Functional resilience of soil microbial communities depends on both soil structure and microbial community composition

The effects of soil structure and microbial community composition on microbial resistance and resilience to stress were found to be interrelated in a series of experiments. The initial ability of Pseudomonas fluorescens to decompose added plant residues immediately after a copper or heat stress (resistance) depended significantly on which of 26 sterile soils it was inoculated into. Subsequent studies showed that both the resistance and subsequent recovery in the ability of P. fluorescens to decompose added plant residues over 28 days after stress (resilience) varied significantly between a sandy and a clay-loam soil. Sterile, sandy and clay-loam soil was then inoculated with a complex microbial community extracted from either of the soils. The resulting microbial community structure depended on soil type rather than the source of inoculum, whilst the resistance and resilience of decomposition was similarly governed by the soil and not the inoculum source. Resilience of the clay-loam soil to heat stress did not depend on the water content of the soil at the time of stress, although the physical condition of the soil when decomposition was measured did affect the outcome. We propose that soil functional resilience is governed by the physico-chemical structure of the soil through its effect on microbial community composition and microbial physiology.     

Biological and physical resilience of soil amended with heavy metal-contaminated sewage sludge

Stability and resilience of a variety of soil properties and processes are emerging as key components of soil quality. We applied recently developed measures of biological and physical resilience to soils from an experimental site treated with metal‐contaminated sewage sludge. Soils treated with cadmium‐, copper‐ or zinc‐contaminated, digested or undigested sewage sludge were studied. Biological stability and resilience indices were: (i) the time‐dependent effects of either a transient stress (heating to 40°C for 18 hours) or a persistent stress (amendment with CuSO₄) on decomposition, and (ii) the mineralization of dissolved organic carbon (DOC) released by drying–rewetting cycles. Physical stability and resilience measures were: (i) compression and expansion indices of the soils, and (ii) resistance to prolonged wetting and structural regeneration through drying–rewetting cycles. Soil total carbon and DOC levels were greater in the sludge‐amended soils, but there were no differential effects due to metal contamination of the sewage sludge. Effects of metals on physical resilience were greater than effects on soil C, there being marked reductions in the expansion indices with Cd‐ and Cu‐contaminated sludge, and pointed to changes in soil aggregation. The rate of mineralization of DOC released by drying and wetting was reduced by Zn contamination, while biological resilience was increased in the Zn‐contaminated soil and reduced by Cd contamination. We argue that physical and biological resilience are potentially coupled through the microbial community. This needs to be tested in a wider range of soils, but demonstrates the benefits from a combined approach to the biological and physical resilience of soils.     

Estimating soil stress distribution by using depth‐dependent soil bulk‐density data

Depth‐dependent soil bulk density (BDS) is usually affected by soil‐specific factors like texture, structure, clay mineralogy, soil organic‐matter content, soil moisture content, and composition of soil solution and is also affected by external factors like overburden‐stress history or hydrological fluxes. Generally, the depth‐dependent BDS cannot be predicted or extrapolated precisely from a limited number of sampling depths. In the present paper, an easy method is proposed to estimate the state of soil mechanical stress by analyzing the packing characteristics of the profile using soil bulk‐density data. Results for homogeneous loess profiles exposed to the site‐specific climatic conditions show that the depth‐dependent relation of void ratio vs. weight of overburden soil can be described systematically so that deviations from the noncompacted reference state can be detected. We observed that precompaction increased from forest soils (reference) to agricultural soils with decreasing depth.     

Functional resilience of microbial communities from perturbed upland grassland soils to further persistent or transient stresses

The microbial functioning of soils following perturbation was assessed at a temperate upland grassland site, maintained by the Soil Biodiversity and Ecosystem Function Programme at Sourhope Research Station, Scotland. Published results indicated that the soil microbial communities were resilient to these initial perturbations; in this paper we tested whether they were equally resilient to a subsequent perturbation. Soil samples were taken from field plots receiving treatments that represented different forms of perturbation, viz. reseeding, application of sewage-sludge, biocide or nitrogen plus lime, and a non-perturbed control. Functional resilience following further perturbation comprising a transient heat or persistent copper perturbation was assessed over 28 days, by monitoring the short-term decomposition of added plant residues. Bacterial community structure was assessed by DGGE separation of eubacterial 16S rDNA PCR products. PCR-DGGE did not distinguish any significant difference (P>0.05) between the bacterial communities of soils under different treatments, showing differences only between treated soils and the untreated, control soils.Two days after the application of stresses, functional capability differed in soils under different treatments. Soil samples from all the treated plots were less resilient to heat stress than soil from control plots. The initial reduction in decomposition following the addition of copper differed between treatments, but function had not recovered in any of the Cu-amended soils within 28 days. Soil resilience varied according to the type and duration of stress applied, microbial activity, soil characteristics and treatment regimes. The initial resistance of function to stress was not predictive of recovery of function over time.     

不同土壤水吸力与耕作方式对土壤压缩—回弹特性的影响

[目的]合理耕作方式是缓解土壤压实、提升土壤生产能力的有效措施,而土壤水分是影响土壤机械物理性能的重要因素,直接影响土壤耕作质量.通过研究不同土壤水吸力和耕作方式下土壤压缩曲线及模型拟合效果,分析土壤回弹—再压缩曲线变化及机械力学参数(预固结压力、压缩指数和回弹指数)差异,以期为农田土壤耕作和培肥提供科学依据.[方法]...     

Balanced Fertilization along with Farmyard Manures Enhances Abundance of Microbial Groups and Their Resistance and Resilience against Heat Stress in a Semi-arid Inceptisol

The impact of long-term (36-year) application of balanced fertilizers and farmyard manures (FYM) on the abundance of microbial groups (bacteria, fungi, actinomycetes, Pseudomonas, Azotobacter, ammonia-oxidizing bacteria) and their resistance and resilience against heat stress was investigated in a semi-arid Inceptisol at New Delhi, India. Surface soils from selected treatments [control, nitrogen (N), N and phosphorus (P), NP and potassium (K), NPK + FYM] under a maize crop were assessed immediately after sampling (0-day) and at 1, 14, 28, and 56 day(s) after heat stress (48 °C for 24 h). The heat stress significantly reduced the microbial groups by 20 to 80%. Recovery after stress was 60 to 100% within 56 days. Resistance and resilience of fungi and actinomycetes were greater than other groups of organisms. Ammonia-oxidizing bacteria (AOB) were found to be most sensitive with the lowest resistance index. Application of NPK + FYM was most effective in enhancing the resistance and resilience of soil microorganisms against heat stress.     

Soil carbon dynamics in saline and sodic soils: a review

Soil salinity (high levels of water-soluble salt) and sodicity (high levels of exchangeable sodium), called collectively salt-affected soils, affect approximately 932 million ha of land globally. Saline and sodic landscapes are subjected to modified hydrologic processes which can impact upon soil chemistry, carbon and nutrient cycling, and organic matter decomposition. The soil organic carbon (SOC) pool is the largest terrestrial carbon pool, with the level of SOC an important measure of a soil's health. Because the SOC pool is dependent on inputs from vegetation, the effects of salinity and sodicity on plant health adversely impacts upon SOC stocks in salt-affected areas, generally leading to less SOC. Saline and sodic soils are subjected to a number of opposing processes which affect the soil microbial biomass and microbial activity, changing CO₂ fluxes and the nature and delivery of nutrients to vegetation. Sodic soils compound SOC loss by increasing dispersion of aggregates, which increases SOC mineralisation, and increasing bulk density which restricts access to substrate for mineralisation. Saline conditions can increase the decomposability of soil organic matter but also restrict access to substrates due to flocculation of aggregates as a result of high concentrations of soluble salts. Saline and sodic soils usually contain carbonates, which complicates the carbon (C) dynamics. This paper reviews soil processes that commonly occur in saline and sodic soils, and their effect on C stocks and fluxes to identify the key issues involved in the decomposition of soil organic matter and soil aggregation processes which need to be addressed to fully understand C dynamics in salt-affected soils.     

Functional stability,substrate utilisation and biological indicators of soils following environmental impacts

Stability of a soil property to perturbation comprises both resistance and resilience. Resistance is defined as the ability of the soil to withstand the immediate effects of perturbation, and resilience the ability of the soil to recover from perturbation. Functional stability is used here to describe the stability of a biological function to perturbation, rather than the stability of physical structure or chemical properties. The function chosen for this study was the short-term decomposition of added plant residues, and the perturbations were copper and heat stresses. Previous studies had shown that functional stability was reduced greatly in soils with experimentally reduced biodiversity. The objective of this study was to determine the relative sensitivity of functional stability and potential indicators of biological status to detect alteration of field soils by various environmental impacts. Functional stability, protozoan populations and substrate mineralisation kinetics, were measured on paired soils with: high or low plant species diversity; hydrocarbon pollution or not; extensive or intensive agricultural management practices. Substrate mineralisation kinetics were poorly related to the soil’s antecedent conditions and were stimulated significantly by hydrocarbon pollution. Protozoan populations were potentially useful for detecting differences within soil type, but will require greater taxonomic input to be most useful. Functional stability, particularly resistance, was able to quantify differences between and within soils. The potential development of the technique in relation to soil health is discussed.     

Measurements of the yield surfaces and critical state of some unsaturated agricultural soils

Yield surfaces and the critical-state condition have been measured on unsaturated agricultural soils using a standard uniaxial compression test and a constant-volume direct shear test. The yield surfaces and critical-state line are all readily described in terms of applied or total stresses, and such an approach offers practical advantages over approaches based on effective stresses. Four soils were tested, these being a silt, a tilth with aggregates mostly from 5-15 mm, a cracking clay and a red-brown earth.
Each soil was tested at a single constant moisture content in all tests, although the moisture content differed from soil to soil. The range of saturation covered by the four soils was approximately 20-98%. All the soils displayed yield and deformation behaviour qualitatively consistent with the critical-state concept. All approach a condition of shear with no volume change (the critical state) under continuing shear. All show collapse with shear in states looser than critical, and expansion with shear in states denser than critical. The silt, being a non-cohesive soil, cannot support shear stresses much above the critical-state line, whereas the other three soils can support much higher shear stresses in the overconsolidated condition. The yield surfaces of the silt and the tilth, which were tested at low saturation, are similar in shape with increasing stress level. However, the other two soils, tested when near saturation, display yield surfaces that are not constant in shape with increasing stress level. While the critical-state concept is applicable both qualitatively and quantitatively, unsaturated soils may be considered to have properties that differ in detail from those of saturated soils.     

生物炭引起的土体回弹变化：土壤质地和生物炭用量的影响

Biochars are,amongst other available amendment materials,considered as an attractive tool in agriculture for carbon sequestration and improvement of soil functions.The latter is widely discussed as a consequence of improved physical quality of the amended soil.However,the mechanisms for this improvement are still poorly understood.This study investigated the effect of woodchip biochar amendment on micro-structural development,micro-and macro-structural stability,and resilience of two differently textured soils,fine sand (FS) and sandy loam (SL).Test substrates were prepared by adding 50 or 100 g kg-1 biochar to FS or SL.Total porosity and plant available water were significantly increased in both soils.Moreover,compressive strength of the aggregates was significantly decreased when biochar amount was doubled.Mechanical resilience of the aggregates at both micro-and macro-scale was improved in the biochar-amended soils,impacting the cohesion and compressive behavior.A combination of these effects will result in an improved pore structure and aeration.Consequently,the physicochemical environment for plants and microbes is improved.Furthermore,the improved stability properties will result in better capacity of the biochar-amended soil to recover from the myriad of mechanical stresses imposed under arable systems,including vehicle traffic,to the weight of overburden soil.However,it was noted that doubling the amendment rate did not in any case offer any remarkable additional improvement in these properties,suggesting a further need to investigate the optimal amendment rate.     

A soil quality index based on the equilibrium between soil organic matter and biochemical properties of undisturbed coniferous forest soils of the Pacific Northwest

Recent studies have suggested that the organic matter contents of undisturbed soils (under natural vegetation) are in equilibrium with biological and biochemical properties. Accordingly, we hypothesised that such equilibria should be disrupted when soils are subjected to disturbance or stress, and that measurement of this disruption can be expressed mathematically and used as a soil quality index. In this study, we evaluated these hypotheses in soils from the H.J. Andrews Experimental Forest in Oregon. Both O and A horizons were sampled from nine sites in Spring 2005 and Fall 2006. Soil samples were analyzed for enzyme activities (phosphatase, β-glucosidase, laccase, N-acetyl-glucosaminidase, protease and urease), and other biological and chemical properties including N-mineralization, respiration, microbial biomass C (MBC), soil organic carbon (SOC) and total nitrogen content. In addition, soil samples from one old-growth site were manipulated in the laboratory to either simulate chemical stresses (Cu addition or pH alteration) or physical disturbances (wet-dry or freeze-thaw cycles). The results showed variation in biological and biochemical soil properties that were closely correlated with SOC. Multiple regression analysis of SOC levels against all soil properties showed that a model containing only MBC and phosphatase activity could account for 97% of the SOC variation among the sites. The model fit was independent of spatial and temporal variations because covariates such as site, stand age, sampling date, and soil horizon were found to be not statistically significant. Although the application of stress/disturbance treatments inconsistently affected most of the individual biochemical properties, in contrast, the ratio of soil C predicted by the model (C_p), and soil C measured (C_m) was consistently reduced in soils submitted to at least one level of stress and disturbance treatments. In addition, C_p/C_m was more affected in soils submitted to wet-dry cycles and Cu contamination than to freeze-thaw cycles or shifts in soil pH. Our results confirm previous evidence of a biochemical balance in high quality undisturbed soils, and that this balance is disrupted when the soil is submitted to disturbances or placed under stress conditions. The C_p/C_m ratio provides a simple reference value against which the degrading effects of pollutants or management practices on soil quality can be assessed.     

Does microbial habitat or community structure drive the functional stability of microbes to stresses following re-vegetation of a severely degraded soil?

Re-vegetation of eroded soil restores organic carbon concentrations and improves the physical stability of the soil, which may then extend the range of microhabitats and influence soil microbial activity and functional stability through its effects on soil bacterial community structure. The objectives of this study were (i) to evaluate the restorative effect of re-vegetation on soil physical stability, microbial activity and bacterial community structure; (ii) to examine the effects of soil physical microhabitats on bacterial community structure and diversity and on soil microbial functional stability. Soil samples were collected from an 18-year-old eroded bare soil restored with either Cinnamomum camphora (“Eroded Cc”) or Lespedeza bicolour (“Eroded Lb”). An uneroded soil planted with Pinus massoniana (“Uneroded Pm”) and an eroded bare soil served as references. The effect of microhabitats was assessed by physical destruction with a wet shaking treatment. Soil bacterial community structure and diversity were measured using a terminal restriction fragment length polymorphism (T-RFLP) approach, while soil microbiological stability (resistance and resilience) was determined by measuring short-term (28 days) decomposition rate of added barley (Hordeum vulgare) powder following copper and heat perturbations. The results demonstrated that re-vegetation treatment affected the recovery of physical and biological stability, microbial decomposition and the bacterial community structure. Although the restored soils overshot the Uneroded Pm sample in physical stability, they had lower microbial decomposition and less resilience to copper and heat perturbations than the Uneroded Pm samples. Soil physical destruction by shaking had the same effect on soil physical stability, but different effects on soil microbial functional stability. There were significant effects of vegetation treatment and perturbation type, and interactive effects among vegetation treatment, shaking and perturbation type on bacterial community structure. The destruction of aggregate structure increased resilience of the Eroded Lb sample and also altered its bacterial community structure. Both copper and heat perturbations resulted in significantly different community structure from the unperturbed controls, with a larger effect of copper than heat perturbation. Bacterial diversity (Shannon index) increased following the perturbations, with a more profound effect in the Uneroded Pm sample than in the restored soils. The interactive effects of vegetation treatment and shaking on microbial community and stability suggest that soil aggregation may contribute to the generation of bacterial community structure and mediation of biological stability via the protection afforded by soil organic carbon. Differential effects of re-vegetation treatment suggest that the long-term effects are mediated through changes in the quality and quantity of C inputs to soil.     

Shrinkage behaviour of transiently- and constantly-loaded soils and its consequences for soil moisture release

Soils under loaded conditions may have different shrinkage behaviour from that of load‐free soils. In this study, we applied two kinds of mechanical stress (σ) on repacked homogeneous soil samples: transient and constant stresses, simulating the traffic load during tillage and the overburden pressure, respectively. Three transient stresses were applied on the soil surface with 150, 400 and 1400 kPa, while the constant stresses ranged from 1.8, 3.8, 5.5, to 7.3 kPa. We hypothesized that the two stresses play different roles in soil shrinkage behaviour as depicted by void ratio (e) and moisture ratio (?), as compared with load‐free soil. Thus, our aim was to build up the relationship between e, ? and σ. For a swelling soil, total pores can be divided into rigid and non‐rigid components according to their swelling and shrinkage capacity relative to soil moisture. The non‐rigid pores compacted by the transient stress can be regained in the subsequent wetting at load‐free conditions, whereas the compacted rigid pores do not recover. The reduction in rigid pores does not alter the soil pore shrinkage capacity. The shrinkage curves of transiently‐loaded soils are therefore parallel to each other with an identical coefficient of linear extensibility (COLE) and the same shrinkage slope, although their structural shrinkage phase narrows with an increase of stress. However, the constant stress compresses non‐rigid pores readily through suppressing their swelling capacity during wetting as well as compacting rigid pores. If the change of rigid pores is negligible, the shrinkage curves of constantly‐loaded soils converge at the zero shrinkage or the dry‐end point with the load‐free soil shrinkage. If the reductions of rigid and non‐rigid pores are both considered, the soil shrinkage combines the part of parallel shrinkage derived from the reduced rigid pores and the intersected shrinkage resulted from the altered non‐rigid pores. On the basis of different shrinkage behaviours resulting from the two mechanical stresses, we propose numerical formulae to illustrate a series of curves for the e‐?‐σ relationship. The different changes in rigid and non‐rigid pores cause soil water release differently.     

An investigation of the re-expansion of unsaturated, structured soils during cyclic static loading

A procedure is presented that quantifies soil resilience to compressive stress, through elastic deformation or re-expansion after stress removal, with a single numerical index. This was achieved by comparing the three parameter coefficients of static-loaded and rebound compression lines (normal stress range = 0–1.0 MPa) which had been fitted to a non-linear density-stress model equation. The difference between the static-loaded and rebound values of one of these coefficients was significantly correlated to the clay and organic matter contents, the gravimetric moisture content and the initial dry bulk density of the 33 soils sampled as intact cores at field moisture content (coefficients of determination=0.533–0.973, P<0.05). The magnitude of the sample rebound observed varied between 0.018 and 0.075 Mg m⁻³ at the maximum applied stress of 1.0 MPa. This is likely to be a significant component of the error in prediction inherent in compaction models based on static-loaded compression data. The data further support the segregation of soils into groupings of comparable mechanical behaviour for soil compaction modelling purposes. The implications of these findings for improving soil resilience to compressive stress through soil and crop management are discussed.     

Potential of multi‐objective models for risk‐based mapping of the resilience characteristics of soils: demonstration at a national level

Policy makers rely on risk‐based maps to make informed decisions on soil protection. Producing the maps, however, can often be confounded by a lack of data or appropriate methods to extrapolate using pedotransfer functions. In this paper, we applied multi‐objective regression tree analysis to map the resistance and resilience characteristics of soils onto stress. The analysis used a machine learning technique of multiple regression tree induction that was applied to a data set on the resistance and resilience characteristics of a range of soils across Scotland. Data included both biological and physical perturbations. The response to biological stress was measured as changes in substrate mineralization over time following a transient (heat) or persistent (copper) stress. The response to physical stress was measured from the resistance and recovery of pore structure following either compaction or waterlogging. We first determined underlying relationships between soil properties and its resistance and resilience capacity. This showed that the explanatory power of such models with multiple dependent variables (multi‐objective models) for the simultaneous prediction of interdependent resilience and resistance variables was much better than a piecewise approach using multiple regression analysis. We then used GIS techniques coupled with an existing, extensive soil data set to up‐scale the results of the models with multiple dependent variables to a national level (Scotland). The resulting maps indicate areas with low, moderate and high resistance and resilience to a range of biological and physical perturbations applied to soil. More data would be required to validate the maps, but the modelling approach is shown to be extremely valuable for up‐scaling soil processes for national‐level mapping.     

The mechanical behavior of structured and homogenized soil under repeated loading

Soil deformation is increasingly important in crop production since nowadays weights of agricultural machines exceed the bearing capacity of most soils. Often this is counteracted by distributing the weight over more axles leading to an increase in wheeling frequency. Machine passages during one year can, depending on the crop and equipment used, range between two and five times for the majority of the field and up to twenty times and more for a wheeling track. These add up to hundreds of loading events for a crop‐rotation period. In this study, we investigated the effect of multiple loading with the same load in a cyclic‐compression test on soil‐pore‐volume change. The tests were conducted on homogenized soil samples with varying texture and undisturbed soil samples from a field experimental site comparing conventional and conservation‐tillage systems. Of particular interest was the question whether there is significant plastic soil deformation for soil stresses that remained sufficiently below the precompression stress, which is commonly neglected. Our results show that especially for cohesive soils, the assumption of fully elasticity in the recompression range may not be justified since those soils show distinct cyclic‐creep behavior. We found that deformation under cyclic loading follows a logarithmic law. We used the slope of the logarithmic fit of void‐ratio changes vs. loading cycles as a parameter to characterize the sensitivity of soils to cyclic compression. The results suggest that for characterizing the mechanical stability of soils that show cyclic creep, we have (with respect to long‐term deformation effects) to consider both precompression stress and cyclic compressibility.     

Lignin turnover in arable soil and grassland analysed with two different labelling approaches

When modelling the carbon dynamics of temperate soils, soil organic carbon (SOC) is often represented by three kinetic pools, i.e. fast, slow and passive/inert. Lignin is often considered to be relatively resistant to decomposition, thus possibly contributing to the passive SOC pool. One way to assess SOC turnover under natural conditions is to follow the fate of ¹³C-labelled biomass in soils. We used compound-specific isotope analysis to analyse CuO oxidation products of lignin from grassland topsoils and from an arable topsoil that had received a natural (by C3-C4 vegetation change) or an artificial (by fumigation with labelled CO₂) isotopic label for 9–23 years. Results indicate faster apparent turnover for lignin (5–26 years in grassland, 9–38 years in arable soil) compared with bulk SOC (20–26 years in grassland, 51 years in arable soil). Although these calculated lignin turnover times cannot be extrapolated to the whole soil profiles, this paper provides isotopic evidence that lignin in soils is not preferentially preserved, which is a consistent result from both ways of isotopic labelling. It also demonstrates, however, that a considerable proportion of lignin in temperate soils can be stabilized for at least a few decades.     

不同管理方式对黑土农田根际土壤团聚体稳定性的影响

以长期定位试验为平台,采集草地(GL)、作物轮作(当季作物为大豆,RS)和连作(玉米、大豆、小麦连作,CM,CS,CW)共5个样地的根际土壤样品作为实验处理,通过对根际土壤团聚体稳定性及有机碳含量的研究,旨在阐明不同植被覆盖与作物轮作—连作对土壤团聚体稳定性的影响。结果表明,土壤有机碳以如下顺序递减:GL>CS>CM>RS>CW;土壤全氮含量以如下顺序递减:GL>CS>CM>CW>RS。农田经过20年的生态恢复,植被演替成为草地,土壤有机碳含量比农田各处理平均高出7.8%。水稳性大团聚体(WSA_(>0.25mm))质量分数以如下顺序递减:GL>CM>RS>CS>CW,草地WSA_(>0.25mm)的质量分数比农田平均高出18.0%。WSA_(>0.25mm)有机碳含量与土壤总有机碳含量呈显著正相关(r=0.89,p<0.05)。从草地到农田各处理WSA_(>2mm)与其WSA_(>0.25mm)的变化趋势基本一致。WSA_(>2mm)有机碳含量与平均重量直径(MWD)呈极显著正相关(r=0.99.p<0.01),对土壤团聚体的稳定性具有重要作用。根密度与WSA_(>0.25mm)、WSA_(>2mm)质量分数及有机碳含量呈显著或极显著正相关,与MWD的相关性也达极显著水平(r=0.97,p<0.01),表明植物根系在水稳性大团聚体的形成与稳定过程中起着重要作用。     

Using soil organic matter fractions as indicators of soil physical quality

The objective of this study was to evaluate the use of chemical and physical fractions of soil organic matter (SOM ), rather than SOM per se , as indicators of soil physical quality (SPQ ) based on their effect on aggregate stability (AS ). Chemically extracted humic and fulvic acids (HA and FA ) were used as chemical fractions, and heavy and light fractions (HF and LF ) obtained by density separation as physical fractions. The analyses were conducted on medium‐textured soils from tropical and temperate regions under cropland and pasture. Results show that soil organic carbon (SOC ), SOM fractions and AS appear to be affected by land use regardless of the origin of the soils. A general separation of structurally stable and unstable soils between samples of large and small SOC content, respectively, was observed. SOM fractions did not show a better relationship with AS than SOC per se . In both geographical regions, soils under cropland showed the smallest content of SOC , HA and carbon concentration in LF and HF , and the largest HF /LF ratio (proportion of the HF and LF in percent by mass of bulk soil). With significant associations between AS and SOC content (0.79**), FA /SOC (r  = −0.83*^*), HA /FA (r  = 0.58*^*), carbon concentration of LF (r  = 0.69*^*) and HF (r  = 0.70*^*) and HF /LF ratio (r  = 0.80*^*), cropland showed lowest AS . These associations indicate that SOM fractions provide information about differences in SOM quality in relation to AS and SPQ of soils from tropical and temperate regions under cropland and pasture.     

松嫩平原土壤有机碳空间分异

Soil organic carbon (SOC) and its relationship with landscape attributes are important for evaluating current regional, continental, and global carbon stores. Data of SOC in surface soils (0–20 cm) of four main soils, Cambisol, Arenosol, Phaeozem, and Chernozem, were collected at 451 locations in Nongan County under maize monoculture in the Song-Nen Plain, Northeast China. The spatial characteristics of soil organic carbon were studied, using geographic information systems (GIS) and geostatistics. Effects of other soil physical and chemical properties, elevation, slope, and soil type on SOC were explored. SOC concentrations followed a normal distribution, with an arithmetic mean of 14.91 g kg-1 . The experimental variogram of SOC was fitted with a spherical model. There were significant correlations between soil organic carbon and bulk density (r =-0.374**), pH (r = 0.549**), total nitrogen (r = 0.781**), extractable phosphorus (r =-0.109*), exchangeable potassium (r = 0.565**), and cation exchange capacity (r = 0.313**). Generally, lower SOC concentrations were significantly associated with high elevation (r =-0.429**). Soil organic carbon was significantly negatively correlated with slope gradient (r =-0.195**). Samples of the Cambisol statistically had the highest SOC concentrations, and samples of the Arenosol had the lowest SOC value.