Straw incorporation and soil organic matter in macro-aggregates and particle size separates

Two field experiments in which straw has been removed or incorporated for 17 yr (loamy sand) and 10 yr (sandy clay loam) were sampled to examine the effect of straw on the C and N contents in whole soil samples, macro-aggregate fractions and primary particle-size separates. The particle size composition of the aggregate fractions was determined. Aggregates were isolated by dry sieving. Straw incorporation increased the number of 1–20 mm aggregates in the loamy sand but no effect was noted in the sandy clay loam. Straw had no effect on the particle size composition of the various aggregate fractions. After correction for loose sand that accumulated in the aggregate fractions during dry sieving, macro-aggregates appeared to be enriched in clay and silt compared with whole soil samples. Because of the possible detachment of sand particles from the exterior surface of aggregates during sieving operations, it was inferred that the particle size composition of macro-aggregates is similar to that of the bulk soil. The organic matter contents of the aggregate fractions were closely correlated with their clay + silt contents. Differences in the organic matter content of clay isolated from whole soil samples and aggregate fractions were generally small. This was also true for the silt-size separates. In both soils, straw incorporation increased the organic matter content of nearly all clay and silt separates; for silt this was generally twice that observed for clay. The amounts of soil C, derived from straw, left in the loamy sand and sandy clay loam at the time of sampling were 4.4 and 4.5 t ha^?1, corresponding to 12 and 21% of the straw C added. The C/N ratios of the straw-derived soil organic matter were 11 and 12 for the loamy sand and sandy clay loam, respectively.     

Cation exchange capacities of organo-mineral particle-size fractions in soils from long-term experiments

Soils with and without organic manuring from 10 long-term manurial experiments in East Germany were fractionated into organo-mineral particle-size separates by ultrasonic disaggregation and sedimentation/decantation. The cation exchange capacities (CECs) buffered at pH 8.1 were determined for the size fractions fine+medium clay, coarse clay, fine, medium and coarse silt, sand, and for the total soil samples. In the samples from nine field experiments the CECs decreased with increased equivalent diameters (fine+medium clay: 489–8 13 mmol_c kg^?1, coarse clay: 367–749 mmol_c kg^?1, fine silt: 202–587 mmol_c kg^?1. medium silt: 63–345 mmol_c kg^?1, coarse silt: 12–128 mmol_c kg^?1 and sand: 10–156 mmol_c kg^?1. The CECs varied with genetic soil type, mineralogical composition of the <6.3-μm particles, and the C and N contents of the size fractions. In a pot experiment examining the role of various organic materials in the early stages of soil formation, the clay-size fractions had the largest CECs (85–392 mmol_c kg^?1), followed by the medium-silt (1 9-222 mmol_c kg^?1) and fine-silt fractions (23–192 mmol_c kg^?1). The effect of organic amendments on CEC was in general: compost>fresh farmyard manure = straw + mineral fertilizer = mineral fertilizer.     

Long-term impact of reduced tillage and residue management on soil carbon stabilization: Implications for conservation agriculture on contrasting soils

Residue retention and reduced tillage are both conservation agricultural management options that may enhance soil organic carbon (SOC) stabilization in tropical soils. Therefore, we evaluated the effects of long-term tillage and residue management on SOC dynamics in a Chromic Luvisol (red clay soil) and Areni-Gleyic Luvisol (sandy soil) in Zimbabwe. At the time of sampling the soils had been under conventional tillage (CT), mulch ripping (MR), clean ripping (CR) and tied ridging (TR) for 9 years. Soil was fully dispersed and separated into 212–2000 μm (coarse sand), 53–212 μm (fine sand), 20–53 μm (coarse silt), 5–20 μm (fine silt) and 0–5 μm (clay) size fractions. The whole soil and size fractions were analyzed for C content. Conventional tillage treatments had the least amount of SOC, with 14.9 mg C g⁻¹ soil and 4.2 mg C g⁻¹ soil for the red clay and sandy soils, respectively. The highest SOC content was 6.8 mg C g⁻¹ soil in the sandy soil under MR, whereas for the red clay soil, TR had the highest SOC content of 20.4 mg C g⁻¹ soil. Organic C in the size fractions increased with decreasing size of the fractions. In both soils, the smallest response to management was observed in the clay size fractions, confirming that this size fraction is the most stable. The coarse sand-size fraction was most responsive to management in the sandy soil where MR had 42% more organic C than CR, suggesting that SOC contents of this fraction are predominantly controlled by amounts of C input. In contrast, the fine sand fraction was the most responsive fraction in the red clay soil with a 66% greater C content in the TR than CT. This result suggests that tillage disturbance is the dominant factor reducing C stabilization in a clayey soil, probably by reducing C stabilization within microaggregates. In conclusion, developing viable conservation agriculture practices to optimize SOC contents and long-term agroecosystem sustainability should prioritize the maintenance of C inputs (e.g. residue retention) to coarse textured soils, but should focus on the reduction of SOC decomposition (e.g. through reduced tillage) in fine textured soils.     

Decomposability of organic matter in particle size fractions from field soils with straw incorporation

Soils from two field experiments on straw disposal were fractionated according to particle size using ultrasonic dispersion and gravity-sedimentation in water. Samples of whole soils, clay. silt and sand-size fractions were held for 49 days at 20°C and the CO₂ evolution measured on 14 dates by gas chromatography.Recovery of soil solids. C and N was 99, 98 and 93%, respectively. Most of the soil C and N was in the clay (<2μm). (loamy sand, 50% C and 56% N; sandy loam. 65% C and 68% N), the silt (2–20 μm) having smaller proportions (loamy sand, 41% C and 38% N; sandy loam. 29% C and 27% N). The sand fraction (20–6000 μm) accounted for 4–7% of the organic matter, and 1–2% of the C was water soluble. Straw incorporation generally increased the C and N content of whole soils and size fractions.The decomposition rate constants were higher for the sandy loam than for the loamy sand soil. For both soils, the decomposability of the organic matter decreased in the order: sand > clay ⩾ whole soil > silt. Straw incorporation increased the decomposition rate of whole soil and sand organic matter. whereas the effect of straw on clay and silt respiration was small.Between 58 and 73% of the respiration was from clay, 21–25% from silt and 6–19% from the sand size fraction.     

Organic carbon quantity and forms as influenced by tillage and cropping sequence

Abstract

Soil organic matter and its chemical fractions have a profound impact on soil chemical and physical properties. In turn, the effect of management (cropping and tillage) on the quantity and chemical properties of soil organic matter can be substantial. The objective of this study was to compare the effects of specific tillage regimes and crop sequences commonly used in the central Great Plains of the United States on the quantity, quality, and distribution with depth of soil organic carbon (SOC). Soils were sampled in 1 cm or 2 cm increments to a depth of 10 cm from experimental field plots on a Sharpsburg silty clay loam (fine, montmorillonitic, mesic Typic Argiudoll). The plots had been under 6 continuous tillage regimes since 1978 and cropped to continuous corn, continuous soybean, or corn‐soybean in rotation since 1985. Soils were analyzed for total SOC, fulvic acid (FA) carbon, and humic acid (HA) carbon. No‐till and continuous corn (Zea mays L.) management generally had the highest SOC, with a sharp reduction in SOC below 2 cm. Only no‐till increased FA, which also decreased with depth, especially between 2 and 4 cm. Humic acid concentration was highest under continuous corn but was unaffected by tillage. Humic acid also was highest in the 1‐ to 2‐cm increment of continuous corn. Two ratios which are used as indices of degree of humification, HA/FA and (HA+FA)/SOC, gave different estimates of the effect of management. Only continuous com increased HA/FA, suggesting increased humification. No treatment affected (HA+FA)/SOC. Overall, continuous corn and no‐till contributed the greatest amounts of residue and maintained a soil environment conducive to preserving the resulting organic matter. These management options increase not only total SOC, but also alter the quality of that SOC as measured by HA and FA. These changes in SOC characteristics may have implications for long‐term soil quality and soil productivity.     

Restoration Affect Soil Organic Carbon and Nutrients in Different Particle‐size Fractions

Desertification is reversible and can often be prevented by adopting measures to control the causal processes. Desertification has generally decreased in most of the arid and semiarid areas of China during the last few decades because of the restoration of degraded vegetation and soil nutrients. However, little is known about the responses of soil nutrients in different particle‐size fractions to the restoration process and about the importance of this response to the restoration of bulk‐soil nutrients. In this study, we separated bulk‐soil samples in different sieve fractions: coarse‐fine sand (2·0–0·1 mm), very fine sand (0·10–0·05 mm) and silt + clay (<0·05 mm) fractions. Soil organic carbon (SOC), N, P and K contents stored in the silt + clay were greater than the contents of non‐protected nutrients in the coarser fractions. During the restoration of desertified land, the content and stability of bulk‐soil SOC, total N and P and available N, P and K increased with increasing nutrient contents in all fractions. Topsoil nutrients stored in coarse‐fine sand and very fine sand fractions were more sensitive than those stored in the silt + clay fraction to the fixation of mobile sandy lands and vegetation recovery. The changes of bulk‐soil nutrients and their stability were decided by the soil nutrients associated with all particle‐size fractions. Path analysis revealed that SOC and total nutrients in very fine sand and available nutrients in coarse‐fine sand were the key factors driving the soil recovery. These results will help us understand soil recovery mechanisms and evaluate the degree of recovery. Copyright © 2015 John Wiley & Sons, Ltd.     

Molecular composition and structure of organic matter in density fractions of soils amended with corn straw for five years

Corn straw is an important source of carbon (C),and when applied to soil,it alters the accumulation and distribution of organic C.However,the mechanistic pathways by which newly added C is stored and stabilized in soil remain a subject of interest and debate among scholars.In this study,we investigated the chemistry of organic matter in different density fractions of Haplic Cambisol (sandy clay loam) in a field experiment with corn straw at8 900 kg ha^-1year^-1under no tillag...     

Distribution of organic carbon in physical fractions of soils as affected by agricultural management

Soil organic carbon (SOC) is distributed heterogeneously among different-sized primary particles and aggregates. Further, the SOC associated with different physical fractions respond differently to managements. Therefore, this study was conducted with the objective to quantify the SOC associated with all the three structural levels of SOC (particulate organic matter, soil separates and aggregate-size fractions) as influenced by long-term change in management. The study also aims at reevaluating the concept that the SOC sink capacity of individual size-fractions is limited. Long-term tillage and crop rotation effects on distribution of SOC among fractions were compared with soil from adjacent undisturbed area under native vegetation for the mixed, mesic, Typic Fragiudalf of Wooster, OH. Forty five years of no-till (NT) management resulted in more SOC accumulation in soil surface (0–7.5 cm) than in chisel tillage and plow tillage (PT) treatments. However, PT at this site resulted in a redistribution of SOC from surface to deeper soil layers. The soils under continuous corn accumulated significantly more SOC than those under corn–soybean rotation at 7.5–45 cm depth. Although soil texture was dominated by the silt-sized particles, most of the SOC pool was associated with the clay fraction. Compared to PT, the NT treatment resulted in (i) significantly higher proportion of large macroaggregates (>2,000 μm) and (ii) 1.5–2.8 times higher SOC concentrations in all aggregate-size classes. A comparative evaluation using radar graphs indicated that among the physical fractions, the SOC associated with sand and silt fractions quickly changed with a land use conversion from native vegetation to agricultural crops. A key finding of this study is the assessment of SOC sink capacity of individual fractions, which revealed that the clay fraction of agricultural soils continues to accumulate more SOC, albeit at a slower rate, with progressive increase in total SOC concentration. However, the clay fraction of soil under native woodlot showed an indication for SOC saturation. The data presented in this study from all the three structural levels of SOC would be helpful for refining the conceptual pool definitions of the current soil organic matter prediction models.     

Carbon content in soil particle size and consequence on cation exchange capacity of Alfisols

Abstract

Many of the cultivated soils of sub‐Saharan Africa typically have a surface horizon low in clay and with a low cation exchange capacity (CEC). In these soils, CEC is largely due to the soil organic matter (SOM). Measurements made on long‐term trials show that changes in CEC and SOM are positively correlated to one another, but not of same magnitude, suggesting that not all of the SOM plays an equal role as regards the soil CEC. To study the influence of the different SOM size fractions on the CEC, soils with or without application of manure or compost coming from trials in Chad and Côte d'Ivoire were separated without destruction of the SOM into five organo‐mineral fractions: “coarse sand”;, “fine sand”;, “coarse silt”;, “fine silt”;, and “clay”; made up of particles of sizes between 2,000 and 200, 200 and 50, 50 and 20, 20 and 2, and 2 and 0 μm, respectively. Fractionation was carried out by mechanical dispersion of the soil, wet sieving of the fractions larger than 20 μm, and decanting of the “clay”; and “fine silt”; fractions. The CEC of these fractions increases inversely with their size. The “clay”; fraction which contains half of the SOM contributes about 80% of the CEC of the soils. The CEC of the fractions is largely a function of their carbon (C) content, but the organic CEC per unit C of the “clay”; fraction appears to be four times greater than that of the other fractions (1,000 as against 270 cmol_c kg^‐1). Applications of manure or compost increase the CEC of the soils by increasing the soil C only when this C increase concerns the fine fractions of the SOM.     

Effect of Clay Minerals and Organic Matter on the Cation Exchange Capacity of Silt Fractions

The cation exchange capacity (CEC) and specific surface properties were investigated in four particle‐size fractions < 50 μm from three loess (one Kastanozem and two Phaeozems), a holocene (Fluvisol) and a basalt soil (Nitisol) before and after destruction of organic matter. Particle‐size fractions were separated by sedimentation after chemical and physical dispersion of the soil samples. Illite, amorphous minerals, mixed layers, smectite and kaolinite were the predominant clay minerals. They were detected in all size fractions. The CEC increased with increasing organic matter contents and this effect was more pronounced in coarser fractions. The organic matter content per unit surface area was two or three times larger in coarse silt than in clay, irrespective of the soil type.     

Distribution of Microorganisms,Biomass ATP,and Enzyme Activities in Organic and Mineral Particles of a Long‐Term Wastewater Irrigated Soil

The aim of the study was to elucidate the spatial distribution of soil microorganisms and enyzme activities in a long‐term wastewater treated soil. Soil was sampled from a plough layer of the Ah horizon of a sandy Haplic Luvisol which was either (1) irrigated with municipal wastewater for almost 100 years, or (2) no more irrigated since 20 years, or (3) never received wastewater. The samples were fractionated by wet sieving to obtain seven size fractions of organic and mineral soil particles, and a separate silt+clay fraction. The individual soil samples contained between 1.2% (never irrigated) and 4.1% (long‐term irrigated) organic particles by weight, but these particles harboured up to 47.8% of the total soil carbon and 41.7% of nitrogen, and thus represented an important storage of energy and nutrient for microorganisms. In total, however, the highest C and N amounts were accumulated in the silt+clay fraction, whereas coarser mineral particles which dominanted by weight in the Haplic Luvisol were low in C and N. The highest numbers of bacteria, actinomycetes and fungi per gram of the individual soil fractions were found in organic particles of the long‐term irrigated soil. Less nutrient‐dependent oligotrophic bacteria were for the most part associated with the silt+clay fraction, irrespective of the soil treatment with wastewater. Similar to microbial counts, also the ATP content, as a measure of active microbial biomass, and the activities of β‐glucosidase, β‐acetylglucosaminidase, and proteinase were higher in the long‐term irrigated soil than in that which was never irrigated. In most cases slightly enhanced values of microbiological and biochemical parameters were still detectable 20 years after the wastewater irrigation was terminated. The values of the individual parameters decreased in all soil samples under testing in general gradually with decrease in size of the organic soil particles. In conclusion, the coarse soil organic particles > 5 mm and the silt+clay fraction < 0.05 mm represent the sites with the highest microbial inhabitance, ATP contents and enzyme activities in the Ah horizon of an Haplic Luvisol. Long‐term wastewater irrigation resulted in an increase of microbial counts, total biomass and soil enzyme activities.     

Comparison of Soil Organic‐Matter Characteristics under the Energy Crop Giant Reed,Cropping Sequence and Natural Grass

The objective of this study was to determine, on the same pedological, topographic, and climatic conditions, the effects of a continuous giant reed cropping (GR), a cropping sequence (CS), and untilled natural grass (NG) on some soil characteristics. Selected characteristics included total soil organic carbon (TOC), light fraction carbon (LFC), dissolved organic carbon (DOC), and microbial biomass carbon (MBC). The various humic fractions and their ratios, humification ratio (HR), humification degree (HD), and humification index (HI) were also evaluated. Results indicate that GR, characterized by low tillage intensity, showed greater values than tilled management system (CS) for all the previously mentioned properties, with the exception of the humification parameters, which were not statistically different between the two systems. Regarding the difference between GR and NG (the latter usually considered as a reference or baseline for evaluating attributes of soil quality), GR generally showed greater values for all organic parameters, with the exception of MBC and humus characteristics.     

Organic matter turnover in soil physical fractions following woody plant invasion of grassland: Evidence from natural C and N

Soil physical structure causes differential accessibility of soil organic carbon (SOC) to decomposer organisms and is an important determinant of SOC storage and turnover. Techniques for physical fractionation of soil organic matter in conjunction with isotopic analyses (δ¹³C, δ¹⁵N) of those soil fractions have been used previously to (a) determine where organic C is stored relative to aggregate structure, (b) identify sources of SOC, (c) quantify turnover rates of SOC in specific soil fractions, and (d) evaluate organic matter quality. We used these two complementary approaches to characterize soil C storage and dynamics in the Rio Grande Plains of southern Texas where C₃ trees/shrubs (δ¹³C=−27‰) have largely replaced C₄ grasslands (δ¹³C=−14‰) over the past 100-200 years. Using a chronosequence approach, soils were collected from remnant grasslands (Time 0) and from woody plant stands ranging in age from 10 to 130 years. We separated soil organic matter into specific size/density fractions and determined their C and N concentrations and natural δ¹³C and δ¹⁵N values. Mean residence times (MRTs) of soil fractions were calculated based on changes in their δ¹³C with time after woody encroachment. The shortest MRTs (average=30 years) were associated with all particulate organic matter (POM) fractions not protected within aggregates. Fine POM (53-250 μm) within macro- and microaggregates was relatively more protected from decay, with an average MRT of 60 years. All silt+clay fractions had the longest MRTs (average=360 years) regardless of whether they were found inside or outside of aggregate structure. δ¹⁵N values of soil physical fractions were positively correlated with MRTs of the same fractions, suggesting that higher δ¹⁵N values reflect an increased degree of humification. Increased soil C and N pools in wooded areas were due to both the retention of older C₄-derived organic matter by protection within microaggregates and association with silt+clay, and the accumulation of new C₃-derived organic matter in macroaggregates and POM fractions.     

Carbon storage in loess derived surface soils from Central Germany: Influence of mineral phase variables

The influence of the soil mineral phase on organic matter storage was studied in loess derived surface soils of Central Germany. The seven soils were developed to different genetic stages. The carbon content of the bulk soils ranged from 8.7 to 19.7 g kg^—1. Clay mineralogy was confirmed to be constant, with illite contents > 80 %. Both, specific surface area (SSA, BET‐N₂‐method) and cation exchange capacity (CEC) of bulk soils after carbon removal were better predictors of carbon content than clay content or dithionite‐extractable iron. SSA explained 55 % and CEC 54 % of the variation in carbon content. The carbon loadings of the soils were between 0.57 and 1.06 mg C m^—2, and therefore in the ”︁monolayer equivalent” (ME) level. The increase in SSA after carbon removal (ΔSSA) was significantly and positively related to carbon content (r² = 0.77). Together with CEC of carbon‐free samples, ΔSSA explained 90 % of the variation in carbon content. Clay (< 2 μm) and fine silt fractions (2—6.3 μm) contained 68—82 % of the bulk soil organic carbon. A significantly positive relationship between carbon content in the clay fraction and in the bulk soil was observed (r² = 0.95). The carbon pools of the clay and fine silt fractions were characterized by differences in C/N ratio, δ¹³C ratio, and enrichment factors for carbon and nitrogen. Organic matter in clay fractions seems to be more altered by microbes than organic matter in fine silt fractions. The results imply that organic matter accumulates in the fractions of smallest size and highest surface area, apparently intimately associated with the mineral phase. The amount of cations adhering to the mineral surface and the size of a certain and specific part of the surface area (ΔSSA) are the mineral phase properties which affect the content of the organic carbon in loess derived arable surface soils in Central Germany most. There is no monolayer of organic matter on the soil surfaces even if carbon loadings are in the ME level.     

Studies of nitrogen immobilization and mineralization in calcareous soils—II: Mineralization of immobilized nitrogen from soil fractions of different particle size and density

¹⁵NO^?₃ was immobilized in a calcareous sandy soil and a calcareous clay soil each incubated with glucose and wheat straw. Net mineralization of organic-¹⁵N was more rapid in the sandy soil, irrespective of C amendment, and in soils amended with glucose. Intermittent drying and wetting of soils during incubation stimulated mineralization of ¹⁵N-labelled and native soil organic-N in all treatments. The availability (percentage mineralization) of recently-immobilized ¹⁵N consistently exceeded that of the native soil N. Ratios of the availability of labelled and unlabelled N were similar in the sandy and clay soils but varied according to C amendment, drying and wetting cycle and incubation period.Changes in the distribution of immobilized N amongst soil extracts and soil fractions of different particle size and density were determined during periods of net N mineralization. In straw-amended soils, the organic-¹⁵N of a light fraction, sp.gr. < 1.59, decomposed relatively rapidly during the late mineralization period. Decreases of organic ¹⁵N of the fine clay fraction were also recorded. In glucose-amended soils, net N mineralization was accompanied by significant decreases in the concentrations of organic-¹⁵N of the silt and fine clay fractions.Drying and rewetting of soils hastened or magnified changes occurring in the organic-¹⁵N of soil fractions, but qualitatively, the pattern of change was similar to that observed with soils incubated under uniformly-moist conditions.The percentage distribution of labelled and unlabelled N suggested that in the long term, the silt fraction will accumulate an increasing proportion of the more stable nitrogenous residues.     

Effect of Continuous Rice–Wheat Rotation on Soil Properties from Four Agro‐ecosystems of Indian Punjab

In Indian Punjab, rice–wheat is a dominant cropping system in four agro‐ecosystems, namely undulating subregion (zone 1), Piedmont alluvial plains (zone 2), central alluvial plains (zone 3), and southwestern alluvial plains (zone 4), varying in rainfall and temperature. Static and temporal variabilities in soil physical and chemical properties prevail because of alluvial parent material, management/tillage operations, and duration of rice–wheat rotation. A detailed survey was undertaken to study the long‐term effect of rice–wheat rotation on soil physical (soil separates, bulk density, modulus of rupture, saturated and unsaturated hydraulic conductivities, soil water content, and suction relations) and chemical (organic carbon, pH, electrical conductivity) properties of different textured soils (sandy clay loam, loam, clay loam, and silty clay loam) in these four zones of Punjab. Soil samples (of 0‐ to 30‐cm depth) from 45 sites were collected during 2006 and were analyzed for physical and chemical properties. The results showed that sand content and pH increased whereas silt and organic carbon decreased significantly from zones 1 to 4. Compared to other textures, significantly greater organic carbon, modulus of rupture, and pH in silty clay loam; greater bulk density in clay loam, and greater saturated hydraulic conductivity in sandy clay loam were observed. Irrespective of zone and soil texture, in the subsurface soil, there was a hard pan at 15–22.5 cm deep, which had high soil bulk density, modulus of rupture, more silt and clay contents (by 3–5%) and less organic carbon and hydraulic conductivity than the surface (0–15 cm) layer. These properties deteriorated with fineness of the soil texture and less organic carbon content. Continuous rice–wheat cropping had a deleterious effect on many soil properties. Many of these soils would benefit from the addition of organic matter, and crop yields may also be affected by the distinct hardpan that exists between 15 and 22.5 cm deep.     

Influence of tillage and rotation systems on distribution of organic carbon associated with particle-size fractions in Chernozemic soils of Saskatchewan, Canada

The effects of several dominant tillage and rotation systems on soil organic C content of different particle-size fractions were studied in Chernozemic soils from southwestern and east-central Saskatchewan, Canada. In an Orthic Brown Chernozem in southwestern Saskatchewan, 7 years of no-till cereal–fallow, imposed on a long-term tillage fallow–wheat rotation soil, resulted in 0.1 Mg C ha⁻¹ more organic C mass in the sand + organic matter (OM) fraction of the 0- to 5-cm layer, whereas organic C associated with coarse silt (CS), fine silt (FS), coarse clay, and fine clay of 0- to 5- and 5- to 10-cm layers was less than that of the comparable tilled cereal–fallow system. Conversion of tilled fallow–wheat rotation soil to continuous cropping had a slight effect, whereas the organic C mass in all the size fractions was significantly increased in both 0- to 5- and 5- to 10-cm layers after alfalfa was introduced on tilled fallow–wheat as perennial forage for 10 years. In an Orthic Black Chernozem in east-central Saskatchewan that was cultivated and tilled using a cereal–fallow rotation for 62 years, organic C mass decreased in sand + OM, CS, and FS of 0- to 10-cm depth. Conversion of the tilled cereal–fallow cropland soil back to seeded grassland resulted in significantly more soil organic C in sand + OM fraction after 12 years of grass seed-down. The sand + OM fraction appears to be the size fraction pool initially most sensitive to adoption of management practices that are liable to sequester carbon in the soil.     

Soil organic carbon and nutrient content in aggregate‐size fractions of a subtropical rice soil under variable tillage

The effects of tillage on soil organic carbon (SOC) and nutrient content of soil aggregates can vary spatially and temporally, and for different soil types and cropping systems. We assessed SOC and nutrient levels within water‐stable aggregates in ridges with no tillage (RNT) and also under conventional tillage (CT) for a subtropical rice soil in order to determine relationships between tillage, cation concentrations and soil organic matter. Surface soil (0–15 cm) was fractionated into aggregate sizes (>4.76 mm, 4.76–2.00 mm, 2.00–1.00 mm, 1.00–0.25 mm, 0.25–0.053 mm, <0.053 mm) under two tillage regimes. Tillage significantly reduced the proportion of macroaggregate fractions (>2.00 mm) and thus aggregate stability was reduced by 35% compared with RNT, indicating that tillage practices led to soil structural change for this subtropical soil. The patterns in SOC, total N, exchangeable Ca²⁺, Mg²⁺ and total exchangeable bases (TEB) were similar between tillage regimes, but concentrations were significantly higher under RNT than CT. This suggests that RNT in subtropical rice soils may be a better way to enhance soil productivity and improve soil C sequestration potential than CT. The highest SOC was in the 1.00–0.25 mm fraction (35.7 and 30.4 mg/kg for RNT and CT, respectively), while the lowest SOC was in microaggregate (<0.025 mm) and silt + clay (<0.053 mm) fractions (19.5 and 15.7 mg/kg for RNT and CT, respectively). Tillage did not influence the patterns in SOC across aggregates but did change the aggregate‐size distribution, indicating that tillage affected soil fertility primarily by changing soil structure.     

流动沙丘造林对土壤物理组分有机碳分配的影响

(1)根据中国知网(CNKI)的《中国学术期刊影响因子年报(自然科学与工程技术.2010版)计量指标统计表》,《水土保持通报》综合统计源统计的总被引频次为3 446次(2009年版中为1 358次);复合影响因子为0.955;期刊综合影响因子为0.568(2008年为0.493),在所统计     

STUDIES OF SOIL ORGANO-MINERAL FRACTIONS

Sand-, silt-, and clay-size organo-mineral fractions were isolated in bulk from surface horizons of five soils following ultrasonic dispersion in water. Good clay separation was achieved for all except one highly organic, calcareous clay soil. Organic-N and -C were concentrated in the clay and silt fractions but for each soil the organic C : N ratio decreased in the order sand > silt > whole soil > clay. Acid hydrolysis of the silt and clay fractions revealed a slight concentration of amino acid-N and NH₄-N in the clays but only small differences in the distribution of individual amino acids were observed. The results suggest that both silt and clay fractions may be important in the stabilization of soil organic matter.