生物炭对日光大棚土壤团聚体结构的影响

[目的]探明果木生物炭对杨凌地区日光大棚土壤团聚体结构的影响,提出适宜该地区的生物炭添加量,为改良日光温室大棚土壤结构,提升作物产量提供科学依据。[方法]设置10,30,50,70,90 t/hm~2共5个生物炭添加量处理,以未添加生物炭处理为对照,采用干筛法和湿筛法,对比分析不同处理的团聚体几何平均直径、平均重量直径、破坏率和分形维数等指标。最后通过分析不同生物炭添加量下的作物产量,综合考虑土壤团聚体指标,提出最优的生物炭添加量。[结果]添加生物炭后,土壤机械稳定性大团聚体含量增加0.6～4.6 mg/kg,机械稳定性微团聚体含量降低4.0%～32.6%;添加生物炭后,粒径为3～2 mm,2～1 mm,1～0.5 mm水稳性大团聚体含量分别增加25.3%～41.2%,22.7%～74.2%,9.1%～46.4%,粒径为0.5～0.25 mm水稳性大团聚体含量降低2.1%～18.1%。生物炭能够促进小粒径微团聚体的形成,但对微团聚体稳定性和总含量没有显著影响。添加生物炭后菠菜鲜重显著提升(68.7%～214.9%)。[结论]生物炭能够改良土壤团聚体结构。综合考虑团聚体、作物产量因素,在日光大棚添加70 t/hm~2生物炭效果最好。     

氮添加对亚高山针叶林土壤结构及水分入渗性能的影响

为探究大气氮沉降增加导致的土壤团聚体结构变化规律及其对土壤水分入渗性能的影响,在贡嘎山针叶成熟林和中龄林内分别设计2种氮肥形态[(NH₄)₂SO₄、KNO₃]和4个浓度水平(0,10,20,40 kg/(hm²·a)N)的添加试验,研究不同形态大气氮沉降对亚高山森林土壤团聚体结构和土壤水分入渗特征的影响。结果表明：(1)随施氮量增加,中龄林土壤大团聚体含量逐渐增加,土壤持水量、孔隙度和水分稳定入渗率逐渐增大;成熟林土壤大团聚体含量、土壤持水量、孔隙度和水分稳定入渗率均表现出先增加后降低的趋势;(2)不同形态氮添加对土壤团聚体结构、孔隙度及水分稳定入渗率的作用无显著差异;(3)土壤持水能力、孔隙度和团聚体结构是土壤入渗性能的主要影响因子,外源氮添加增加中龄林土壤持水量和孔隙度,改善中龄林土壤团聚体结构稳定性和水分入渗性能。     

Disruption of soil aggregates by varied amounts of ultrasonic energy in fractionation of organic matter of a clay Latosol: carbon,nitrogen and δ13C distribution in particle‐size fractions

Ultrasonic energy has been widely used to disrupt soil aggregates before fractionating soil physically when studying soil organic matter (SOM). Nevertheless, there is no consensus about the optimum energy desirable to disrupt the soil. We therefore aimed (i) to quantify the effect of varied ultrasonic energies on the recovery of each particle‐size fraction and their C, N and δ¹³C distribution, and (ii) to determine an ideal energy to fractionate SOM of a specific soil. Our results show that the 2000–100 μm particle‐size fraction was composed mainly of unstable aggregates and the 100–2 μm fraction of stable aggregates. Energies of 260–275 J ml^?1 were sufficient to disrupt most of the unstable aggregates and leave stable aggregates. The use of this threshold energy combined with particle‐size fractionation was not satisfactory for all purposes, since litter‐like material and relatively recalcitrant organic carbon present in stable aggregates > 100 μm were recovered in the same pool. An ultrasonic energy of 825 J ml^?1 was not sufficient to stabilize the redistribution of soil mass and organic matter among particle‐size fractions, but at energies exceeding 260–275 J ml^?1 relatively stable aggregates would fall apart and cause a mixture of carbon with varied nature in the clay fraction.     

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.     

长期施用化肥条件下土团聚体中有机碳与养分分布

[目的]探究长期施肥与小麦—玉米轮作下(蝼)土表层水稳性团聚体组成及对团聚体中有机碳和养分分布的影响,为评价长期施肥对改善(蝼)土肥力状况的影响提供科学依据.[方法]利用田间长期施用化肥与轮作定位试验,通过湿筛法分离土壤水稳性团聚体,得到土壤团聚体构成,并测定不同粒径团聚体中有机碳和养分的含量.[结果]结果表明长期施用化肥显著影响土壤水稳性团聚体含量,长期施肥降低了＞2 mm的水稳性团聚体数量,增加了＜1 mm的水稳性团聚体含量.施肥在一定程度上提高了水稳性团聚体有机碳的含量,但施用高量氮、磷下0.25～0.5 mm和＜0.25 mm水稳性团聚体中有机碳含量明显低于施用低量氮、磷肥料.不同施肥处理土壤水稳性团聚体全氮含量变化趋势与水稳性团聚体有机碳含量基本一致.磷素在不施肥处理各粒径团聚体中均匀分布,低氮、磷处理各粒径团聚体中全磷含量差异较小,高氮、磷处理各粒径土壤团聚体中全磷含量变化无明显趋势.长期施肥降低了＞2 mm水稳性团聚体中全K含量,增加了＜2 mm的水稳性团聚体中全K含量.[结论]水稳性团聚体关系土壤有机碳、氮的数量,水稳性团聚体及其中有机碳、氮含量在氮磷化肥的长期施用下变化无明显规律,并且,长期施用氮、磷化肥下土壤磷素和钾素在土壤中的保存及供应能力也受到影响.因此,需要合理施肥管理,促进农田生产力的可持续发展.     

喀斯特坡地浅层岩溶裂隙土壤团聚体稳定性与养分垂向变化特征

浅层岩溶裂隙（SKF）为植物提供生长空间、水分和养分,是石漠化地区的重要生境类型。以矩形和漏斗形SKF剖面为研究对象,采用干、湿筛分法和Le Bissonnais法,分析了不同土层土壤团聚体稳定性特征和破坏机理,测定了团聚体中土壤有机质（SOM）、碱解氮（AHN）和有效磷（AP）的含量。结果表明：SKF剖面粒径＞0.25 mm的团聚体均超过90%,PAD值范围为0.01%~4.75%。干、湿筛作用下,MWD值变化范围分别为4.63~7.69 mm和1.33~4.24 mm,团聚体分形维数D范围分别为1.57~2.18和1.55~2.15。SKF土壤团聚体的稳定性随剖面深度加深而降低,矩形SKF土壤团聚体的稳定性要强于漏斗形SKF,快速湿润产生的消散作用是造成团聚体破碎的主要机制。团聚体破坏率（PAD）、团聚体分形维数（D）和平均重量直径（MWD）这三类指标均表明,SKF土壤团聚体水稳定性、通透性均较好。SKF剖面30 cm以下土层,团聚体SOM、AHN和AP含量相较0~20 cm土层大幅下降,含量范围分别为13.27±0.94~37.53±3.47 g?kg-1、71.58±3.27~198.54±22.63 mg?kg-1和0.15±0.03~ 0.38±0.10 mg?kg-1,土壤AP十分贫乏。SKF形态会影响SOM含量随土层深度的变化,矩形SKF 30 cm以下土层含量随深度加深而降低,而漏斗形SKF则没有显著性差异。随土层深度加深,矩形和漏斗形SKF剖面AP含量的变化趋势一致,AHN含量的变化趋势则与SKF形态之间没有明显关联。SOM、AHN和AP含量越高,SKF剖面团聚体水稳定性越强。     

Changes in selected soil physical properties caused by sodicity of soil and irrigation water

Abstract

Sodic water and spring water percolated through clay, clay loam, and sandy loam (SL) soils with exchangeable sodium percentages (ESPs) of 0, 10, 30, and 50. Reduction in saturated hydraulic conductivity and water stable aggregates recorded at higher ESPs. At ESP ≈30, application of sodic and spring water to clay soil (C) reduced saturated hydraulic conductivity from 1.2 to 3 mm hr^?1, whereas in SL soil, the values were 2.8 and 6.2 mm hr^?1, respectively. Results indicated that at any ESP and water source, the highest free swelling obtained was in the C soil. This study has practical importance to the management of irrigation water quality with respect to soil deterioration.     

Eine Methode zur Ermittelung der wasserspannungsabhängigen Änderung des Sauerstoffpartialdruckes und der Sauerstoffdiffusion in einzelnen Bodenaggregaten

A method to determine oxygen partial pressure and oxygen diffusion in single soil aggregates as a function of soil moisture tension Anaerobic zones occur even in unsaturated soils of silty or clayey texture, that are aerated sufficiently in their macropore system. These zones can be related to the inner parts of soil aggregates. To describe the oxygen balances in soils it is necessary to measure not only in soil profiles but as well in single soil aggregates within a range of soil matrix potentials. Therefore oxygen partial pressure in single soil aggregates of different texture was measured continuously as a function of soil matrix potential. For that purpose we developed an oxygen sensitive microelectrode with a tip diameter of 0.5 mm, that is sturdy enough to measure even in sandy soils. One microtensiometer (diameter of the tip < 0.5 mm) and one oxygen microelectrode were placed in water saturated soil aggregates. Soil water potential and oxygen partial pressure were measured continuously during soil drying. The results show an aeration of primarily anoxic soil aggregates at different soil matrix potentials due to different texture and structure. The clayey polyhedral aggregates of the Vertisol were aerated at significantly lower soil matrix potentials than the loamy prisms of the Fluvisol. These show higher values of oxygen partial pressure even at soil water potentials less than 150 hPa. In the aggregates of the Vertisol, that have a fine texture, values of rel. aparent diffusion D_s/D_o were in the range of 1 · 10^?3 at soil water potentials < ?     

Aggregate associated carbon, nitrogen and sulfur and their ratios in long-term fertilized soils

Farmyard manure (FYM) and fertilizer applications are important management practices used to improve nutrient status and organic matter in soils and thus to increase crop productivity and carbon (C) sequestration. However, the long-term effects of fertilization on C, nitrogen (N) and sulfur (S) associated with aggregates, especially on S are not fully understood. We investigated the effects of more than 80 years of FYM (medium level of 40 Mg ka⁻¹ and high level of 60 Mg ka⁻¹) and mineral fertilizer (NPKS and NK) on the concentrations and pools of C, N, and S and on their ratios in bulk soil, dry aggregates and water stable aggregates on an Aquic Eutrocryepts soil in South-eastern Norway. A high level of FYM and NPKS application increased the proportion of small dry aggregates (<0.6 mm) by 8%, compared with the control (without fertilizer). However, both medium and high level of FYM application increased the proportion of large water stable aggregates (>2 mm) compared with mineral fertilizer (NPKS and NK). The total C and N pools in bulk soils were also increased in FYM treatments but no such increase was seen with mineral fertilizer treatments. The increased total S pool was only found under high level of FYM application. Water stable macroaggregates (>2 and 1–2 mm) and microaggregates (<0.106 mm) contained higher concentrations of C, N and S than the other aggregate sizes, but due to their abundance, medium size water stable aggregates (0.5–1 mm) contained higher total pools of all three elements. High level of FYM application increased the C concentration in water stable aggregates >2, 0.5–1 and <0.106 mm, and increased the S concentration in most aggregates as compared with unfertilized soils. Higher C/N, C/S and N/S ratios were found both in large dry aggregates (>20 and 6–20 mm) and in the smallest aggregates (<0.6 mm) than in other aggregate sizes. In water stable aggregates, the C/N ratio generally increased with decreasing aggregate size. However, macroaggregates (>2 mm) showed higher N/S ratios than microaggregates (<0.106 mm). We can thus conclude, that long-term application of high amounts of FYM resulted in C, N and S accumulation in bulk soil, and C and S accumulation in most aggregates, but that the accumulation pattern was dependent on aggregate size and the element (C, N and S) considered.     

Fate of carbon and nitrogen in water-stable aggregates during decomposition of 13C15N-labelled wheat straw in situ

When incorporated in soil, plant residues and their decomposition products are in close contact with mineral particles with which they can be bound to form aggregates. We measured the incorporation of carbon (C) and nitrogen (N) derived from crop residues in water-stable aggregate fractions of a silty soil in a field experiment in Northern France using ¹³C¹⁵N-labelled wheat straw (Triticum aestivum L.). Soil samples were taken seven times for 18 months and separated into slaking-resistant aggregate size fractions which were analysed for total C and N contents, and ¹³C and ¹⁵N enrichments. During the early stages of decomposition (approximately 200 days), the enrichment of ¹³C increased rapidly in the macro aggregates (> 250 pm) but decreased thereafter. The macro aggregates represented only < 20% of the soil mass and at any one time, they accounted for <25% of the residual ¹³C in the soil. The proportion of ¹³C recovered in the <50-μm and 50–250-μm fractions increased during decomposition of the residues; at day 574, the 50–250-μm fraction accounted for close to 50% of the residual ¹³C. A greater proportion of ¹⁵N than ¹³C was recovered in the <50-μm fraction. The results indicate that during decomposition in soil, C and N from crop residues become rapidly associated with stable aggregates. In this silty soil the 50–250-μm stable aggregates appear to be involved in the storage and stabilization of C from residues.     

Impact of long‐term mineral and organic fertilizer application on the water stability,wettability and porosity of aggregates obtained from two loamy soils

The impact of fertilizer application on soil aggregate stability is of increasing interest to soil scientists. Aggregate water stability depends primarily on soil organic matter. We studied silty loam and loamy sand aggregates from three long‐term fertilizer treatments (control, pig manure and NPK) which significantly altered the quantity of organic matter. A new approach to examining aggregate stability was used: soil aggregates were immersed in methanol‐water solutions with methanol at 0, 20, 40 and 60% concentration (C), and non‐disrupted aggregates were separated after 30 minutes. The aggregate resistance R(C) against each solution was taken as the percentage of stable aggregates. Overall resistance of the aggregates was taken as the R_tot parameter given by the product of four R(C) values. The R(C) values of all aggregates were positively correlated with C. The R(60) values were independent of the applied fertilizer. The R(0) value for silty loam aggregates ranged from 28% (pig manure) to 7% (NPK), while that for loamy sand equalled 7–9% in all cases. The R(20) values were most effective at differentiating the soils and the fertilizer treatments. However, the R_tot value was a better indicator of aggregate stability. Greater differences in stabilities of aggregates were noted in loamy sand. Mineral fertilizer application seemed to decrease aggregate resistance in both soils. Total organic carbon and nitrogen content in all non‐disrupted aggregates were negatively correlated with methanol concentration (C) of the solution applied for aggregate separation. The largest decrease was for the pig manure treatment, and the smallest was for the control. Porosity and pore size distributions of the aggregates were derived from micro‐tomography and approximated to lognormal pore size distributions. Larger porosities and pores were found in water‐stable aggregates than in methanol‐stable aggregates. It seems that the dominant mechanisms for aggregate instability during fast wetting were not related only to the pore air compression, but to weakening of attractive forces between aggregate particles by water.     

苎麻对红壤旱地土壤团聚体及其特性的影响

采用干、湿筛法研究了种植苎麻和花生对红壤旱地土壤团聚体及其特性的影响,并比较分析了土壤团聚体及土壤理化性质与地表径流和土壤侵蚀量的关系。结果表明:(1)与花生地相比,苎麻地有机质、田间持水量、总孔隙度、沙粒分别升高了28.44%,10.06%,5.65%和53.13%,土壤容重、粉粒和黏粒则分别降低了7.20%,14.85%和34.95%,均达显著性差异水平(p0.05)。(2)团聚体平均重量直径(MWD)、稳定性指数(ASI)显著升高(p0.01),苎麻地土壤团聚体稳定性优于花生地;(3)两处理均以0.25~1mm粒径团聚体保存几率最大,抗水蚀能力最强。(4)地表径流量和土壤侵蚀量与土壤有机质、沙粒含量、1mm的干团聚体、0.5mm的水稳性团聚体、MWD以及ASI呈极显著负相关关系(p0.01),而与粉粒、黏粒、0.25mm干团聚体、0.053mm的水稳性团聚体、呈极显著正相关关系(p0.01)。     

Relevance and limitations of biogenic and physicogenic classification: a comparison of approaches for differentiating the origin of soil aggregates

Although freshly formed or unaltered biogenic aggregates are easily recognized, identifying the origin of aggregates altered by physical and biological processes remains empirical and prone to error. The aim of this study was to distinguish between biogenic (BIO) and physicogenic (PHYS) aggregates in various states of fragmentation or size classes using visual, physical and chemical characteristics. Casts produced by Amynthas khami (BIO) and surrounding soil aggregates without visible biological activity (PHYS) were left to disaggregate by natural rainfall events and then separated into five size classes of >10, 10–5, 5–2, 2–0.5 and <0.5 mm. We then analysed aggregate morphology, elemental and stable isotope composition and soil stability, and used near‐infrared spectroscopy (NIRS) to determine their chemical characteristics. Although visual assessment is the method most commonly used in the field to distinguish between BIO and PHYS, our study found that the results obtained were always prone to error and that the classification was arbitrary for BIO and PHYS aggregates smaller than 5 and 2 mm in size, respectively. Soil structural stability was only useful for identifying BIO aggregates larger than 2 mm. While C content and δ¹³C in BIO were always different from PHYS, regardless of soil aggregate size, N content and δ¹⁵N were similar. NIRS was the most effective method because it clearly discriminated soil aggregates on the basis of size and origin. The NIRS characteristics of BIO were also more uniform than those of PHYS, suggesting that BIO aggregates have a simpler organization and as a consequence more homogeneous ecological functions. Thus, our findings suggest that information may be lost when only the physical aspect of aggregates is used to quantify the activity of ecosystem engineers in soil. After fragmentation, BIO aggregates become hidden and although it may be impossible to distinguish them visually from PHYS aggregates they retain some of their specific chemical characteristics.     

Influence of soil tillage systems on aggregate stability and the distribution of C and N in different aggregate fractions

Soil aggregation is influenced by the tillage system used, which in turn affects the amount of C and N in the different aggregate fractions. This study assessed the impact of different tillage systems on soil aggregates by measuring the aggregate stability, the organic carbon (C_org) and the total nitrogen (N_tot) contents within different aggregate fractions, and their release of dissolved organic carbon (DOC). Soil samples were collected from the top 0 to 10 cm of a long-term tillage experiment at Fuchsenbigl (Marchfeld, Austria) where conventional tillage (CT), reduced tillage (RT), and minimum tillage (MT) treatments were applied to a Chernozem fine sandy loam. The stable aggregates (1000–2000 μm) were subject to dispersion by the soil aggregate stability (SAS or wet sieving) method after Kemper and Rosenau (1986), and the ultrasonic method of Mayer et al. (2002). Chemical analysis of the soil was obtained for the aggregate fractions 630–1000, 250–630 and 63–250 μm gathered from the ultrasonic method. Using the SAS method, CT and RT had the least amounts of stable aggregates (18.2% and 18.9%, respectively), whereas MT had twice as much stable aggregates (37.6%). Using the ultrasonic method, MT also had the highest amount of water stable aggregates in all three fractions (1.5%, 3.7%, and 35%, respectively), followed by RT (1%, 2.3%, 32.3%), and CT (0.8%, 1.7%, 29.1%). For comparison, a reference soil, EUROSOIL 7 (ES-7) was also analysed (40%, 6.7%, and 12.1%). The highest amounts of C_org and N_tot were measured under MT in all three fractions, with 8.9%, 3.8%, and 1.3% for C_org, and 0.4%, 0.3%, and 0.1% for N_tot. Apart from the fraction 630–1000 μm, the aggregates of RT and CT contained <50% of the C_org and N_tot values of MT. The C/N ratio was least favourable for CT (42.6) in the aggregate fraction 630–1000 μm. The DOC release from stable aggregates after 10 min of ultrasonic dispersion was highest from MT soil (86.7 mg l⁻¹). The values for RT and CT were 21% and 25% below this value. The results demonstrate that tillage type influences both aggregate stability and aggregate chemical composition. This research confirms that CT interferes more with the natural soil properties than RT and MT. Furthermore, MT has the highest potential to sequester C and N in this agriculturally used soil.     

Simple Modification of the Clod Method for Determining Bulk Density of Very Gravelly Soils

Abstract

The accuracy of the clod method in determining fine‐earth bulk density in very gravelly soils can be affected by the difficulty of fine‐earth separation from both coarse fragments and the coating substance. A modification of the paraffin‐coated clod method is presented that accurately determines fine‐earth bulk density of very gravelly soils. The modification involves washing the paraffin‐coated clod in boiling water to separate paraffin from gravel and hardened soil aggregates. Samples were analyzed with an existing gravel correction method, herein referred to as the hand removal method (HRM), and the proposed modification, which is named the gravel washing method (GWM). Bulk density means for HRM and GWM were 1.29 and 1.41 g cm^?3, respectively. This trend of higher bulk densities for GWM was consistent across pedons sampled in the study and was attributed to its ability to completely remove wax from gravels and hardened soil aggregates, effectively separating the fine‐earth fraction.     

Aggregates stability and water erosion in Andosols of the Canary Islands

The aim of this paper is to assess the mechanisms of water erosion in andic soils using two tests, which in a certain way simulate the two principal mechanisms of aggregate destruction in the process of water erosion—water dispersion and raindrops impact—and compare them with the aggregation observed in material dettached by inter‐rill erosion (sediments) in experimental plots with natural rain. In accordance with the obtained results, the erosive process in these soils seems to come about through a picking off of surface material of larger aggregates, due to the impact of raindrops. The intensity of pull off and fragment size from larger aggregates depends on the kinetic energy of the drops (rain intensity), but the size generally ranges between 0ċ2 and 0ċ5 mm. Therefore inter‐rill erosion initially proceeds by a washing down of smaller aggregates (<0ċ5 mm) (of less bulk density than larger aggregates 0ċ4 Mg m⁻³ against 0ċ9 Mg m⁻³), enriching the soil in larger sized aggregates which, on being fragmented by picking off of raindrops, supply new material for washing down by inter‐rill erosion. Copyright © 2002 John Wiley & Sons, Ltd.     

Soil‐aggregate formation as influenced by clay content and organic‐matter amendment

Naturally occurring wetting‐and‐drying cycles often enhance aggregation and give rise to a stable soil structure. In comparatively dry regions, such as large areas of Australia, organic‐matter (OM) contents in topsoils of arable land are usually small. Therefore, the effects of wetting and drying are almost solely reliant on the clay content. To investigate the relations between wetting‐and‐drying cycles, aggregation, clay content, and OM in the Australian environment, an experiment was set up to determine the relative influence of both clay content (23%, 31%, 34%, and 38%) and OM amendments of barley straw (equivalent to 3.1 t ha^–1, 6.2 t ha^–1, and 12.4 t ha^–1) on the development of water‐stable aggregates in agricultural soil. The aggregate stability of each of the sixteen composite soils was determined after one, three, and six wet/dry cycles and subsequent fast and slow prewetting and was then compared to the aggregate stabilities of all other composite soils. While a single wet/dry cycle initiated soil structural evolution in all composite soils, enhancing macroaggregation, the incorporation of barley straw was most effective for the development of water‐stable aggregates in those soils with 34% and 38% clay. Repeated wetting‐and‐drying events revealed that soil aggregation is primarily based on the clay content of the soil, but that large straw additions also tend to enhance soil aggregation. Relative to untreated soil, straw additions equivalent to 3.1 t ha^–1 and 12.4 t ha^–1 increased soil aggregation by about 100% and 250%, respectively, after three wet/dry cycles and fast prewetting, but were of less influence with subsequent wet/dry cycles. Straw additions were even more effective in aggregating soil when combined with slow prewetting; after three wet/dry cycles, the mean weight diameters of aggregates were increased by 70% and 140% with the same OM additions and by 160% and 290% after six wet/dry cycles, compared to samples without organic amendments. We suggest that in arable soils poor in OM and with a field texture grade of clay loam or finer, the addition of straw, which is often available from preceding crops, may be useful for improving aggregation. For a satisfactory degree of aggregate stability and an improved soil structural form, we found that straw additions of at least 6.2 t ha^–1 were required. However, rapid wetting of straw‐amended soil will disrupt newly formed aggregates, and straw has only a limited ability to sustain structural improvement.     

Effect of Hormogaster elisae (Oligochaeta; Hormogastridae) on the stability of soil aggregates

The production and stability of soil aggregates produced by laboratory cultures of the endogeic earthworm Hormogaster elisae was studied using three different techniques: the determination of the soil mean weight diameter (MWD), the aggregate tensile strength, and by the Blanchart method, which involves three different tests. The MWD index of soils was higher in microcosms containing H. elisae. Tensile strength was significantly higher in earthworm casts than in naturally formed aggregates. The Blanchart method also showed aggregates produced by earthworms to be more stable. The results of all three methods concur in that aggregates produced by H. elisae are larger and more stable than those produced in control microcosms without earthworms.     

A new approach to determining water stable aggregation

Abstract

A new method is introduced to measure water stability of soil aggregates. The wrist‐action shaker is a simple, inexpensive tool that provides highly accurate data for the assessment of soil erodibility. Three soils from Hawaii (two Oxisols and one Vertisol) with different mineralogies, management histories, and potassium (K)‐factors were examined in this study. Six indices of water stable aggregation were determined after rapid immersion of air‐dry aggregates, followed by gentle wet‐sieving. Single‐sieve indices of percent water stable aggregates (WSA) < 0.063 mm, > 0.25 mm, and > 1.00 mm, were highly correlated. Additionally, these indices were highly correlated with three multiple sieve indices, namely geometric mean aggregate diameter (GMAD), arithmetic mean aggregate mass diameter (MAMD), and the coarse‐to‐fine index (CFI = % WSA > 1.00 mm / % WSA < 0.063 mm). Analysis of WSA data indicated that the relative soil erodibility ranking, from high to low, would be: Lualualei Vertisol > Molokai Oxisol > Kaneloa Oxisol. Discriminant analysis using GMAD and % WSA > 1.00 mm correctly classified 55 of 56 soil samples into their respective soil series.     

生物炭对豫西丘陵区农田土壤团聚体稳定性及碳、氮分布的影响

为研究生物炭对豫西丘陵地区农田土壤团聚体分布、稳定性及其碳、氮在团聚体中分布的影响,进一步探明生物炭对丘陵区农田土壤结构和养分的长期作用效果。采用田间长期定位试验,生物炭用量为0(C0),20(C20),40(C40)t/hm~2 3个处理,研究生物炭施用5年后对土壤团聚体组成及稳定性的影响,探究土壤团聚体中有机碳和全氮分布特性。结果表明:施加20,40 t/hm~2生物炭可提高0—20,20—40 cm土层的机械性0.5 mm以上粒级和水稳性0.053 mm以上粒级团聚体含量。在0—20 cm土层中,C20和C40处理下0.25 mm的机械性团聚体(DR_(0.25))分别较对照增加3.78%和6.83%,0.25 mm水稳性团聚体(WR_(0.25))分别较对照增加31.0%和49.45%,土壤不稳定团粒指数(E_(LT))分别较对照降低4.30%和6.85%,土壤团聚体破坏率(PAD)分别较对照降低9.71%和14.77%,土壤团聚体平均质量直径(MWD)分别较对照增加28.44%和45.34%,几何平均直径(GMD)分别较对照增加32.04%和54.92%。各粒级的有机碳和全氮含量随生物炭施用量的增加而增加,有机碳和全氮含量都以0.25～0.053 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土层的有机碳和全氮含量;随着生物炭施用量的增加,2,2～0.25,0.25～0.053 mm粒级团聚体有机碳和全氮贡献率随之增加,而0.053 mm粒级微团聚体有机碳和全氮贡献率随之降低。总体来说,生物炭能够改善豫西丘陵地区农田土壤的团聚体结构,增加土壤大团聚体的含量,增强团聚体的稳定性,提高土壤团聚体中碳、氮含量,有利于豫西地区农田土壤肥力的保持和持续健康发展。