Soil aggregation: Influence on microbial biomass and implications for biological processes

Our 1988 paper, describing the effects of cultivation on microbial biomass and activity in different aggregate size classes, brought together the ‘aggregate hierarchy theory’ and the ‘microbial biomass concept’. This enabled us to identify the relationships between microbial and microhabitat (aggregate) properties and organic matter distribution and explain some of their responses to disturbance. By combining biochemical and direct microscopy based quantification of microbial abundance with enzyme activities and process measurements, this study provided evidence for the role of microbial biomass (especially fungi) in macroaggregate dynamics and carbon and nutrient flush following cultivation. In the last ten years environmental genomic techniques have provided much new knowledge on bacterial composition in aggregate size fractions yet detailed information about other microbial groups (e.g. fungi, archaea and protozoa) is lacking.We now know that soil aggregates are dynamic entities – constantly changing with regard to their biological, chemical and physical properties and, in particular, their influences on plant nutrition and health. As a consequence, elucidation of the many mechanisms regulating soil C and nutrient dynamics demands a better understanding of the role of specific members of microbial communities and their metabolic capabilities as well as their location within the soil matrix (e.g. aggregates, pore spaces) and their reciprocal relationship with plant roots. In addition, the impacts of environment and soil type needs to be quantified at the microscale using, wherever possible, non-destructive ‘in situ’ techniques to predict and quantify the impacts of anthropogenic activities on soil microbial diversity and ecosystem level functions.     

Sedimentary aggregates in the Peoria Loess of Nebraska, USA

Loess grain size data used to infer transport direction or wind strength are generally derived from vigorously disaggregated samples. However, these data may not adequately represent the effective particle size distribution during loess transport, if the transported dust contained aggregates of fine-grained material. Thin sections of minimally altered C and BC horizons in the late Pleistocene Peoria Loess of Nebraska, USA, indicate the presence of aggregates with diameters of 30–1000 μm. The larger aggregates (>250 μm) are unlikely to have been transported, and are interpreted as the result of soil faunal activity and other pedogenic processes after deposition. Aggregates smaller than 250 μm could have a similar origin, but laser diffraction particle size analysis suggests that many are sedimentary particles. Comparison of minimally and fully dispersed particle size distributions from each sampling site was used to estimate the modal diameter of aggregates. The aggregate modal diameter becomes finer with decreasing loess thickness, representing increasing distance from the source. A similar trend was observed in the modal diameter of fully dispersed particle size distributions, which represents the mode of sand and silt transported as individual grains. We interpret both trends as the result of sorting during transport, supporting the interpretation that many of the aggregates were transported rather than formed in place. Aggregate content appears to increase with distance from the source, explaining a much more rapid downwind increase in clay content than would be expected if clay were transported as particles smaller than 2 μm diameter. Although the Peoria Loess of Nebraska contains sedimentary aggregates, many of the coarse silt and sand grains in this loess were transported as primary particles, were thoroughly exposed to sunlight and are potentially well suited for luminescence dating.     

Multivariate effects of plant canopy, soil physico-chemistry and microbiology on Sclerotinia stem rot of soybean in relation to crop rotation and urban compost amendment

The effects of canopy, soil physico-chemical and microbiological variables on Sclerotinia stem rot (SSR) on soybean were assessed in two soils (clay loam and sandy loam) using multiple regression and canonical redundancy analysis (RDA) and their partial form to control for the rotation (2 or 3-y-corn/soybean monoculture) and fertilization (mineral/urban compost) or spatial variables effects. The models revealed the minimal sets of variables that best explain the variance of the survival of Sclerotinia sclerotiorum’s sclerotia, carpogenic germination, disease severity and their associations. In clay loam, the 3-y-corn rotation reduced disease severity mainly through the reduction of weed biomass that favoured carpogenic germination. Urban compost has a conducive effect explained by a better soil surface drainage. Additionally, total N was found suppressive to sclerotial survival. In sandy loam, the carpogenic germination was negatively correlated with high C mineralization quotient and aggregate stability but correlated positively with Ca. Sclerotial survival was negatively correlated with pH and Ca, and positively correlated with biological fertility index. Aggregate stability, Ca and pH were associated with the urban compost. The regression and RDA analyses allowed to identify key variables that drived SSR development and explain their relationship with the cultural practices, soil health, as well as the spatial variation of disease variables.     

Flow-microcalorimetry measurements of aerobic and anaerobic soil microbial activity

Heat output can be used as an indicator of microbial activity and is usually measured in a microcalorimeter with closed ampoules. In long-term experiments particularly, interpretation of the data is hindered by the changing environment in the closed ampoules because of O₂ consumption and CO₂ enrichment. We used a combination of a flow-microcalorimeter and a gas chromatograph to measure the heat flux and CO₂ and N₂O production rates under controlled conditions. Simultaneous detection of the heat output and CO₂ emission allowed calculation of the calorimetric: CO₂ (Cal/CO₂) ratio. A mean ratio of-435 kJ mol^-1 CO₂ was detected in six different soils amended with glucose and incubated under aerobic conditions. This ratio indicated that CO₂ was the end-product of catabolism. In wet 10–12 mm soil aggregates of a gleyic vertisol amended with glucose, values of-285 kJ mol^-1 CO₂ under an aerobic and-141 kJ mol^-1 CO₂ under a N₂ atmosphere was determined. These findings indicated that fermentative metabolism occurred. The Cal/CO₂ ratio was not affected when enough NO _inf3 ^sup- was available and denitrification processes (N₂O production) were possible.     

Aggregate stabilization in kaolinitic soils by low rates of anionic polyacrylamide

Abstract. The stability of aggregates can be improved by amending the soil with charged synthetic polymers. We have evaluated the effects of addition of low rates of anionic polyacrylamide (PAM) on the stability of aggregates from three predominately kaolinitic soils, and investigated whether the applied PAM penetrated into the aggregates or was adsorbed onto exterior surfaces only. Aggregates (6.3–9.5 mm) from the Ap horizons of a Cecil sandy loam and a Cecil loamy sand (Typic Kanhapludult) and from the Bt horizon of a Davidson clay (Rhodic Kandiudult) were treated with a high molecular weight(2 × 10⁷Da), negatively charged (20% hydrolysis) PAM at rates of 0, 10 or 20 kg/ha. The treated aggregate were dried and exposed to 20 mm of high kinetic energy (23.0 kJ/m³) simulated rain. Some of the PAM-treated Cecil loamy sand aggregates (5 or 10 kg/ha) were broken in half to expose untreated aggregate interiors to the rain. Percentage stable aggregates (weight/weight) in the PAM treatments ranged from 52 to 97%, vs. 20-32% in the control. PAM addition significantly increased the percentage of >4 mm sized aggregates compared with the untreated aggregates. PAM was more effective at stabilization in the light to medium textured Cecil soils than in the clayey Davidson. When internal surfaces of PAM treated aggregates were exposed, the percentage of stable aggregates exceeded the amount expected if the polymer was only adsorbed on to external surfaces. This suggested that some PAM had penetrated to some degree into aggregate interiors, thereby stabilizing both external and internal aggregates surfaces. Based on these results it is envisaged that use of PAM could be a viable alternative to the commonly used soil and water conservation practices (e.g. mulching, dyking, contour tillage).     

土地利用变化对海南热带土壤团聚体的影响

运用物理分组的方法对海南岛的次生林、水稻田、甘蔗地土壤（剖面分0～10cm，10-20cm，20-50cm。50-100cm四层）进行物理组分及其有机C含量进行研究。结果表明：土地利用方式由次生林变为农用地后．大团聚体占土壤的质量百分数显著降低，微团聚体和粉粒与粘粒大小的团聚体占土壤的质量百分数显著增加。几种团聚体中微团聚体的变幅最大；由次生林变为水田比变为甘蔗地对土壤不同团聚体所占的比例影响大；次生林改为水稻田和甘蔗地后，水稻田和甘蔗地土壤各物理组分的有机碳含量都显著降低．且水田土壤团聚体有机碳降低的幅度较大。土地利用变化主要使0-50cm土壤产生显著退化。     

黏土矿物改良剂对新垦砂壤质耕地土壤有机碳积累的影响

为了解黏土矿物改良剂在新垦耕地地力提升中的作用,选择膨润土、高岭石、沸石和硅藻土4种黏土矿物,在添加量为10%的条件下,采用培养试验研究其对有机肥料矿化及土壤有机碳积累的影响。结果表明,与对照比较,添加黏土矿物可明显增加土壤有机碳的积累,其效果由强到弱为膨润土、高岭石、硅藻土、沸石;团聚体分析和重液分组结果表明,添加黏土矿物可显著增加土壤水稳定性团聚体数量,促进矿物结合态有机碳的形成。研究认为,黏土矿物增加有机碳稳定性是其促进土壤有机碳积累的主要原因。     

长期施肥对潮土团聚体有机碳分子结构的影响

采取长期施用有机肥（CM）、一半化肥氮和一半有机肥氮（HCM）、化肥（NPK）和不施肥对照（CK）的土壤,用湿筛法分为大团聚体（2000~250 μm）、微团聚体（250~53 μm）和粉砂 黏粒组分（<53 μm）,利用固态13C-核磁共振技术分析了土壤和团聚体中有机质的分子结构特征。结果表明,随着团聚体粒径减小,烷基碳/烷氧碳比值逐渐提高,并与土壤C/N呈显著负相关（R2 = 0.421,p = 0.022）,表明随着团聚体粒径减小,有机质的分解程度不断增加。与对照土壤相比,长期施用有机肥（HCM和CM处理）提高了土壤中烷氧碳和羰基碳的相对含量,烷氧碳的增加主要是由于大团聚体中甲氧基和含氮烷基碳相对含量的增加,羰基碳则主要在大团聚体和微团聚体中积聚。施用化肥土壤提高了烷氧碳和烷基碳的相对含量,烷氧碳增加主要是由于大团聚体中甲氧基和含氮烷基碳以及微团聚体中含氧烷基碳相对含量的提高,烷基碳增加主要发生在大团聚体。有机肥和化肥处理土壤中芳基碳相对含量降低1.8%~4.6%,主要是大团聚体和微团聚体中芳基碳比例下降引起的。而在粉砂 黏粒组分中芳基碳和酚基碳均增加,烷基碳相对含量降低5.9%~7.1%,表明施肥更利于芳香碳在小粒径组分中积累,减弱烷基碳在小粒径组分中的积累。结果表明长期施用有机肥可通过大团聚体和微团聚体物理保护肥料带入的大量碳水化合物和有机酸从而提高土壤有机碳含量。     

Influence of spring preparation date and soil water content on seedbed physical conditions of a clayey soil in Sweden

Secondary tillage performed under inadequate soil water contents usually leads to a poor seedbed. Under normal Swedish weather conditions, clayey soils ploughed during autumn form a very dry top layer in spring, which acts as an evaporation barrier so that deeper layers remain wet. Thus, the conventional approach considering soil workability in relation to a single value of soil water content is difficult to apply. Hence, a field experiment was carried out to study the effect of seedbed preparation date, the associated soil water contents and traffic consequences on the physical properties of a spring seedbed. The field was autumn ploughed and the experiment started as soon as the field was trafficable after winter thawing. The seedbed preparation consisted of three harrowing operations on plots 8 m×8 m (three replications) with a spring tined harrow and a tractor mounted with dual tyres and was performed on 10 occasions from the beginning of April to the middle of May. With the exception of some short periods after rain, the soil had a clear water stratification during the experiment, with a very dry superficial layer (5–20 mm thick) contrasting to water contents over 300 g kg⁻¹ from only 40 mm depth. After the harrowing operation, the seedbed aggregate fraction less than 2 mm increased from about 40% at the beginning of April to about 60% for the last four treatments in May. Contributing factors to the rise were attributed to the lower water contents of the top layer (<40 mm) and the drying–wetting and freezing–thawing cycles that occurred in the surface layer during April. There were no significant differences in bulk density after harrowing between the treatments but an increase in penetration resistance up to a depth of 180 mm in the harrowed plots was statistically significant (P<0.001). In the non-harrowed soil, penetration resistance also increased, including in those soil layers where water contents kept nearly constant.

In conclusion, the seedbed preparation dates had only a minor effect on soil compaction, as measured by bulk density and penetration resistance, due to the slow drying beneath the dry top layer. The fraction of fine aggregates in the seedbed increased with time. Thus, the optimal time for seedbed preparation depended mainly on soil friability and not on the risk of compaction.     

Soil texture affects soil microbial and structural recovery during grassland restoration

Many biotic and abiotic factors influence recovery of soil communities following prolonged disturbance. We investigated the role of soil texture in the recovery of soil microbial community structure and changes in microbial stress, as indexed by phospholipid fatty acid (PLFA) profiles, using two chronosequences of grasslands restored from 0 to 19 years on silty clay loam and loamy fine sand soils in Nebraska, USA. All restorations were formerly cultivated fields seeded to native warm-season grasses through the USDA’s Conservation Reserve Program. Increases in many PLFA concentrations occurred across the silty clay loam chronosequence including total PLFA biomass, richness, fungi, arbuscular mycorrhizal fungi, Gram-positive bacteria, Gram-negative bacteria, and actinomycetes. Ratios of saturated:monounsaturated and iso:anteiso PLFAs decreased across the silty clay loam chronosequence indicating reduction in nutrient stress of the microbial community as grassland established. Multivariate analysis of entire PLFA profiles across the silty clay loam chronosequence showed recovery of microbial community structure on the trajectory toward native prairie. Conversely, no microbial groups exhibited a directional change across the loamy fine sand chronosequence. Changes in soil structure were also only observed across the silty clay loam chronosequence. Aggregate mean weighted diameter (MWD) exhibited an exponential rise to maximum resulting from an exponential rise to maximum in the proportion of large macroaggregates (>2000 μm) and exponential decay in microaggregates (<250 μm and >53 μm) and the silt and clay fraction (<53 μm). Across both chronosequences, MWD was highly correlated with total PLFA biomass and the biomass of many microbial groups. Strong correlations between many PLFA groups and the MWD of aggregates underscore the interdependence between the recovery of soil microbial communities and soil structure that may explain more variation than time for some soils (i.e., loamy fine sand). This study demonstrates that soil microbial responses to grassland restoration are modulated by soil texture with implications for estimating the true capacity of restoration efforts to rehabilitate ecosystem functions.