbance due to cultivation and removal of top soil reduces the accumulation of soil C, N, and P in bulk soil and decreases water transmission properties. On the other hand, aggregate‐associated C, N and P accumulations are dependent on the level of soil surface disturbance and aggregate sizes.