The influence of freeze–thaw cycles and soil moisture on aggregate stability of three soils in Norway

Winter conditions with seasonally frozen soils may have profound effects on soil structure and erodibility, and consequently for runoff and erosion. Such effects on aggregate stability are poorly documented for Nordic winter conditions. The purpose of this study was to quantify the effect of variable freeze–thaw cycles and soil moisture conditions on aggregate stability of three soils (silt, structured clay loam—clay A and levelled silty clay loam—clay B), which are representative of two erosion prone areas in southeastern Norway. A second purpose was to compare aggregate stabilities measured by the Norwegian standard procedure (rainfall simulator) and the more widely used wet-sieving procedure. Surface soil was sampled in autumn. Field moist soil was sieved into the fraction 1–4 mm and packed into cylinders. The water content of the soil was adjusted, corresponding to matric potentials of − 0.75, − 2 and − 10 kPa. The soil cores were insulated and covered, and subjected to 0, 1, 3 or 6 freeze–thaw cycles: freezing at − 15 °C for 24 h and thawing at 9 °C for 48 h. Aggregate stability was measured in a rainfall simulator (all soils) and a wet-sieving apparatus (silt and clay B). The rainfall stability of silt was found to be significantly lower than of clay A and clay B. Clay A and clay B had similar rainfall stabilities, even though it was expected that the artificially levelled clay B would have lower stability. Freezing and thawing decreased the rainfall stability of all soils, but the effect was more severe on the silt soil. There was no evident effect of water content on the stability, probably due to experimental limitations. The same effects were observed for wet-sieved soil, but the wet-sieving resulted in less aggregate breakdown than the rainfall simulator. Rainfall impact seemed to be more detrimental than wet-sieving on more unstable soil, that is, on silt soil and soil subjected to many freeze–thaw cycles. Such conditions are expected to occur frequently during field conditions in unstable winters.     

Changes in some soil properties in a Vertic Argiudoll under short-term conservation tillage

The purpose of this work was to determine whether some soil physical and chemical properties, and microbial activity were affected by two conservation tillage systems in a Chernozemic clay loam soil (Vertic Argiudoll), after 5 years of trial initiation. Two crop sequences, corn (Zea mays L.)–wheat (Triticum aestivum L.)/soybean (Glycine max (L.) Merr.) and wheat/soybean, under chisel plowing (ChP) and no till (NT) were evaluated. Physical and chemical properties were also analyzed taking the same soil without disturbance as reference. The Hénin instability index (HI) was larger in ChP than in NT in both corn–wheat/soybean (C–W/S) and wheat/soybean (W/S) sequences (P≤0.05). The C–W/S sequence differed from W/S (P≤0.01) in total organic carbon (TOC). As regards organic carbon fractions, no differences were found in labile organic carbon (LOC), while W/S under ChP showed the lowest value (P≤0.01) of humified organic carbon (HOC). No differences were found in microbial respiration either in crop sequences or in tillage systems. Soil physical and chemical properties differentiated crop sequences and tillage treatments from the undisturbed soil when a Student’s t-test was performed. Five years elapsed since the beginning of this trial was time enough to detect changes in some of the soil properties as a consequence of management practices. An important reduction in the soil structural stability was observed as related to the undisturbed soil. However, the C–W/S sequence under NT resulted in lower soil degradation with respect to the other treatments.     

Organic matter addition, N, and residue burning effects on infiltration, biological, and physical properties of an intensively tilled silt-loam soil

Seventy years of different management treatments have produced significant differences in runoff, erosion, and ponded infiltration rate in a winter wheat (Triticum aestivum L.)–summer fallow experiment in OR, USA. We tested the hypothesis that differences in infiltration are due to changes in soil structure related to treatment-induced biological changes. All plots received the same tillage (plow and summer rod-weeding). Manure (containing 111 kg N ha⁻¹), pea (Pisum sativum L.), vine (containing 34 kg N ha⁻¹), or N additions of 0, 45 and 90 kg ha⁻¹ were treatment variables with burning of residue as an additional factor within N-treatments. We measured soil organic C and N, water stability of whole soil, water stable aggregates, percolation through soil columns, glomalin, soil-aggregating basidiomycetes, earthworm populations, and dry sieve aggregate fractions. Infiltration was correlated (r = 0.67–0.95) to C, N, stability of whole soil, percolation, and glomalin. Basidiomycete extracellular carbohydrate assay values and earthworm populations did not follow soil C concentration, but appeared to be more sensitive to residue burning and to the addition of pea vine residue and manure. Dry sieve fractions were not well correlated to the other variables. Burning reduced (p < 0.05) water stability of whole soil, total glomalin, basidiomycetes, and earthworm counts. It also reduced dry aggregates of 0.5–2.0 mm size, but neither burning nor N fertilizer affected total C or total N or ponded infiltration rate. Water stability of whole soil and of 1–2-mm aggregates was greater at 45 kg N ha⁻¹ than in the 0 and 90 kg N ha⁻¹ treatments. Zero N fertilizer produced significantly greater 0.5–2.0 mm dry aggregate fractions. We conclude that differences in infiltration measured in the field are related to relatively small differences in aggregate stability, but not closely related to N or residue burning treatments. The lack of an effect of N fertilizer or residue burning on total C and N, along with the excellent correlation between glomalin and total C (r = 0.99) and total N (r = 0.98), indicates that the major pool of soil carbon may be dependent on arbuscular mycorrhizal fungi.     

Measuring and modelling the actual energy involved in aggregate breakdown

This work investigates the distribution of soil aggregates for Vertisols and Ferrasols as a function of the actual energy involved in dispersion, known as the dispersive energy. For Vertisols showing an aggregate hierarchy, the breakdown of aggregates 2–50 μm is modelled using the aggregate liberation and dispersion characteristic curve indicating a stepwise breakdown of soil aggregates. Meanwhile, for Ferrasols, the breakdown of 2–50 μm aggregates increases monotonically with increasing dispersive energy, which is indicative of the direct release of silt and clay from the disruption of aggregates. For soils displaying an aggregate hierarchy, the relative rate constant of aggregate liberation is much larger than the relative rate constant of the aggregate dispersion. Furthermore, the redistribution of aggregates for a number of size ranges within the 2–50 μm fraction illustrates a number of different pathways in aggregate breakdown, and the assumption that aggregates follow an exponential decay may not detect the presence of a weakly expressed aggregate hierarchy. The exponential decrease in ultrasonic power over time for the Ferrasols, as opposed to the prominent drop or steps in the curves for the Vertisols, confirms the lack of a prominent aggregate hierarchy.     

Particle size fractionation of fungal and bacterial biomass in subalpine grassland and forest soils

Characterization of soil aggregates according to particle size fractions is a useful tool in process-oriented research into soil organic matter and biological properties. Substrate-induced respiration (SIR) inhibition was used to quantify microbial, fungal and bacterial biomass in particle size fractions of soils ranging from forest to grassland in a subalpine region of central Taiwan. In addition, ergosterol content was determined in the same samples to verify fungal biomass measured by SIR inhibition technique. Surface soil (0–10 cm) was fractionated into four particle size fractions: coarse sand (250–2000 μm), fine sand (53–250 μm), silt (2–53 μm) and clay (0.2–2 μm). The larger sized fractions (>250 μm and 53–250 μm) contained higher levels of fungal ergosterol than the smaller sized ones (2–53 μm and 0.2–2 μm). The largest particle size fraction (250–2000 μm) from all studied habitats showed the highest level of microbial biomass, with no clear trend in microbial biomass level among the other size fractions. SIR-calculated fungal biomass level and ergosterol converted fungal biomass content were positively correlated (r=0.71, p<0.05), and such correlation decreased as biomass levels were high. Ratios of fungi to bacteria ranged between 0.6 and 1.3 in fractions obtained in this study. This study indicates a high variability of microbial (fungal and bacterial) biomass level among particle size fractions in soil, and that the large-sized fractions tend to contain a high level of microbial biomass in a given ecosystem.     

Carbohydrate composition in relation to structural stability, compactibility and plasticity of two soils in a long-term experiment

The effects of tillage on soil organic carbon content, carbohydrate content, monosaccharide composition, aggregate stability, compactibility and plasticity were investigated in a field experiment on a gleysol and on a cambisol under winter barley in South-East Scotland. Two long-term treatments (direct drilling and conventional mouldboard ploughing for 22 years) were compared with short-term direct drilling and broadcast sowing plus rotavation for 5 years. Carbohydrate released sequentially to cold water, hot water, 1.0 M HCl and 0.5 M NaOH was determined after hydrolysis as reducing sugar equivalent to glucose in both fresh and air-dried samples. All other measurements were made on dry soils only. About 3% of the soluble carbohydrate was extracted by cold water, 10% by hot water, 12% by HCl and 75% by NaOH from both the dry and fresh soils. The total reducing sugars of the fractions were proportional to the total organic carbon determined by dichromate oxidation or C analysis. Organic carbon and carbohydrates were concentrated near the surface of the direct drilled soil, but were more uniformly distributed with depth in the ploughed soil. The surface soil under direct drilling was more stable, less compactible and had greater plasticity limits than under ploughing. However, particle size distributions were unaffected by tillage so that differences in soil properties were attributed to differences in the quantity and quality of organic matter. Differences in compactibility, structural stability and plasticity limits between depths and tillage treatments correlated with total carbon and with total carbohydrates. The hot water extractable carbohydrate fraction correlated best with aggregate stability and the NaOH fraction correlated best with compactibility and plastic limit. Both fractions were greatest in the long-term direct drilled soil. The hot water fraction had a galactose plus mannose over arabinose plus xylose ratio of 1.0–1.6 in comparison to 0.4–0.7 in the NaOH fraction indicating that the microbial contribution within the hot water-soluble fraction was the greater. The hot-water fraction was likely to contain more exocellular microbial polysaccharides involved in the stabilizing of soil aggregates. The hot-water and NaOH carbohydrate fractions may be good indicators of soil organic matter quality relevant to the preservation of good soil physical conditions.     

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.     

Effects of agricultural management systems on soil organic carbon in aggregates of Ustolls and Usterts

Soil erosion contributes to the removal and redistribution of soil organic C from cultivated fields. The soil organic C content of wind erodible and water unstable aggregates is an important factor in determining the amount of carbon loss occurring in erosion processes. The relative distribution of organic carbon among aggregate size fractions may also affect the response of soils to erosion. Soil organic C distribution is dependent on the chosen management system. The effects of no-till, till, and grassland management systems on organic C content of erodible and non-erodible aggregates were examined in six Ustolls and two Usterts of central South Dakota. Organic C contents were related to dry- and wet-sieving to represent the potential influence of wind and water erosion on C loss in the absence of vegetative cover. Loss of aggregate stability in cultivated soils was associated with organic C loss. Most structural characteristics developed under tilled systems persisted after 6–16 years of no-till. Changes in distribution of organic C due to management systems were most evident in Ustolls where cultivation resulted in net soil C losses. Soil organic C was not significantly increased by the no-tillage practices applied in this on-farm study (in Ustolls 49 Mg ha⁻¹ in no-till versus 41 Mg ha⁻¹ in till, for 0–0.20 m depth). Soil properties of Usterts were less affected by land use and management practices due to the high shrink swell action and self-mixing. In both soil orders the greater concentration of organic C in the wind erodible (<1 mm) dry aggregate size fraction implies a high potential for organic C loss by erosion in addition to organic C loss from mineralization after tillage. Grassland when compared to cultivated topsoil showed the largest amounts of organic carbon stored and the minimal potential for erosion loss of soil organic C.     

严重退化红壤植被恢复后有机质富集和团聚体稳定性

Three types of soils： an eroded barren soil under continuous fallow, an eroded soil transplanted with Lespedeza shrubs （Lespedeza bieolor）, and an eroded soil transplanted with camphor tree （Cinnaraomum camphora） were investigated to quantify organic matter pools and aggregates in reforested soils using physical fractionation techniques and to determine aggregate stability in relation to the enrichment of soil organic carbon （SOC）. Soil organic matter （SOM） was physically fractionalized into free particulate organic matter （fPOM）, occluded particulate organic matter （oPOM）, and mineralassociated organic matter （mOM）. The SOM was concentrated on the surface soil （0 5 cm）, with an average C sequestration rate of 20-25 g C m^-2 year^-1 over 14 years. As compared to the eroded barren land, organic C content of fPOM, oPOM, and mOM fractions of the soil under Lespedeza and under camphor tree increased 12-15, 45-54, and 3.1-3.5 times, respectively. A linear relationship was found between aggregate stability and organic C （r^2 = 0.45, P 〈 0.01）, oPOM （r^2 = 0.34, P 〈 0.05）, and roOM （r^2 = 0.46, P 〈 0.01） of aggregates. The enrichment of organic C improved aggregate stability of the soil under Lespedeza but not that under camphor tree. However, further research is needed on the physical and biological processes involved in the interaction of soil aggregation and SOC sequestration in ecosystem.     

Distribution of nutrient elements within water-stable aggregates of two tropical agro-ecological soils under different land uses

This study evaluated carbon and nutrient distributions within water-stable aggregates (WSA) of soils of two contrasting ecosystems under different land uses. Surface soil samples were collected from uncultivated and cultivated land in rainforest and savannah agro-ecological areas and separated by wet-sieving technique into 4.76–2.0, 2.0–1.0, 1.0–0.50, 0.50–0.25 and <0.25 mm aggregate fractions. The results show that irrespective of the agro-ecological area, cultivation significantly (p < 0.05) reduced the macroaggregate fractions (>0.25 mm) to smaller diameters. Distribution of organic carbon (C), total nitrogen (N) and available phosphorus (P) within the WSA showed preferential enrichment of these elements in the large macroaggregate fraction (4.76–2.0 mm) for the uncultivated soils and microaggregate fraction (<0.25 mm) for the cultivated soils. The overall pattern indicates higher accumulation of C, N and P in the WSA of the uncultivated soils over the cultivated soils. Average distribution of total exchangeable bases (TEB), i.e., sum of Ca²⁺, Mg²⁺, K⁺ and Na⁺, within WSA of the uncultivated soils of the rainforest region were 7.35 and 7.39 cmol/kg for 4.76–2.0 and <0.25 mm fractions, respectively. The distributions of TEB for cultivated soils of the rainforest region were 2.76 cmol/kg (4.76–2.0 mm fraction) and 7.73 cmol/kg for <0.25 mm fraction. This showed that cultivation significantly (p < 0.05) led to 62% reduction in these nutrients in the 4.76–2.0 mm fraction and 5% increase in concentrations of these cations in <0.25 mm fraction. For savannah soils, distributions of TEB were 7.44 and 6.77 cmol/kg for 4.76–2.0 and <0.25 mm fractions, respectively, in uncultivated sites, whereas TEB were 2.19 cmol/kg (4.76–2.0 mm) and 6.35 cmol/kg (<0.25 mm) for cultivated savannah. This indicated that cultivation significantly (p < 0.05) led to 71% and 6% reductions in Ca²⁺, Mg²⁺, K⁺ and Na⁺ concentrations within the 4.76–2.0 and <0.25 mm aggregate fractions, respectively. However, there were 18% and 50% increase in these elements in the 2.0–1.0 and 1.0–0.50 mm fractions of the cultivated soils of the savannah region, respectively. The general trend showed that in uncultivated soils, the 4.76–2.0 and <0.25 mm fractions were preferentially enriched with Ca²⁺, Mg²⁺, K⁺ and Na ⁺; whereas, cultivation led to redistribution of these elements into the smaller aggregates. Since smaller aggregates are preferentially removed by erosion, this study underscores the need for sustainable soil management practices that would minimize nutrient loss when forest or fallow lands are converted to cropland.     

灰质白云岩土壤有机碳的团聚体保护

采集喀斯特地区灰质白云岩发育的乔木林下土壤,全部湿筛分为>5mm,5～2mm,2～1mm,1～0.5mm,0.5～0.25mm共5个粒级团聚体,再将5个粒级团聚体进行碳水化合物提取后后再次分别湿筛,收集>5mm,5～2mm,2～1mm,1～0.5mm,0.5～0.25mm共5个粒级的团聚体样品.对两次湿筛中5个粒级的土壤分别进行团聚体含量、土壤有机碳、土壤可氧化态有机碳测定,分析土壤团聚体稳定性与土壤有机碳、土壤可氧化态有机碳的关系.结果表明:灰质白云岩乔木林下土壤在经过提取碳水化合物的第二次湿筛后,大粒级团聚体(>5mm,5～2mm)向小粒级(2～1mm,1～0.5mm,0.5～0.25mm)转移;有机碳主要存在于较大粒级团聚体中,但各粒级团聚体有机碳并不随之转移;各粒径团聚体可氧化态碳含量均减少,但较大粒级(>5mm,5～2mm)可氧化态有机碳含量多,较小粒级(2～1mm,1～0.5mm,0.5～0.25mm)可氧化态有机碳含量少,故推测较大粒级团聚体(>2mm)保护土壤活性有机碳能力比较小粒级团聚体(<2mm)强.     

Long-term farmyard manure application effects on properties of a silty clay loam soil under irrigated wheat–soybean rotation

Increasing importance has been placed on the use of agricultural soils for the mitigation of atmospheric CO₂ through sequestration of soil C. Although crop productivity is sustained mainly through the application of organic manure in the Indian Himalayas, little information is available on C sequestration, C content in different aggregate size fractions and soil water transmission properties (infiltration and saturated hydraulic conductivity) as affected by long-term manure addition. We analyzed results of an 8-year experiment, initiated in 1995–1996 on a silty clay loam soil, to determine the influence of fertilizer and fertilizer + farmyard manure (FYM) application on those important soil properties. The overall increase in soil organic C (SOC) content in the 0–45 cm soil depth in NPK + FYM treatment as compared to NPK and control treatments was 11.0 and 13.9 Mg C ha⁻¹ at the end of 8 years, respectively. Application of FYM significantly reduced soil bulk density and increased mean weight diameter (MWD) and SOC contents in different aggregate size fractions. Soil organic C content in macroaggregates was greater than in microaggregates. The response of SOC content to FYM application was dependent upon inorganic fertilization and more upon balanced application of NPK than N only. Steady state infiltration rate under NPK + FYM (1.98 cm h⁻¹) was higher than under unfertilized (0.72 cm h⁻¹) and NPK (1.2 cm h⁻¹). Soil water sorptivity (calculated from Philip's equation) under NPK + FYM (1.06 cm min^−0.5) was higher than under NPK (0.61 cm min^−0.5). We conclude that hill farmers in northern India should be encouraged to use FYM along with chemical fertilizers to increase SOC content and improve soil physical properties.     

Effect of aggregate size and soil water content on emergence of soybean (Glycine max, L. Merr.) and maize (Zea mays, L.)

Rapid wetting of irrigated soils often leads to slaking and slumping, and on drying a surface crust and hard-set conditions may occur. This results in reduced crop emergence unless the surface is kept moist. The effect of aggregate size and water content on the emergence of soybean and maize from an Entic chromustert (heavy cracking clay) was determined using pots of sieved aggregates with size ranges less than 1, 1–2, 2–5 and 5–15 mm at soil water contents of 15, 20 and 25 g (100 g)⁻¹. Unsieved soil was used as a control. Greatest emergence tended to occur from fine (1–2 mm) seedbeds compared with coarse (5–15 mm) seedbeds for both crops. A covered treatment, simulating a stubble mulch, resulted in greater emergence than an uncovered treatment for all water contents and aggregate sizes. Earlier emergence occurred from finer (less than 1 mm and 1–2 mm) seedbeds than from coarse (5–15 mm) seedbeds, and at the greatest water content used. Soil strength, measured with a shear vane, decreased with increasing water content and tended to be less on fine (1–2 mm) seedbeds compared with very fine (less than 1 mm) or coarse (5–15 mm) seedbeds. It is recommended that, for good emergence from this Entic chromustert, seedbeds be brought to a water content of 25 g (100 g)⁻¹ by capillary wetting to prevent hardsetting and consist of 1–2 or 2–5 mm aggregates for soybean and maize, respectively, and have a stubble mulch on the surface. This corresponds to an equivalent depth of water of 15 mm and 9 mm for soybean and maize, respectively, in the top 50 mm of the profile.     

Short-term compost effect on macroaggregation in a sandy soil under low rainfall in the valley of Mexico

Several studies have shown the importance of organic material in the formation and stability of soil aggregates. The organic matter of soil (SOM) is affected among other factors by the application of farmyard waste and compost, as well as tillage and crop rotation. This paper examines the aggregation and stability of a sandy soil (Haplic Fluvisol) in the valley of Mexico when treated with either 40 Mg ha⁻¹ of compost or urea (80 kg ha⁻¹ of N) and sown to amaranth (Amaranthus hypochondriacus L.) under dryland conditions. The application of compost resulted in a significantly larger proportion of aggregates in the fractions >1 mm (1.0–2.0, 2.0–2.3, 2.3–4.7 mm) than in the smaller fraction (<1 mm). However the stability of the macroaggregates >1 mm in the compost treatment was not higher than in contrasting treatments which did not include organic matter. Compost, which was applied under drought conditions, did not increase the aggregate stability of the soil probably because of the restricted transformation of the compost and microorganism activity.     

The effect of aggregate size on water retention and pore structure of two silt loam soils of different genesis

Aggregate size distribution and pore structure affect many soil functions and root growth. The aggregate structure is associated with soil genesis and management practices applied. In this study the effects of various size ranges of aggregates (<0.25, 0.25–0.5, 0.5–1, 1–3, 3–5, and 5–10 mm) and undisturbed soil from the plough layer (0–15 cm) of two types of soils (Haplic Phaeozem and Eutric Fluvisol) of the same silty loam textural group on water retention curves (WRC) and pore size distribution (PSD) were assessed. A greater bulk density and lower humus content characterized the Eutric Fluvisol as compared to the Haplic Phaeozem. The WRC was determined using standard Richards chambers in drying process and expressed as the degree of saturation. Equivalent PSD was derived from the WRC. Resin impregnated sections from the layer of 0–8 cm showed that the Eutric Fluvisol, compared with the Haplic Phaeozem, had coarser pores and aggregates. The degree of saturation in beds of aggregates <0.25, 0.25–0.5 and 0.5–1 mm compared to beds of aggregates 1–3, 3–5 and 5–10 mm was greater at higher values of pressure head for both soils, and for undisturbed soil it was greater for the Haplic Phaeozem than for the Eutric Fluvisol at lower values of pressure head. The inverse relationship was true at higher values of pressure head. The derivative curves of PSD showed that the beds of aggregates and undisturbed soils exhibited multi-peak PSD. The pore radius peaks within the textural (primary) pore system were more defined in beds of aggregates <0.25 mm than in beds of coarser aggregates, whereas in the case of the structural and macropore peaks it was often the reverse. Greater magnitude and narrower shape of the peaks in the undisturbed Haplic Phaeozem compared to the Eutric Fluvisol indicated a more heterogeneous nature of the pore system in the former. The PSD data are discussed in relation to aggregate size distribution and stability of the soil aggregates. The results of this study can be helpful in predicting storage and transmission functions of surface aggregated soils.     

Structural stability and carbohydrate contents of an ultisol under different management systems

An understanding of the dynamics of soil carbohydrate pools is necessary for assessing the impact of organic residue management in organic matter build up and structural stability in tropical ecosystems. The objectives of this study were to evaluate temporal changes in aggregate stability and cold water-soluble, hot water-soluble and acid-soluble carbohydrate fractions of a sandy soil under different organic residue management practices. The soil is an Nkpologu sandy clay loam (fine-loamy, kaolinitic, isohyperthermic, typic kandiustult) at Nsukka in southeastern Nigeria. In July 1995, it was incorporated with complete fertiliser (N:P:K = 12:12:17 at 480 kg/ha) (F); rice mill wastes (RW, 10 t/ha); RW+F; poultry manure (PM, 10 t/ha) and RW (5 t/ha)+PM (5 t/ha) up to the 0–20 cm depth. A control, tilled up to the 0–20 cm depth, was also included. Surface soil samples (0–20 cm), collected at 3, 6 and 12 months after residue applications were used to measure changes in aggregate stability by mean weight diameter (MWD), total OC and carbohydrate pools.

In all treatments MWD increased whereas the concentrations of acid-soluble, hot water-soluble and cold water-soluble carbohydrates decreased with sampling time. Also irrespective of the type of amendment, the carbohydrate concentrations at each sampling period varied in the order, acid-soluble>hotwater-soluble>coldwater-soluble. Aggregate stability correlated very poorly with all the carbohydrate fractions and OC. The correlation coefficient values were rather low and did not mean much in the physical interpretation of these results. This shows that these carbohydrate pools were not very effective in stabilizing the soil aggregates.     

Impact of sugarcane cultivation on soil carbon fractions, consistence limits and aggregate stability of a Yellow Latosol in Northeast Brazil

The effects of continuous sugarcane (Saccharum officinarum) cropping on the properties of a cohesive Yellow Latosol were studied in the region of the Coastal Tablelands, Northeast Brazil. Four areas were studied at Caeté mill, municipality of São Miguel dos Campos, Alagoas State, involving a native forest (Tn), and sugarcane fields cultivated for periods of 2 years (T2), 18 years (T18) and 25 years (T25). Samples were collected from each area at 0–0.2 and 0.2–0.4 m depth, to determine total organic C, physical fractionation of soil organic matter and consistence limits. Undisturbed samples were collected to determine wet aggregate mean weight–diameter, dry mean weight diameter and aggregate stability. In relation to the soil under native forest, total organic C and particulate organic matter contents were reduced after 2 years of cultivation. Sugarcane cropping for a longer period promoted a recuperation of soil organic matter content. The decrease of total organic C and reduction in aggregate stability and plastic limit after 2 years of sugarcane cultivation rendered the soil more susceptible to compaction.     

Long-term fertilization, manure and liming effects on soil organic matter and crop yields

Yield decline or stagnation and its relationship with soil organic matter fractions in soybean (Glycine max L.)–wheat (Triticum aestivum L.) cropping system under long-term fertilizer use are not well understood. To understand this phenomenon, soil organic matter fractions and soil aggregate size distribution were studied in an Alfisol (Typic Haplustalf) at a long-term experiment at Birsa Agricultural University, Ranchi, India. For 30 years, the following fertilizer treatments were compared with undisturbed fallow plots (without crop and fertilizer management): unfertilized (control), 100% recommended rate of N, NP, NPK, NPK+ farmyard manure (FYM) and NPK + lime. Yield declined with time for soybean in control (30 kg ha⁻¹ yr⁻¹) and NP (21 kg ha⁻¹ yr⁻¹) treatments and for wheat in control (46 kg ha⁻¹ yr⁻¹) and N (25 kg ha⁻¹ yr⁻¹) treatments. However, yield increased with time for NPK + FYM and NPK + lime treatments in wheat. At a depth of 0–15 cm, small macroaggregates (0.25–2 mm) dominated soil (43–61%) followed by microaggregates (0.053–0.25 mm) with 13–28%. Soil microbial biomass carbon (SMBC), nitrogen (SMBN) and acid hydrolysable carbohydrates (HCH) were greater in NPK + FYM and NPK + lime as compared to other treatments. With three decades of cultivation, C and N mineralization were greater in microaggregates than in small macroaggregates and relatively resistant mineral associated organic matter (silt + clay fraction). Particulate organic carbon (POC) and nitrogen (PON) decreased significantly in control, N and NP application over fallow. Results suggest that continuous use of NPK + FYM or NPK + lime would sustain yield in a soybean–wheat system without deteriorating soil quality.     

Cattle manure effects on structural stability and water retention capacity of a granitic sandy soil in Zimbabwe

The evaluation of soil aggregate stability and water retention is important in the assessment of soil management options. A 3-year study was conducted in 1999 to determine the effects of two cattle manure application methods on soil aggregate stability and water retention capacity of a sandy soil (Haplic Lixisol). Manure application increased soil organic C by 10–38% in the 0–10 cm layer. Compared with the control, manure management treatments increased the aggregate stability of soil as measured by the mean weight diameter (MWD) and aggregates between 2 and 10 mm (AGG2) indices from 0.243 to 0.733–0.926 mm, and from 27.3 to 128.3–148.3 g kg⁻¹, respectively. The readily available water (RAW) capacity of the soil was significantly increased by manure addition, whereas the increase in AWC was not significant. The increase in water retention capacity in the soil was more affected at low suctions and this was related to the effects of manure on macroporosity. It was concluded that cattle manure was beneficial to the structural stability and water retention of this soil.     

Tillage impacts on aggregate stability and crop productivity in a clay-loam soil in central Iran

To take advantage of conservation tillage systems (including direct drilling and non-inversion) in central Iran, it is important to study the effects of different cultivation practices on soil structural stability as a physical indicator. A four-year study was conducted to investigate the effects of seven tillage systems on aggregate properties of a clay-loam soil (Calcic Cambisol) with continuous wheat (Triticum aestivum L.) production. Crop productivity was also evaluated. Tillage treatments were moldboard plowing+disking (MD) as conventional tillage; chisel plowing +disking (CD); chisel plowing+rotary tilling (CR); chisel plowing (twice)+disking (2CD); plowing with Khishchi (a regional rigid cultivator)+disking (KD) as non-inversion methods; and till-planting with cultivator combined drill (TP); and no-till (NT) as direct drilling methods. A randomized complete block design consisting of four replications was used. Samples were taken from three different soil depths. A wet sieving method was used to determine aggregate size distribution (ASD), and mean weight diameter (MWD) as indices of soil aggregate stability. Soil organic carbon was also determined. For the first three years of the experiment, ASD and MWD at 0–15 cm were similar in different tillage treatments, except for direct drilling which had a significantly higher amount of aggregate greater than 2 mm and 2–1 mm diameter compared to the conventional method. At the second and third sampling depths all treatments had similar influence on ASD and MWD. Tillage treatments showed a significant effect on ASD and MWD in the fourth year of the experiment in all three depths. Almost 70% of the aggregates in the MD system were less than 0.25 mm, while only 55% of the aggregates in the direct drilling methods were less than 0.25 mm diameter. The four-year yield average for conventional and non-inversion tillage systems was 7264 and 6815 kg ha⁻¹, respectively. Although, direct drilling improved soil structural stability, its lower yield (5608 and 4731 kg ha⁻¹ for TP and NT, respectively) potential would indicate that reduced tillage systems (i.e. CD) appear to be the accepted alternative management compared to conventional practice (MD).