利用方式和土壤肥力对土壤团聚体和养分的影响

The size distribution of water-stable aggregates and the variability of organic C, N and P contents over aggregate size fractions were studied for orchard, upland, paddy, and grassland soils with high, medium, and low fertility levels. The results showed that > 5 mm aggregates in the cultivated upland and paddy soils were 44.0% and 32.0%, respectively, less than those in the un-tilled orchard soil. Organic C and soil N in different size aggregate fractions in orchard soil with high fertility were significantly higher than those of other land uses. However, the contents of soil P in different size aggregates were significantly greater in the paddy soil as compared to the other land uses. Soil organic C, N and P contents were higher in larger aggregates than those in smaller ones. The amount of water-stable aggregates was positively correlated to their contribution to soil organic C, N and P. For orchard and grassland soils, the > 5 mm aggregates made the greatest contribution to soil nutrients, while for upland soil, the 0.25-0.053 mm aggregates contributed the most to soil nutrients. Therefore, the land use with minimum disturbance was beneficial for the formation of a better soil structure. The dominant soil aggregates in different land use types determined the distribution of soil nutrients. Utilization efficiency of soil P could be improved by converting other land uses to the paddy soil.     

不同利用方式旱地红壤水稳性团聚体及其碳、氮、磷分布特征

针对江西红壤地区不同利用方式引起的土壤质量和肥力的相应变化,研究了不同肥力水平、不同利用方式下红壤旱地水稳性团聚体含量及其养分分布规律。研究表明,荒地土壤中>5 mm水稳性团聚体含量显著高于其他利用方式,花生地和果园土壤则以0.25～0.053 mm的水稳性团聚体为主。各肥力水平下,菜地土壤中除>5 mm水稳性团聚体外,各粒级团聚体中有机碳、全氮和全磷含量均显著高于花生地、果园和荒地土壤。说明菜地土壤长期大量施肥,导致土壤碳、氮、磷养分含量均相对丰富。不同利用方式旱地红壤中,有机碳、全氮主要分布在>5 mm、5～2 mm和2～1 mm的较大粒径水稳性团聚体中。说明随着团聚体粒径增大,其有机碳含量增加,土壤全氮的消长趋势和有机碳一致。土壤全磷较均匀地分布在水稳性团聚体中,如高肥力菜地和荒地土壤各粒级团聚体中全磷含量间均无显著性差异。各利用方式旱地红壤中2～1 mm和1～0.5 mm的水稳性团聚体含量与土壤有机碳、全氮和全磷含量间均达到了极显著正相关。     

Land-use effects on organic carbon,nitrogen, and sulphur concentrations in macroaggregates of differently textured Brazilian oxisols

The objective of this study was to assess the influence of land use on soil aggregate size distribution and the consequences for organic C, N, and S concentrations in bulk soil and macroaggregates. The properties of a loamy and a clayey Oxisol used for continuous cropping, pasture and reforestation were compared with those of the native savannah (“Cerrado”). We measured aggregate size fractionation, C, N, and S concentrations in bulk soil, small (0.25–2 mm, SMA), and large macroaggregates (2–8 mm, LMA), and carried out a mineralization experiment with intact and crushed LMA. The aggregate size distribution of pastures was not different from native Cerrado. Reforestation and plowing caused higher percentages of smaller aggregates which was more pronounced in the loamy than in the clayey soil. Total concentrations of C, N, and S were higher in the clayey (C: 21.5–23.3 g kg^?1; N: 1.2–1.4 g kg^?1; S: 178–213 mg kg^?1) than in the loamy soil (C: 7.8–10.3 g kg^?1; N 0.5–0.7 g kg^?1; S: 87–132 mg kg^?1). LMA of the loamy soil had higher C, N, and S concentrations than the bulk soil. SMA and both macroaggregate fractions of the clayey soil did not differ from the bulk soil. 71 % of potentially mineralizable N in LMA of the loamy soil were only mineralized after aggregate disruption. In contrast, there were only small differences between crushed and intact LMA of the clayey soil. Therefore, we considered conventional tillage suitable for clayey soils. The loamy soil would require a more soil conserving system like no-till or crop-pasture rotation to improve sustainability.     

Fate of airborne metal pollution in soils as related to agricultural management: 2. Assessing the role of biological activity in micro‐scale Zn and Pb distributions in A,B and C horizons

This work assesses relationships between characteristic aggregate microstructures related to biological activity in soils under different long‐term land use and the distribution and extractability of metal pollutants. We selected two neighbouring soils contaminated with comparable metal loads by past atmospheric deposition. Currently, these soils contain similar stocks, but different distributions of zinc (Zn) and lead (Pb) concentrations with depth. One century of continuous land use as permanent pasture (PP) and conventional arable (CA) land, has led to the development of two soils with different macro‐ and micro‐morphological characteristics. We studied distributions of organic matter, characteristic micro‐structures and earthworm‐worked soil by optical microscopy in thin sections from A, B and C horizons. Concentrations and amounts of total and EDTA‐extractable Zn and Pb were determined on bulk samples from soil horizons and on size‐fractions obtained by physical fractionation in water. Large amounts of Zn and Pb were found in 2–20‐µm fractions, ascribed to stable organo‐mineral micro‐aggregates influenced by root and microbial activity, present in both soils. Unimodal distribution patterns of Zn, Pb and organic C in size‐fractions were found in horizons of the CA soil. In contrast, bimodal patterns were observed in the PP soil, because large amounts of Zn and Pb were also demonstrated in stable larger micro‐aggregates (50–100‐µm fractions). Such differing distribution patterns characterized all those horizons markedly influenced by earthworm activity. Larger earthworm activity coincided with larger metal EDTA‐extractability, particularly of Pb. Hence, land use‐related biological activity leads to specific soil microstructures affecting metal distribution and extractability, both in surface and subsurface horizons.     

Stability of soil aggregates in relation to organic constituents and soil water content

The effects of soil organic matter content, soil water content and duration of wet-sieving on aggregate stability of soils with contrasting cropping histories were investigated. Long-term pasture samples had a greater aggregate stability than long-term arable samples. However, air-drying aggregates before wet-sieving increased the aggregate stability of long-term pasture samples, but decreased that of long-term arable samples. With increasing duration of wet-sieving, the proportion of water-stable aggregates declined until a near-constant value was reached for each sample. Thus, within a sample there are aggregates possessing a wide range of stabilities; with increasing time under arable cropping there is an increase in the proportion of unstable aggregates present, and the measured aggregate stability, therefore, declines. Unstable aggregates (defined as those dispersed after wet-sieving for 1 min) generally had lower organic matter content than stable ones (those still intact after sieving for 15 min). The aggregate stability of a regrassed site (13 years of arable plus 2 years of pasture) was markedly higher than that of a corresponding site from 15 years of arable cropping. Nonetheless, levels of organic matter (organic C, total N and hydrolysable carbohydrate) were almost identical at the two sites. However, aggregates from the regrassed site did have a higher biomass C and water-extractable carbohydrate content than those from the 15-year arable site. For a group of soils with varying cropping histories, aggregate stability was significantly more closely correlated with hot water-extractable carbohydrate content than with organic C or hydrolysable carbohydrate content. It is suggested that the hot water-extractable carbohydrate fraction may represent a pool of carbohydrate involved in the formation of stable aggregates.     

Storage and stability of organic matter and fossil carbon in a Luvisol and Phaeozem with continuous maize cropping: A synthesis

Quantitative information about the amount and stability of organic carbon (OC) in different soil organic‐matter (OM) fractions and in specific organic compounds and compound‐classes is needed to improve our understanding of organic‐matter sequestration in soils. In the present paper, we summarize and integrate results performed on two different arable soils with continuous maize cropping (a) Stagnic Luvisol with maize cropping for 24 y, b) Luvic Phaeozem with maize cropping for 39 y) to identify (1) the storage of OC in different soil organic‐matter fractions, (2) the function of these fractions with respect to soil‐OC stabilization, (3) the importance and partitioning of fossil‐C deposits, and (4) the rates of soil‐OC stabilization as assessed by compound‐specific isotope analyses. The fractionation procedures included particle‐size fractionation, density fractionation, aggregate fractionation, acid hydrolysis, different oxidation procedures, isolation of extractable lipids and phospholipid fatty acids, pyrolysis, and the determination of black C. Stability of OC was determined by ¹³C and ¹⁴C analyses. The main inputs of OC were plant litter (both sites) and deposition of fossil C likely from coal combustion and lignite dust (only Phaeozem).     

A comparison of two methods for the isolation of free and occluded particulate organic matter

Various methods exist for the isolation of particulate organic matter (POM), one of the soil‐organic‐matter (SOM) fractions reacting most sensitive on land‐use or soil‐management changes. A combination of density separation and ultrasonic treatment allows to isolate two types of POM: (1) free POM and (2) POM occluded in soil aggregates. POM fractions are closely linked to their biochemical function for the formation and stabilization of aggregates, therefore methods using different aggregate sizes may result in different POM fractions isolated. We evaluated two physical fractionation procedures to reveal whether they yield different POM fractions with respect to amount and composition, using grassland and arable soils with sandy‐loam to sandy–clay‐loam texture and thus low macroaggregate stability. Method I used air‐dried aggregates of <2.0 mm size and a low‐energy sonication for aggregate disruption, method II used field‐moist aggregates <6.3 mm and a high‐energy–sonication procedure for aggregate disruption. POM fractions were analyzed by elemental analysis (C, N) and CPMAS ¹³C‐NMR spectroscopy. With both methods, about similar proportions of the SOM are isolated as free or occluded POM, respectively. The free‐ and occluded‐POM fractions obtained with method I are also rather similar in C and N concentration and composition as shown by ¹³C‐NMR spectroscopy. Method II isolates a free‐ and occluded‐POM fraction with significantly different C and N concentrations. NMR spectra revealed significant differences in the chemical composition of both fractions from method II, with the occluded POM having lower amounts of O‐alkyl C and higher amounts of aryl C and alkyl C than the free POM. Due to the use of larger, field‐moist aggregates with minimized sample pretreatment, two distinctly different POM fractions are isolated with method II, likely to be more closely linked to their biochemical function for the formation and stabilization of aggregates. High‐energy sonication as in method II also disrupts small microaggregates <63 µm and releases fine intraaggregate POM. This fraction seems to be a significant component of occluded POM, that allows a differentiation between free and occluded POM in sandy soils with significant microaggregation. It can be concluded, that microaggregation in arable soils with sandy texture is responsible for the storage of a more degraded occluded POM, that conversely supports the stabilization of fine microaggregates.     

SOIL ORGANIC CARBON STOCK AND FRACTIONS IN RELATION TO LAND USE AND SOIL DEPTH IN THE DEGRADED SHIWALIKS HILLS OF LOWER HIMALAYAS

The proportional differences in soil organic carbon (SOC) and its fractions under different land uses are of significance for understanding the process of aggregation and soil carbon sequestration mechanisms. A study was conducted in a mixed vegetation cover watershed with forest, grass, cultivated and eroded lands in the degraded Shiwaliks of the lower Himalayas to assess land‐use effects on profile SOC distribution and storage and to quantify the SOC fractions in water‐stable aggregates (WSA) and bulk soils. The soil samples were collected from eroded, cultivated, forest and grassland soils for the analysis of SOC fractions and aggregate stability. The SOC in eroded surface soils was lower than in less disturbed grassland, cultivated and forest soils. The surface and subsurface soils of grassland and forest lands differentially contributed to the total profile carbon stock. The SOC stock in the 1.05‐m soil profile was highest (83.5 Mg ha⁻¹) under forest and lowest (55.6 Mg ha⁻¹) in eroded lands. The SOC stock in the surface (0–15 cm) soil constituted 6.95, 27.6, 27 and 42.4 per cent of the total stock in the 1.05‐m profile of eroded, cultivated, forest and grassland soils, respectively. The forest soils were found to sequester 22.4 Mg ha⁻¹ more SOC than the cultivated soils as measured in the 1.05‐m soil profiles. The differences in aggregate SOC content among the land uses were more conspicuous in bigger water‐stable macro‐aggregates (WSA > 2 mm) than in water‐stable micro‐aggregates (WSA < 0.25 mm). The SOC in micro‐aggregates (WSA < 0.25 mm) was found to be less vulnerable to changes in land use. The hot water soluble and labile carbon fractions were higher in the bulk soils of grasslands than in the individual aggregates, whereas particulate organic carbon was higher in the aggregates than in bulk soils. Copyright © 2012 John Wiley & Sons, Ltd.     

侵蚀环境退耕撂荒地水稳性团聚体演变特征及土壤养分效应

研究了侵蚀环境退耕撂荒地生态恢复过程中水稳性团聚体的变化规律及其与土壤养分状况及土壤微生物量的关系，结果表明，侵蚀环境退耕撂荒地土壤水稳性团聚体含量迅速提高，在退耕撂荒的第一个10年，水稳性团聚体增大约253．45％。随着退耕撂荒年限的延长，其增大幅度趋缓，第10～50年，水稳性团聚体仅增大16．42％，之后增大幅度更加缓慢，接近一个稳定的水平。相关性分析和回归分析表明侵蚀环境退耕地撂荒后土壤中〉0．25mm水稳性团聚体与土壤的有机碳、全氮、速磷、碳酸钙、微生物量（C，N，P）、CEC关系密切，达到显著（P〈0．05）或极显著水平（P〈0．01），而与速钾没有明显的相关性。要保持侵蚀环境退耕撂荒土壤良好的结构状况，必须坚持长期封禁，维持土壤有机碳变化处于平衡状态。     

长期施肥对高原农业生态系统土壤有机碳和氮的影响

The effects of fertilization on the distributions of organic carbon (OC) and nitrogen (N) in soil aggregates and whether these effects vary with cropping system have not been well addressed.Such information is important for understanding the sequestration of OC and N in agricultural soils.In this study,the distributions of OC and N associated with soil aggregates were analyzed in different fertilization treatments in a continuous winter wheat cropping system and a legume-grain rotation system in a 27-year field experiment,to understand the effects of long-term fertilization on the distributions of OC and N in aggregates and to examine the recovery of soil OC and N in a highland agroecosystem.Manure fertilizer significantly decreased soil bulk density but increased the amount of coarse fractions and their associated OC and N stocks in the soils of both systems.Fertilizers N + phosphorus (P) and manure had similar effects on total soil OC and N stocks in both systems,but had larger effects on the OC and N stocks in ＞ 2 mm aggregates in the legume-grain rotation system than in the continuous winter wheat system.The application of P increased the OC and N stocks in ＞ 2 mm aggregates and decreased the loss of N from chemical fertilizers in the legume-grain rotation system.The results from this study suggested that P fertilizer should be applied for legume-included cropping systems and that manure with or without chemical fertilizers should be applied for semiarid cropping systems in order to enhance OC and N accumulation in soils.     

Dynamics of soil organic carbon and nitrogen associated with physically separated fractions in a grassland-cultivation sequence in the Qinghai-Tibetan plateau

This study is aimed at quantifying organic carbon (C) and total nitrogen (N) dynamics associated with physically separated soil fractions in a grassland-cultivation sequence in the Qinghai-Tibetan plateau. Concentrations of organic C and N of soil, free and occluded particulate organic matter (OM), and aggregate- and mineral-associated OM in different land uses are increased in the following order: 50 years cultivation < 12 years cultivation ≤ native grassland. The prolonged cropping of up to 50 years markedly affected the concentrations of free and occluded particulate OM and mineral-associated OM. After wet-sieving, 43% of native grassland soil mass was found in >1−10 mm water-stable aggregates that stored 40% of bulk soil organic C and N; only 16% and 7% of soil mass containing 16% and 7% of bulk soil organic C and N was >1−10 mm water-stable aggregates of soils cultivated for 12 years and 50 years, respectively. This indicated that losses of soil organic C and N following cultivation of native grassland would be largely related to disruption of >1–10 mm size aggregates and exposure of intra-aggregate OM to microbial attack. Organic C and N concentrations of soil aggregates were similar among aggregate size fractions (>0.05−10 mm) within each land use, suggesting that soil aggregation process of these soils did not follow the hierarchy model. The increase of the C-to-N ratio of free and occluded particulate fractions in the cultivated soils compared to the grassland soil indicated a greater loss of N than C.     

不同土壤管理措施下塿土团聚体的大小分布及其稳定性

土壤团聚体是土壤的重要组成部分,其大小分布影响土壤的功能。本文利用22年土长期定位试验,研究不同土壤管理措施和不同施肥对土壤机械稳定性和水稳性团聚体的分布及其稳定性的影响。土壤管理措施包括裸地休闲、 撂荒和小麦/玉米轮作体系,其中小麦/玉米轮作体系中有9种施肥处理,分别为不施肥（CK）,化肥氮（N）、 磷（P）和钾（K）不同配施5个处理（N、 NP、 NK、 PK、 NPK）,秸秆还田与化肥配合（SNPK）,有机肥与化肥配施2个处理（M1NPK、 M2NPK）。结果表明,不同管理措施显著影响表层（010 cm）和亚表层（1020 cm）土壤的机械稳定性和水稳性团聚体的分布。与作物体系比较,长期休闲可显著增加机械稳定性微团聚体（0.25 mm）的含量,对水稳性团聚体的含量和分布没有显著影响。而长期撂荒显著增加了大于2 mm 的团聚体含量及团聚体的稳定性。长期不同施肥显著影响 030 cm 土层的机械稳定性和水稳性团聚体的分布,总趋势为施肥比不施肥处理降低了1 mm的团聚体含量,增加了0.25~1 mm的团聚体含量,但对土壤团聚体的稳定性没有显著影响。因此,土撂荒22年后显著增加了土壤团聚体的稳定性,而种植作物和不同施肥处理对土壤团聚体的稳定性影响甚微。     

复种模式对豫西褐土团聚体稳定性及其碳、氮分布的影响

  【目的】  长期单一的玉米–小麦复种模式会引起土壤结构破坏、农田生产力下降。探究不同复种模式对农田土壤团聚体稳定性及其碳、氮分布的影响,为维持土壤结构稳定,实现农业可持续发展提供科学依据。  【方法】  定位试验在河南洛阳褐土上进行。设置冬小麦–夏玉米 (T1)、冬小麦–夏花生 (T2)、冬小麦–夏玉米||花生间作 (2行玉米间作4行花生,T3) 3个复种模式处理。试验始于2014年6月,2019年10月夏季作物收获后 (共11茬作物),采集0—20和20—40 cm土层土壤样品,利用湿筛法和干筛法分析土壤团聚体组成、团聚体稳定性、有机碳和全氮在不同粒级团聚体中的含量及分配比例。  【结果】  与T1相比,在0—20 cm土层中,T2和T3处理土壤中 > 0.25 mm粒级的机械性团聚体 (DR_0.25) 占比分别增加了5.9%和9.9%,> 0.25 mm粒级的水稳性团聚体 (WR_0.25) 占比分别增加了50.3%和57.9%,不稳定性团粒指数 (E_LT) 分别较T1减少了33.2%和50.6%,土壤团聚体破坏率 (PAD) 分别较T1减少49.3%和51.4%,土壤团聚体平均质量直径 (MWD) 分别较T1增加36.4%和47.0%,几何平均直径 (GMD) 分别较T1增加100.0%和120.0%。在20—40 cm土层中,T2和T3处理土壤中不稳定性团粒指数 (E_LT) 分别较T1减少了13.2%和18.0%,土壤团聚体破坏率 (PAD) 分别较T1减少21.4%和28.8%,土壤团聚体平均质量直径 (MWD) 分别较T1增加4.8%和6.0%,几何平均直径 (GMD) 分别较T1增加11.5%和7.7%。各粒级的有机碳和全氮含量均以2～0.25 mm粒级最高,且0—20 cm土层的有机碳和全氮含量高于20—40 cm土层。与T1处理相比,T3处理显著提高了0—20 cm土层各粒级土壤全氮的贡献率和 > 0.25 mm粒级土壤有机碳的贡献率,降低了 < 0.25 mm粒级土壤有机碳的贡献率;T2处理显著提高了0—20 cm土层除 >2和<0.053 mm粒级外的土壤全氮的贡献率和 > 0.25 mm粒级土壤有机碳的贡献率,降低了0.25～0.053 mm粒级土壤有机碳的贡献率。与T1处理相比,T2和T3处理提升了0—20和20—40 cm土层土壤总有机碳、全氮、有效磷和速效钾的含量,3个处理的土壤容重和pH无显著差异。  【结论】  冬小麦–夏花生、冬小麦–夏玉米||花生复种模式较传统冬小麦–夏玉米复种模式明显增加了土壤大团聚体含量,增强了团聚体的机械稳定性和水稳定性,还可显著提高土壤团聚体 (特别是 > 0.25 mm粒级团聚体) 的碳、氮含量,提高土壤有效磷和速效钾含量,更有利于豫西褐土区农田土壤肥力保持。且冬小麦–夏玉米||花生复种的效果优于冬小麦–夏花生复种。     

Influence of land use on soil properties in a forest region of Southern Nigeria

Abstract. To assess the effect of different land uses on soil properties, five land use types (fallow, Gmelina , arable, secondary forest and cocoa plots) on a sandy loam Alfisol (Typic Kandiudalf) were compared in terms of surface (0–15 cm) soil pH, exchangeable acidity, K, Ca and Mg, extractable P, total N, organic matter, gravimetric moisture, temperature and bulk density. There were significant differences ( P ≤ 0.05- P ≤ 0.001) between the land use types for all the properties except exchangeable acidity and moisture. All the land use types differed significantly from each other in at least four properties. Fallow and secondary forest differed in nine properties, fallow and cocoa in seven and fallow and Gmelina in six. In terms of the number of properties with high variability (CV ≥ 35%), the order was arable, secondary forest and cocoa (4) > Gmelina (3) > fallow (1).     

轮作休耕模式对土壤团聚体及有机碳含量的影响

为探究轮作休耕模式对红壤坡耕地团聚体稳定性及有机碳含量的影响,通过田间试验设置了休闲地、玉米单作、玉米-豌豆-玉米和玉米-苕子-玉米4个处理,分析了不同种植模式下土壤团聚体稳定性及有机碳含量,结果表明:(1)不同种植模式下土壤机械稳定性和水稳定性团聚体组成均以≥0.25 mm粒径占比最高,机械稳定性团聚体占比在80.68%以上,水稳定性团聚体达到了77.05%以上,且≥0.25 mm团聚体所占比例大小顺序为:休闲地>玉米-豌豆-玉米>玉米-苕子-玉米>玉米单作。(2)根据不同团聚体稳定指数(MWD,GMD,R_0.25和PAD)显示,休闲地的团聚体稳定性最好,与玉米单作之间差异显著,且休闲地和玉米轮作两种模式下团聚体稳定性均优于玉米单作。(3)玉米单作土壤团聚体有机碳含量最低,玉米轮作和休闲地能显著提高有机碳含量,并且玉米-苕子-玉米轮作效果最优; ≥0.25 mm粒径范围的团聚体储存的有机碳含量最高。(4)通过对土壤水稳性团聚体稳定指数与有机碳含量的相关性分析表明,4个不同团聚体稳定指数与有机碳含量都呈极显著正相关关系,与R_0.25相关程度最高。研究结果可为试验区域坡耕地水土流失防治提供理论依据。     

Long-term changes in land use impact the accumulation of microbial residues in the particle-size fractions of a Mollisol

To test the impact of a range of long-term land use types on the partitioning of microbial residues among soil particles, samples from a Mollisol with plots under 100 years of continuous arable cropping, 30 years of simulated overgrazing to severely degraded bare soil, or 30 years of grassland restoration were investigated. The microbial residues, which were assessed among three particle-size fractions (<2, 2–20, and 20–250 μm) by amino sugar extraction, exhibited change with particle size and land use. Converting arable cropping to bare soil induced substantial depletion of amino sugars associated with the clay-size fraction, as a proportion of total carbon (C) and total soil mass, but not the silt- and sand-size fractions. Alternatively, switching arable soil to grassland increased amino sugar stocks in both the clay- and sand-size fractions. Analysis of the relative input of fungal and bacterial derived amino sugars indicated that fungal sources are the most dynamic with respect to land use change. These results highlight the selective vulnerability of microbial C pools in finer fractions under low plant C input and the selective recovery in specific fractions upon restoration, emphasizing the importance of the conversion of plant organic matter into mineral-associated microbial residues to promote stable soil organic C.     

Improvement of the physical fertility of a degraded Alfisol with planted and natural fallows under humid tropical conditions

Abstract. Topsoil (0–15 cm) bulk density, aggregate stability, soil dispersibility, water retention and infiltration were measured between 1989 and 1996 on an Alfisol under rehabilitation in southwestern Nigeria. The planted leguminous species were Pueraria phaseoloides, Senna siamea, Leucaena leucocephala, Acacia leptocarpa and A. auriculiformis. Also, plots with natural fallow and maize/cassava intercropping were included. Level (minimum) and mound tillage with hoes was adopted for the cultivated areas under study after 4 and 6 year fallow periods. Under fallow, the soil bulk density decreased from1.56 to 1.11 t m73.The continuously cropped treatment (level tillage) had significantly higher bulk density than the fallowed subplots after 6 years. Mean soil penetrometer resistance ranged from 75 to 157 kPa for fallowed plots and from 192 to 295 kPa for the continuously cropped (level tillage) subplot. Surface soilwater contentswere similar for all the treatments during the soil strength measurements. Although soil aggregates were generally of low stability and not well formed, they were improved by fallowing.
Soil structural improvement by planted fallows was similar to that by natural fallow, but the trees were more promising for long-term fallow (>6 years) than the herbaceous P. phaseoloides. However, the improvement in soil structure after 4 or 6 year fallow could not be maintained in subsequent cropping. Furthermore, the significant improvement in soil bulk density caused by A. auriculiformis and natural fallow was more rapidly lost on the cultivated subplots compared with other fallow treatments. Thus, soil structure recovery under a fallow does not imply a sustained improvement when stress is applied to this soil. Post-fallow soil management options such as residue incorporation and tillage to ameliorate compaction or soil strength will be necessary to enhance the improvements by fallow species.     

长期培肥过程中塿土物理性质演变规律

通过对长期培肥试验田不同处理土壤的比重、容重、孔隙度、团聚体等物理性质的系统分析,企图揭示在长期培肥过程中土壤物理性质演变规律。研究结果表明:未进行培肥作用的土壤(对照)小区仍维持着原来土壤有机质含量水平,该试验小区土壤所具有的是最基础的物理性质,其它不同培肥处理的土壤比重和容重均随有机质积累而明显减小,孔隙度随土壤有机质含量增加而升高。不同培肥措施都明显地促进土壤大团聚体的形成,特别是增加了>0.25mm水稳性团聚体的含量,降低了团聚体破坏率,逐渐地改善了土壤物理性质。在常年休闲过程中土壤结构性能明显地被破坏。     

Changes in soil aggregate‐associated enzyme activities and nutrients under long‐term chemical fertilizer applications in a phosphorus‐limited paddy soil

Paddy soils in subtropical China are usually deficient in phosphorus (P) and require regular application of chemical fertilizers. This study evaluated the effects of chemical fertilizers on the distribution of soil organic carbon (SOC), total nitrogen (N) and available P, and on the activity of the associated enzymes in bulk soil and aggregates. Surface soils (0–20 cm) were collected from a 24‐yr‐old field experiment with five treatments: unfertilized control (CK), N only (N), N and potassium (NK), N and P (NP), and N, P and K (NPK). Undisturbed bulk soils were separated into >2, 1–2, 0.25–1, 0.053–0.25 and <0.053 mm aggregate classes using wet sieving. Results showed that both NP‐ and NPK‐treated soils significantly increased mean weight diameter of aggregates, SOC, available P in bulk soil and aggregates, as compared to CK. Most SOC and total N adhered to macro‐aggregates (>0.25 mm), which accounted for 64–81% of SOC and 54–82% of total N in bulk soil. The activities of invertase and acid phosphatase in the 1–2 mm fraction were the highest under NPK treatment. The highest activity of urease was observed in the <0.053 mm fraction under NP treatment. Soil organic carbon and available P were major contributors to variation of enzyme activities at the aggregate scale. In conclusion, application of NP or NPK fertilizers promoted the formation of soil aggregates, nutrient contents and activities of associated enzymes in P‐limited paddy soils, and thus enhanced soil quality.     

Composition and stability of soil aggregates in Fluvisols under forest,meadows, and 100 years of conventional tillage

Conversion of meadow and forest ecosystems to agricultural land generally leads to changes in soil structure. This comparative study presents the composition and stability of structural aggregates in humus horizons (0–30 cm) of noncarbonate silty‐clay Fluvisols in the Kolubara River Valley, W Serbia. Aggregates collected from under a native forest were compared to aggregates from meadows and arable fields which underwent crop rotation for > 100 y. The results show that size distribution and stability of structural aggregates in the humus horizons of arable soil are significantly impaired due to long‐term anthropogenization. In the humus horizons, the content of the agronomically most valuable aggregates (0.25–10 mm) decreased by a factor of ≈ 2, from 68%–74% to 37%–39%, while the percentage of cloddy aggregates (>10 mm) increased by a factor of ≈ 2, from 23%–31% to 48%–62%, compared to forest aggregates. The long‐term‐arable soil had significantly (p < 0.05) lower aggregate stability, determined by wet sieving, than meadow and forest soils. The lowest aggregate stability was found in aggregates > 3 mm. Their content is ≈ 2.5–3 times lower in arable soil (13%–16%) than in forest soil (32%–42%) at a depth of 0–20 cm. The largest mean weight diameters of dry aggregates (dMWD) with a range between 12.6 and 14.7 mm were found in arable soil, vs. 9.5–9.9 mm in meadow and 6.5–8.3 mm in forest. The arable soil had significantly lower mean weight diameters of wet‐stable aggregates (wMWD) and a lower structure coefficient (Ks) than forest and meadow soils. The dispersion ratio (DR) of arable soil was significantly higher than that of forest and meadow soils. Forest and meadow showed a significantly higher soil organic‐matter content (SOM) by 74% and 39%, respectively, compared with arable soil, while meadow uses decreased the SOM content by 57% compared with forest at a depth of 0–10 cm. In conclusion, the results showed that long‐term conventional tillage of soils from natural forest and meadow in the lowland ecosystems of W Serbia degraded soil aggregate–size distribution and stability and reduced SOM content, probably resulting in lower productivity and reduced crop yields.