秸秆与氮肥配施对潮土微生物活性及团聚体分布的影响

为了探索秸秆还田与氮肥配施对砂质潮土微生物活性与土壤团聚体的影响.采用室内培养的方法,在定量秸秆条件下,通过控制尿素用量,研究不同氮肥用量与秸秆配施比例对砂质潮土的有机质、土壤微生物量碳、氮含量、土壤酶活性以及土壤团聚体的影响.结果 表明:与CK相比,秸秆配施适量氮肥,能显著提高土壤有机质(29.04％～41.90％)...     

Glucose additions to aggregates subjected to drying/wetting cycles promote carbon sequestration and aggregate stability

Biogeochemical mechanisms at microscale regions within soil macroaggregates strengthen aggregates during repeated DW cycles. Knowledge of additional biogeochemical processes that promote the movement of dissolved organic carbon (DOC) into and throughout soil aggregates and soil aggregate stabilization are essential before we can more accurately predict maximum carbon (C) sequestration by soils subjected to best management practices. We investigated the spatial distribution of ¹³C-glucose supplied to individual soil macroaggregate surfaces and subjected to multiple drying and wetting (DW) cycles. Subsequent distribution of added glucose-C, CO₂ respiration, increased microbial community activity and concomitant changes in soil aggregate stabilization were monitored. Moist macroaggregates were treated with no DW cycles and zero glucose C (Control), 5 DW cycles and zero glucose (DW0G), and 5 DW cycles with additions of 250 μg glucose-¹³C/g soil during each cycle (DW+G). Repeated additions of glucose-C to aggregate surfaces reduced the mineralization of pre-existing soil C by an average of 45% and established concentric gradients of glucose-derived C. It is concluded these increasing gradients promoted the diffusion of soluble C into interior regions and became less available to microbial respiration. Spatial gradients of glucose-derived C within aggregates influenced a shift in the abundance of unique ribotypes spatially distributed within aggregates. Rapid decreases in the mineralization rates of glucose-C during repeated DW cycles suggested greater C sequestration by either physical restriction of microbes or chemical sorption of new C that diffused into aggregates. Aggregate stability decreased significantly following 2-3 DW cycles, when glucose-C was not added. Additions of glucose-C with each DW cycle maintained soil aggregate stability equal to the moist but not cycled control throughout the 5 DW cycles of this study. These data simulate the strengthening of soil aggregates in no tillage agroecosystems which provides continuous additions of DOC compounds generated by decomposing plant residues on the soil surface, and root exudates and decomposition, as well as the mineralization of POM materials within nondisturbed soil profiles.     

Influence of microbial populations and residue quality on aggregate stability

Soil structure mediates many biological and physical soil processes and is therefore an important soil property. Physical soil processes, such as aggregation, can be markedly influenced by both residue quality and soil microbial community structure. Three experiments were conducted to examine (i) the temporal dynamics of aggregate formation and the water stability of the obtained aggregates, (ii) the effect of residue quality on aggregation and microbial respiration, and (iii) the effect of fungi and bacteria on aggregation.In the first experiment, 250 μm sieved air-dried soil, mixed with wheat straw, was incubated for 14 days to allow formation of water-stable macroaggregates (>250 μm). Aggregate stability was measured by wet sieving after four different disruptive treatments: (i) soil at field capacity; (ii) soil air-dried and slowly wetted; (iii) soil air-dried and quickly wetted; (iv) 8 mm sieved soil, air-dried and immersed in water (slaking). After 14 days of incubation, maximum aggregation for soil sieved at field capacity was reached; however, these newly formed aggregates were not yet resistant to slaking.During the second experiment, the effect of low-quality residue (C/N: 108) (with or without extra mineral nitrogen) and high-quality residue (C/N: 19.7) (without extra mineral nitrogen) on macroaggregate formation and fungal and bacterial populations was tested. After 14 days, aggregation, microbial respiration, and total microbial biomass were not significantly different between the low-quality (minus mineral nitrogen) and high-quality residue treatment. However, fungal biomass was higher for the low-quality residue treatment compared to the high-quality residue treatment. In contrast, bacterial populations were favored by the high-quality residue treatment. Addition of mineral N in the low-quality residue treatment resulted in reduced macroaggregate formation and fungal biomass, but had no effect on bacterial biomass. These observations are not conclusive for the function of fungal and/or bacterial biomass in relation to macroaggregate formation. In order to directly discern the influence of soil microflora on aggregation, a third experiment was conducted in which a fungicide (captan) or bactericide (oxytetracycline) was applied to selectively suppress fungal or bacterial populations. The direct suppression of fungal growth by addition of fungicide led to reduced macroaggregate formation. However, suppression of bacterial growth by addition of bactericide did not lead to reduced macroaggregate formation. In conclusion, macroaggregate formation was positively influenced by fungal activity but was not significantly influenced by residue quality or bacterial activity.     

Soil aggregate stabilization by a saprophytic lignin-decomposing basidiomycete fungus I. Microbiological aspects

 We studied the effects of a saprophytic lignin-decomposing basidiomycete isolated from plant litter on soil aggregation and stabilization. The basidiomycete produced large quantities of extracellular materials that bind soil particles into aggregates. These binding agents are water-insoluble and heat-resistant. Water stability of aggregates amended with the fungus and the degrees of biodegradation of the binding agents by native soil microorganisms were determined by the wet-sieving method. The data demonstrated that aggregates supplemented with a source of C (millet or lentil straw) were much more water-stable and resisted microbial decomposition longer than when they were prepared with fungal homogenates alone. Moreover, retrieval of fungal-amended aggregates supplemented with millet during the first 4 weeks of incubation in natural soil exhibited more large aggregate fractions (>2 mm) than the ones supplemented with lentil straw. The possible relationship of the role of basidiomycetes in litter decomposition and soil aggregation is discussed. Received: 27 September 1999     

Long-term tillage effects on the distribution patterns of microbial biomass and activities within soil aggregates

The effects of tillage on the interaction between soil structure and microbial biomass vary spatially and temporally for different soil types and cropping systems. We assessed the relationship between soil structure induced by tillage and soil microbial activity at the level of soil aggregates. To this aim, organic C (OC), microbial biomass C (MBC) and soil respiration were measured in water-stable aggregates (WSA) of different sizes from a subtropical rice soil under two tillage systems: conventional tillage (CT) and a combination of ridge with no-tillage (RNT). Soil (0–20 cm) was fractionated into six different aggregate sizes (> 4.76, 4.76–2.0, 2.0–1.0, 1.0–0.25, 0.25–0.053, and < 0.053 mm in diameter). Soil OC, MBC, respiration rate, and metabolic quotient were heterogeneously distributed among soil aggregates while the patterns of aggregate-size distribution were similar among properties, regardless of tillage system. The content of OC within WSA followed the sequence: medium-aggregates (1.0–0.25 mm and 1.0–2.0 mm) > macro-aggregates (4.76–2.0 mm) > micro-aggregates (0.25–0.053 mm) > large aggregates (> 4.76 mm) > silt + clay fractions (< 0.053 mm). The highest levels of MBC were associated with the 1.0–2.0 mm aggregate size class. Significant differences in respiration rates were also observed among different sizes of WSA, and the highest respiration rate was associated with 1.0–2.0 mm aggregates. The C_mic/C_org was greatest for the large-macroaggregates regardless of tillage regimes. This ratio decreased with aggregate size to 1.0–0.25 mm. Soil metabolic quotient (qCO₂) ranged from 3.6 to 17.7 mg CO₂ g^− 1 MBC h^− 1. The distribution pattern of soil microbial biomass and activity was governed by aggregate size, whereas the tillage effect was not significant at the aggregate scale. Tillage regimes that contribute to greater aggregation, such as RNT, also improved soil microbial activity. Soil OC, MBC and respiration rate were at their highest levels for 1.0–2.0 mm aggregates, suggesting a higher biological activity at this aggregate size for the present ecosystem.     

Structure properties and pore dynamics in aggregate beds due to wetting–drying cycles

Aggregate hierarchy and porosity changes in aggregate beds as a consequence of wetting–drying cycles were studied in two Andisols and one Mollisol from Chile, collected at two depths. Bulk density and indirect tensile strength were measured in aggregates of different sizes. Aggregate beds were prepared in cylinders with two size classes. Six wetting–drying cycles between 0 and –60 hPa were applied. Bulk density (Db) of soil matrix was controlled after each cycle, and the macroporosity was calculated. A repellency index was measured in one of the Andisols. In addition, also the air permeability was measured after the sixth cycle. It could be proofed, that the aggregate strength is an appropriate parameter to evaluate the aggregate hierarchy, and this parameter is also more sensitive than aggregate Db to discriminate between the effects of land‐use intensity. Aggregate strength is furthermore well correlated with changes in pore water pressure and can be applied to relate strength values with aggregate development level. Only if the predrying exceeds pF > 3.0, aggregate strength correlates with Db. The more pronounced is the land‐use, the higher is the increase of Db values for aggregate beds. The decrease of coarse porosity during wetting–drying cycles can be explained by mass differences between saturated and equilibrated water conditions that considers the water around aggregates and within the contact area. Nevertheless, the relation of relative macroporosity change, calculated by P_exped where D_agg is the Db measured by clod method, and the relative Db change, is useful to explain possible presence of coarse pores inside the aggregates. The newly formed porosity prevents the water repellency, but after six cycles of drying, the repellency index increased in the topsoil while we could detect a decrease in the subsoil samples (under defined conditions in the laboratory) which we assume to be caused by microbial activity. The approaching of aggregates by drying cycles generates in Andisols a reduced area to air fluxes, with low values of air permeability.     

Effects of Alfalfa on Aggregate Stability,Aggregate Preserved‐C and Nutrients in Region Mountain Agricultural Soils 1 Year After its Planting

In mountain areas, water erosion plays an important role on soil structure and can strongly affect its functions. Suitable management practices, namely choice of crops, may be able to improve aggregate stability and reduce soil loss by surface runoff. To study the effects of cover crops on aggregation in mountain soils, we investigated a soil planted with wheat (site C) compared with two soils under alfalfa (sites A and B). All investigated soils were Typic Ustorthents. The soil aggregates were isolated by a physical‐functional procedure defined on the basis of aggregate resistance against specific breakdown mechanisms such as slaking and water abrasion. One year after alfalfa plantation, its effect on wet aggregate stability and the amount of organic carbon (OC) and nutrients retained by the isolated aggregates were measured. In the less degraded soils (site B), organic matter decomposition was stimulated, and after 1 year, the total OC (TOC) and TOC/N declined. Consequently, a reduction in aggregate stability and ability to preserve elements occurred, as shown by the decrease of wet aggregate stability indexes and total nutrients (e.g. N, P and K) retained by stable aggregates. In more degraded soil (site A), alfalfa generally positively affected the investigated properties. The effect of alfalfa in the studied crop rotation depends on the degree of land degradation due to water erosion. As regards the investigated breakdown mechanisms, our data showed that the aggregate resistance to water abrasion was related to TOC, while the resistance against fast wetting breakdown is correlated to the microbial biomass. Copyright © 2017 John Wiley & Sons, Ltd.     

Effect of Long-Term Soil Management on the Mutual Interaction Among Soil Organic Matter, Microbial Activity and Aggregate Stability in a Vineyard

Vineyard management practices to enhance soil conservation principally focus on increasing carbon (C) input, whereas mitigating impacts of disturbance through reduced tillage has been rarely considered. Furthermore, information is lacking on the effects of soil management practices adopted in the under-vine zone on soil conservation. In this work, we evaluated the long-term effects (22 years) of alley with a sown cover crop and no-tillage (S + NT), alley with a sown cover crop and tillage (S + T), and under-vine zone with no vegetation and tillage (UV) on soil organic matter (SOM), microbial activity, aggregate stability, and their mutual interactions in a California vineyard in USA. Vegetation biomass, microbial biomass and activity, organic C and nitrogen (N) pools, and SOM size fractionation and aggregate stability were analysed. Soil characteristics only partially reflected the differences in vegetation biomass input. Organic C and N pools and microbial biomass/activity in S + NT were higher than those in S + T, while the values in UV were intermediate between the other two treatments. Furthermore, S + NT also exhibited higher particulate organic matter C in soil. No differences were found in POM C between S + T and UV, but the POM fraction in S + T was characterized by fresher material. Aggregate stability was decreased in the order: S + NT > UV > S + T. Tillage, even if shallow and performed infrequently, had a negative effect on organic C and N pools and aggregate stability. Consequently, the combination of a sown cover crop and reduced tillage still limited SOM accumulation and reduced aggregate stability in the surface soil layer of vineyards, suggesting relatively lower resistance of soils to erosion compared to no-till systems.     

冻融循环对黑土团聚体稳定性与微结构特征的影响

冻融循环作用下的土壤结构变化被认为是融雪期黑土坡面土壤侵蚀加剧的主要原因之一,土壤团聚体稳定性与团聚体微结构是影响土壤可蚀性的关键因子.基于控制条件土壤冻融模拟试验,采用湿筛法、扫描电子显微技术(SEM)和Image-Pro Plus(IPP)图像分析处理相结合的方法,分析了冻融循环过程中黑土团聚体微结构的动态变化特征...     

Effects of straw,vegetable oil and whey on physical and microbiological properties of a chernozem

A soil microcosm study was carried out to compare the effects of straw, vegetable oil or whey additions on physical and microbiological properties of a chernozem after 18 weeks of incubation. The application rates of the different organic materials were based on the addition of carbon at a concentration equivalent to 1% of soil dry mass.Straw, vegetable oil and whey additions to soil increased dry aggregate size distribution during incubation. All organic treatments had significantly higher mean weight diameter (MWD) of dry aggregates than the untreated control. The ability of whey, straw and oil to increase MWD did not differ significantly. Only straw addition significantly increased soil maximum water-holding capacity after 126 days of incubation. In comparison with the untreated control, straw tended to reduce the resistance of aggregates to abrasion, while addition of whey and vegetable oil seemed to have a positive effect on this property. Aggregates of organic matter amended soil materials had higher resistance to the dispersive action of water than those of the untreated control. Whey and straw additions sustained higher soil biomass-C contents and C_mic/C_org ratios than vegetable oil. Biomass development was most favored by whey addition. Fungal contributions to biomass-C dominated over bacterial contributions in whey and straw-amended soil. The opposite was observed for the oil treatment. Increases in MWD of dry aggregates seem to be related to the synthesis of biomass and soil binding metabolites in whey and straw-treated soil. Among the organic treatments, oil-amended soil had the highest respiratory activity and the lowest biomass content. Stabilizing agents derived from the microbial metabolism using oily substrate are discussed.     

Influence of dry–wet cycles on the interrelationship between aggregate,particulate organic matter,and microbial community dynamics

Aggregate dynamics and their relationship to the microbial community have been suggested as key factors controlling SOM dynamics. Dry–wet (DW) cycles are thought to enhance aggregate turnover and decomposition of soil organic matter (SOM), particularly in tilled soils. The objective of this study was to evaluate the effects of DW cycles on aggregate stability, SOM dynamics, and fungal and bacterial populations in a Weld silt loam soil (Aridic Paleustoll). Samples, taken from 250 μm sieved air-dried soil (i.e. free of macroaggregates > 250 μm), were incubated with ¹³C-labeled wheat residue. In one set of soil samples, fungal growth was suppressed using a fungicide (Captan) in order to discern the effect of dry–wet cycles on fungal and bacterial populations. Aggregate formation was followed during the first 14 d of incubation. After this period, one set of soil samples was subjected to four DW cycles, whereas another set, as a control, was kept at field capacity (FC). Over 74 d, total and wheat-derived respiration, size distribution of water stable aggregates and fungal and bacterial biomass were measured. We determined native and labeled C dynamics of three particulate organic matter (POM) fractions related to soil structure: the free light fraction (LF), and the coarse (250–2000 μm) and fine (53–250 μm) intra-aggregate POM fraction (iPOM). In the fungicide treated soil samples, fungal growth was significantly reduced and no large macroaggregates (> 2 mm) were formed, whereas without addition of fungicide, fungi represented the largest part of the microbial biomass (66%) and 30% of the soil dry weight was composed of large macroaggregates. During macroaggregate formation, labeled free LF-C significantly decreased whereas labeled coarse iPOM-C increased, indicating that macroggregates are formed around fresh wheat residue (free LF), which is consequently incorporated and becomes coarse iPOM. The first drying and wetting event reduced the amount of large macroaggregates from 30 to 21% of the total soil weight. However, macroaggregates became slake-resistant after two dry-wet cycles. Fine iPOM-C was significantly lower in soil after two dry–wet cycles compared to soil kept at FC. We conclude that more coarse iPOM is decomposed into fine iPOM in macroaggregates not exposed to DW cycles due to a slower macroaggregate turnover. In addition, when macroaggregates, subjected to dry–wet cycles, became slake-resistant (d 44) and consequently macroaggregate turnover decreased, fine iPOM accumulated. In conclusion, differences in fine iPOM accumulation in DW vs. control macroaggregates are attributed to differences in macroaggregate turnover.     

Hydrophobicity of organic matter in arable soils: influence of management

The affinity of soil organic matter for water influences resistance to microbial degradation, the rate of wetting and adsorption processes. Such properties play key roles in organic matter and microbial biomass dynamics, aggregate stability, water infiltration, leaching of organic and inorganic pollutants, chemical composition and the dynamics of dissolved organic matter (DOM). The hydrophobicity of the organic matter as a function of management have been studied in two soils with contrasting textures using diffuse reflectance infrared fourier transform spectroscopy (DRIFT). The results show that agricultural management clearly influences the amount of aliphatic C-H units and implicitly the hydrophobicity of the soil organic matter. A decrease of organic C due to management is accompanied by a decrease of hydrophobicity as well as of soil microbial activity and aggregate stability. The hydrophobicity index is a sensitive quantity to characterize the‘quality’ of soil organic matter. DRIFT spectroscopy proves to be a rapid and reliable technique to determine quantitatively the hydrophobicity of soil organic matter.     

Variations in soil microbial biomass and crop roots due to differing resource quality inputs in a tropical dryland agroecosystem

The influence of exogenous organic inputs on soil microbial biomass dynamics and crop root biomass was studied through two annual cycles in rice-barley rotation in a tropical dryland agroecosystem. The treatments involved addition of equivalent amount of N (80 kg N ha⁻¹) through chemical fertilizer and three organic inputs at the beginning of each annual cycle: Sesbania shoot (high-quality resource, C:N 16, lignin:N 3.2, polyphenol+lignin:N 4.2), wheat straw (low-quality resource, C:N 82, lignin:N 34.8, polyphenol+lignin:N 36.8) and Sesbania+wheat straw (high-and low-quality resources combined), besides control. The decomposition rates of various inputs and crop roots were determined in field conditions by mass loss method. Sesbania (decay constant, k=0.028) decomposed much faster than wheat straw (k=0.0025); decomposition rate of Sesbania+wheat straw was twice as fast compared to wheat straw. On average, soil microbial biomass levels were: rice period, Sesbania?Sesbania+wheat straw>wheat straw?fertilizer; barley period, Sesbania+wheat straw>Sesbania?wheat straw?fertilizer; summer fallow, Sesbania+wheat straw>Sesbania>wheat straw?fertilizer. Soil microbial biomass increased through rice and barley crop periods to summer fallow; however, in Sesbania shoot application a strong peak was obtained during rice crop period. In both crops soil microbial biomass C and N decreased distinctly from seedling to grain-forming stages, and then increased to the maximum at crop maturity. Crop roots, however, showed reverse trend through the cropping period, suggesting strong competition between microbial biomass and crop roots for available nutrients. It is concluded that both resource quality and crop roots had distinct effect on soil microbial biomass and combined application of Sesbania shoot and wheat straw was most effective in sustained build up of microbial biomass through the annual cycle.     

Decomposition of bacterial polymers in soil and their influence on soil structure

Summary The adherence of soil particles into stable aggregates increases with the addition of monosaccharides or polysaccharde polymers to soil, either as plant residues, microbial metabolites, or as simple carbohydrates. Microbial polysaccharides are one of the most effective organic agents that promote soil aggregate stability, but the effectiveness of these polymers in stabilizing soil particles varies dramatically between microbial strains, the amount present and the prevailing environmental conditions. We conducted glasshouse and laboratory studies to determine the effectiveness of selected microbial polymers in stabilizing soil aggregates. The addition and thorough mixing of 1.0 mg microbial polymer C g^–1 soil of seven bacteria strains (Arthrobacter viscocus, Azotobacter indicus, Bacillus subtilus, Chromobacterium violaceum, Pseudomonas aeruginosa, Pseudomonas strain I, and Pseudomonas strain II), three deuteromycete strains (Cryptococcus laurentii, Hansenula holstii, and Mucor rouxii), and two reference compounds (hydroxyethyl guar and glucose) to an Arlington coarse-loamy soil resulted in stimulated soil respiration, increased aggregate stability, and decreased soil bulk density and modulus of rupture when incubated from 1 to 12 weeks. The monosaccharides present in the added polymers were rapidly decomposed and the sacchride content of the polymer-treated soil returned to the level of the soil control (with no polymer addition) after 2 weeks of incubation, while the maximum increase in soil aggregate stability was noted during the 3rd and 4th weeks of incubation. Statistical analyses showed that the glucose content of the polymers added was significantly correlated with soil aggregation [weeks 1 (r=0.78^***) and 2 (r=0.61^*)], but the extractable soil saccharides were not significantly correlated with increased aggregate stability or decreased soil bulk density during this study. When microbial extracellular polymers were added to soil only a transient increase in soil stability was measured upon decomposition of the added saccharides. This finding suggests that the stabilization of soil aggregates is a result of other microbial processes or metabolites rather than the direct binding effects of the added polysaccharides.     

添加生物质黑炭对红壤结构稳定性的影响

本研究通过室内恒温恒湿控制试验,研究添加黑炭、秸秆在分解过程中对土壤抗破碎性及团聚体稳定性等土壤生物、物理特征的影响。黑炭是在400℃缺氧状态下水稻秸秆焚烧而制成的。实验包括不添加任何物料的对照(CK)、添加黑炭(BC)、黑炭+氮磷钾肥(BC+NPK)、秸秆(Straw)、秸秆+氮磷钾肥(Straw+NPK)等5个处理。研究结果表明,经过55天恒温(25℃)恒湿(田间持水量的40%)培养,黑炭分解速度很慢,通过换算,其周转周期约为1400年,而秸秆周转周期仅为7年,配施氮磷钾能加速黑炭和秸秆的分解。添加黑炭没有提高土壤抗破碎能力和土壤团聚体稳定性(P>0.05),而秸秆则相反(P<0.05)。该研究结果表明生物质黑炭不能像其他有机物料一样提高土壤微生物活性和土壤结构稳定性。     

Resilience of microorganisms and aggregation of a sandy calcareous soil to amendment with organic and synthetic fertilizer

Marginal coastal soils are dependent on appropriate land management to prevent soil erosion, as a result of low soil stability combined with exposure to strong winds. An example of such an area is the machair, a fixed dune system utilized for agriculture in the northwest of Scotland, UK. The separate and combined effects of synthetic NPK fertilizer and a traditional soil conditioner (kelp, a seaweed) on soil structure formation, stabilization and biological parameters were studied on a cropped field on the machair. Soil physical properties examined included water retention at 10 kPa matric suction, water stable aggregates (WSA) >1 mm, aggregate stability, and biological properties including ester-linked fatty acid (ELFA) analysis and β-glucosidase activity for microbial biomass and activity, respectively. Significant treatment effects were few and inconsistent between sampling times, but included kelp and/or NPK fertilizer reducing aggregation, water retention, microbial biomass and activity relative to the unamended control treatment. Furthermore, seasonal variation, which could be attributed to changes in soil water content, was stronger than variation in response to fertilizer treatments. Principal components analysis of the ELFA data showed that ploughing promoted fungal biomass relative to bacteria, and confirmed both the absence of consistent synthetic and organic fertilizer effects and the sensitivity of microbial biomass to season. Overall, the study demonstrated the resilience of a calcareous sandy soil to amendment with fertilizer.     

Straw coverage alleviates seasonal variability of the topsoil microbial biomass and activity

Straw coverage on soil surface is a well-known practice for conserving soil and water. Seasonal variability of microbial biomass carbon (MBC) and basal respiration (BR) in surface soil (0–5 cm) was compared between conventional straw removal (SR) and straw coverage (SC) in a maize (Zea mays L.) field experiment, Northeast China. The straw coverage treatment significantly increased microbial energy, carbon and nutrient sources (soil C and N contents) and improved soil physical environment (moisture and porosity), and thus stimulated MBC and BR across the growth season of maize, as compared to the straw removal treatment. MBC and BR showed similar seasonal trends in soil temperature, with maximum values in summer. In both study years, the straw coverage treatment reduced seasonal variation of soil temperature, therefore it significantly moderated seasonal variability of MBC and BR. Our results demonstrate that straw coverage contributes to stabilizing soil microbial characteristics in season.     

A comparison of aggregate stability and biological activity in earthworm casts and uningested soil as affected by amendment with wheat or lucerne straw

The mechanisms responsible for stabilization of earthworm casts were investigated in a laboratory study. Earthworms (Aporrectodea caliginosa) were fed soil or soil amended with either ground wheat straw or lucerne hay. Cast material and uningested soil material were incubated for 56 days, and changes in aggregate stability (measured by wet sieving), soil biological activity and macronutrient availability were measured periodically. In general, aggregate stability and microbial biomass C tended to increase during the incubation for both cast and soil material, whereas hot-water extractable carbohydrate content declined. For amended treatments, basal respiration rate and microbial metabolic quotient were large at the first sampling (7 days) but declined rapidly thereafter. There was a transitory increase in extractable P in fresh casts compared with uningested material, a sustained increase in mineral N concentrations but no change in exchangeable K content. For unamended treatments, the casts were less stable than soil material, but this difference diminished during incubation. Drying aggregates before analysing them did not reverse this trend. The casts contained more microbial biomass C than soil material did, but the basal respiratory rate, respiratory quotient and hot-water extractable carbohydrate content were less. By contrast, for wheat- and lucerne-amended treatments casts were more stable than soil material, and the microbial biomass was less at all sampling times. For wheat treatments, respiratory quotient and hot-water extractable carbohydrate content were larger for cast than soil material, but the opposite was the case for lucerne treatments. We attribute the stability of casts in amended treatments to the intimate mixing of part-decomposed organic fragments with comminuted soil particles, binding by microbial mucilage associated with the organic fragments and linking and binding by fungal hyphae.     

Interactions between soil organic matter status, cropping history, method of quantification and sample pretreatment and their effects on measured aggregate stability

 The effects of sample pretreatment (field-moist, air-dried or tension rewetted) on aggregate stability measured by wet sieving or turbidimetry were compared for a group of soil samples ranging in organic C content from 20 to 40 g C kg^–1. Concentrations of total N, total and hot-water-extractable carbohydrate and microbial biomass C were linearly related to those of organic C. Aggregate stability measured by wet sieving using air-dried or field-moist samples and that measured by turbidimetry, regardless of sample pretreatment, increased curvilinearly with increasing soil organic C content. However, when tension-rewetted samples were used for wet sieving, aggregate stability was essentially unaffected by soil organic C content. Measurements of aggregate stability (apart from wet sieving using rewetted soils) were closely correlated with one another and with organic C, total and extractable carbohydrate and microbial biomass C content of the soils. The short-term effects of aggregate stability were also studied. Soils from under long-term arable management and those under long-term arable followed by 1 or 3 years under pasture had similar organic C contents, but aggregate stability measured by turbidimetry and by wet sieving using air-dried or field-moist samples increased with increasing years under pasture. Light fraction C, microbial biomass and hot-water-extractable carbohydrate concentrations also increased. It was concluded that both total and labile soil organic C content are important in relation to water-stable aggregation and that the use of tension-rewetted samples to measure stability by wet sieving is unsatisfactory since little separation of values is achieved. Received: 6 January 1999     

Organic Fertilization in Traditional Mediterranean Grapevine Orchards Mediates Changes in Soil Microbial Community Structure and Enhances Soil Fertility

Soil microbial populations and their functions related to nutrient cycling contribute substantially to the regulation of soil fertility and the sustainability of agroecosystems. A field experiment was performed to assess the medium‐term effect of a mineral fertilizer and two organic fertilization systems with different nitrogen sources on the soil microbial community biomass, structure, and composition (phospholipid fatty acids, pattern, and abundance), microbial activity (basal respiration, dehydrogenase, protease, urease, β‐glucosidase, and total amount of phosphomonoesterase activities), and physical (aggregate stability) and chemical (total organic C, total N, available P and water‐soluble carbohydrates) properties in a vineyard under semiarid Mediterranean conditions after a period of 10 years. The three fertilization systems assayed were as follows: inorganic fertilization, addition of grapevine pruning with sheep manure (OPM), and addition of grapevine pruning with a legume cover crop (OPL). Both treatments, OPM and OPL, produced higher contents of total organic carbon, total N, available P, water‐soluble carbohydrates, and stable aggregates. The organic fertilization systems increased microbial biomass, shifted the structure and composition of the soil microbial community, and stimulated microbial activity, when compared with inorganic fertilization. The abundances of fungi and G+ bacteria were increased by treatments OPM and OPL, without significant differences between them. Organic and inorganic fertilization produced similar grapevine yields. The ability of the organic fertilization systems for promoting the sustainability and soil biological and chemical fertility of an agroecosystem under semiarid conditions was dependent of the organic N source. Copyright © 2016 John Wiley & Sons, Ltd.