Field soil aggregate stability kit for soil quality and rangeland health evaluations

Soil aggregate stability is widely recognized as a key indicator of soil quality and rangeland health. However, few standard methods exist for quantifying soil stability in the field. A stability kit is described which can be inexpensively and easily assembled with minimal tools. It permits up to 18 samples to be evaluated in less than 10 min and eliminates the need for transportation, minimizing damage to soil structure. The kit consists of two 21×10.5×3.5 cm plastic boxes divided into eighteen 3.5×3.5 cm sections, eighteen 2.5-cm diameter sieves with 1.5-mm distance openings and a small spatula used for soil sampling. Soil samples are rated on a scale from one to six based on a combination of ocular observations of slaking during the first 5 min following immersion in distilled water, and the percent remaining on a 1.5-mm sieve after five dipping cycles at the end of the 5-min period. A laboratory comparison yielded a correlation between the stability class and percent aggregate stability based on oven dry weight remaining after treatment using a mechanical sieve. We have applied the method in a wide variety of agricultural and natural ecosystems throughout western North America, including northern Mexico, and have found that it is highly sensitive to differences in management and plant community composition. Although the field kit cannot replace the careful laboratory-based measurements of soil aggregate stability, it can clearly provide valuable information when these more intensive procedures are not possible.     

Tillage and drainage impact on soil quality: I. Aggregate stability, carbon and nitrogen pools

Effects of two tillage treatments, tillage (T) with chisel plough and no-till (NT), were studied under un-drained and drained soil conditions. Soil physical properties measured were bulk density (ρ_b), total porosity (ƒ_t), water stable aggregates (WSA), geometric mean diameter (GMD), mean weight diameter (MWD), organic carbon (OC) and total N concentrations in different aggregate size fractions, and total OC and N pools. The experiment was established in 1994 on a poorly drained Crosby silt loam soil (fine mixed, mesic, Aeric Ochraqualf) near Columbus, Ohio. In 2007, soil samples were collected (0–10, 10–20, and 20–30 cm) from all treatments and separated into six aggregate size classes for assessing proportions of macro (5–8, 2–5, 1–2, 0.5–1, 0.25–0.5) and micro (<0.25 mm) aggregates by wet sieving. Tillage treatments significantly (P ≤ 0.05) influenced WSA, MWD, and GMD. Higher total WSA (78.53 vs. 58.27%), GMD (0.99 vs. 0.68 mm), and MWD (2.23 vs. 0.99 mm) were observed for 0–10 cm depth for NT than T treatments. Relative proportion of macro-aggregates (>0.25-mm) was also more in NT than T treatment for un-drained plots. Conversely, micro-aggregates (<0.25-mm) were more in T plots for both drained and un-drained treatments. The WSA, MWD and GMD decreased with increase in soil depth. The OC concentration was significantly higher (P ≤ 0.05) in NT for un-drained (P ≤ 0.01) treatment for all soil depths. Within macro-aggregates, the maximum OC concentrations of 1.91 and 1.75 g kg⁻¹ in 1–2 mm size fraction were observed in NT for un-drained and drained treatments, respectively. Tillage treatments significantly (P < 0.01) affected bulk density (ρ_b), and total porosity (f_t) for all soil depths, whereas tillage × drainage interaction was significant (P < 0.01) for 10–20 and 20–30 cm depths. Soil ρ_b was negatively correlated (r = −0.47; n = 12) with OC concentration. Tillage treatments significantly affected (P ≤ 0.05) OC pools at 10–20 cm depth; whereas drainage, and tillage × drainage significantly (P ≤ 0.05) influenced OC pools for 0–10 cm soil layer. The OC pool in 0–10 cm layer was 31.8 Mg ha⁻¹ for NT compared with 25.9 Mg kg⁻¹ for T for un-drained treatment. In comparison, the OC pool was 23.1 Mg ha⁻¹ for NT compared with 25.2 Mg ha⁻¹ for T for the drained plots. In general, the OC pool was higher in NT system, coupled with un-drained treatment than in drained T plots. The data indicate the importance of NT in improving the OC pool.     

Earthworm avoidance test for soil assessments

For ecotoxicological assessments of contaminated or remediated soils pointing to the habitat function of soils for biocenoses, standardized tests with earthworms (acute test, reproduction test) are available among others. Tests used for routine applications should be sensitive and indicate impacts on test organisms after short test periods. The usually applied earthworm tests do not satisfactorily fulfil these criteria. Therefore, in the present work, a behavioural test with earthworms (test criterion: avoidance) was investigated in detail using uncontaminated, artificially contaminated and originally contaminated soils. It was demonstrated that the avoidance behaviour is primarily determined by pollutants, and not by chemical-physical soil properties. The sensitivity of the presented test reaches the sensitivity of established tests. For waste sites, a considerably higher sensitivity was determined. An avoidance behaviour of at least 80% of the worms leaving the soil to be assessed is proposed as a criterion for toxicity.     

Aggregate stability and microbial community dynamics under drying-wetting cycles in a silt loam soil

Aggregate stability often exhibits a large inter-annual and seasonal variability which occurs regardless of residue treatments and is often larger than the differences between soils or cropping systems. Variations in soil moisture and seasonal stimulation of microbial activity are frequently cited as the major causes. The goal of this paper was to evaluate the effects of drying-rewetting cycles on aggregate stability and on its main microbially mediated agents from a mechanistic point of view. The 3-5 mm aggregates of a silty soil were incubated at 20 °C for 63 days with the following treatments and their combinations: (i) with or without straw input and (ii) with or without exposure to four dry-wet cycles. Microbial activity was followed by measuring the soil respiration. We estimated the microbial agents of aggregate stability measuring hot-water extractable carbohydrate-C, microbial biomass carbon and ergosterol content. We measured the water drop penetration time to estimate the hydrophobicity and aggregate stability according to Le Bissonnais [1996. Aggregate stability and assessment of soil crustability and erodibility: I. Theory and methodology. European Journal of Soil Science 47, 425-437] to distinguish three breakdown mechanisms: slaking, mechanical breakdown and microcracking. The addition of straw stimulated microbial activity and increased the resistance to the three tests of aggregate stability, enhancing the internal cohesion and hydrophobicity of aggregates. All the estimated microbial agents of aggregate stability responded positively to the addition of organic matter and were highly correlated with aggregate stability. Fungal biomass correlated better with aggregate stability than total microbial biomass did, showing the prominent role of fungi by its triple contribution: physical entanglement, production of extracellular polysaccharides and of hydrophobic substances. Dry-wet cycles had less impact on aggregate stability than the addition of straw, but their effects were more pronounced when microbial activity was stimulated demonstrating a positive interaction.     

Aggregate stability and physical protection of soil organic carbon in semi‐arid steppe soils

Spatial inaccessibility of soil organic carbon (SOC) for microbial decay within soil aggregates is an important stabilization mechanism. However, little is known about the stability of aggregates in semiarid grasslands and their sensitivity to intensive grazing. In this study, a combined approach using soil chemical and physical analytical methods was applied to investigate the effect of grazing and grazing exclusion on the amount and stability of soil aggregates and the associated physical protection of SOC. Topsoils from continuously grazed (CG) and ungrazed sites where grazing was excluded from 1979 onwards (UG79) were sampled for two steppe types in Inner Mongolia, northern China. All samples were analysed for basic soil properties and separated into free and aggregate‐occluded light fractions (fLF, oLF) and mineral‐associated fractions. Tensile strength of soil aggregates was measured by crushing tests. Undisturbed as well as artificially compacted samples, where aggregates were destroyed mechanically by compression, were incubated and the mineralization of SOC was measured. For undisturbed samples, the cumulative release of CO₂‐C was greater for CG compared with UG79 for both steppe types. A considerably greater amount of oLF was found in UG79 than in CG soils, but the stabilities of 10–20‐mm aggregates were less for ungrazed sites. Compacted samples showed only a slightly larger carbon release with CG but a considerably enhanced mineralization with UG79. We assume that the continuous trampling of grazing animals together with a smaller input of organic matter leads to the formation of mechanically compacted stable ‘clods’, which do not provide an effective physical protection for SOC in the grazed plots. In UG79 sites, a greater input of organic matter acting as binding agents in combination with an exclusion of animal trampling enhances the formation of soil aggregates. Thus, grazing exclusion promotes the physical protection of SOC by increasing soil aggregation and is hence a management option to enhance the C sequestration potential of degraded steppe soils.     

Aggregate stability as an indicator of soil susceptibility to runoff and erosion; validation at several levels

The evaluation of soil susceptibility to runoff and water erosion in the field is often expensive or time-consuming. Several authors have reported that susceptibility is linked to aggregate stability, whose determination is far easier. However, this susceptibility has generally been deduced from rainfall simulation experiments on sieved soil samples, whose behaviour is not always representative of field-scale phenomena. Our aim was to extend the validity of relationships between soil aggregation and erosion through comparisons of topsoil aggregate stability and field-assessed susceptibility to runoff and erosion.Susceptibility to runoff and erosion was determined at several levels: first, on a southern French Regosol, through measurements of runoff and soil loss from 1-m² microplots under simulated rainfall; second, from 100- to 800-m² runoff plots on a Nitosol in Benin, a Ferralsol in Cameroon and a Regosol in Mexico (with additional data on Syria from the literature); and finally, soil susceptibility to erosion was determined through semi-quantitative assessment of the frequency of erosion features on vineyard hillsides in southern France. Aggregate stability was determined by immersion in water and wet-sieving of 2-mm sieved, air-dried 0- to 10-cm soil samples, which actually tests aggregate resistance to slaking.Under simulated rainfall, runoff depth and soil loss after 30 min were negatively correlated with topsoil content in stable macroaggregates (>0.2 mm). On runoff plots, 3-year runoff rate and soil loss were negatively correlated with topsoil aggregate stability, especially stable macroaggregate content; these correlations were improved when slope gradient and climate aggressiveness were considered in addition to aggregate stability. On vineyard hillsides, the frequency index of erosion features was negatively correlated with topsoil content in stable macroaggregates, especially in the absence of conservation practices.These results confirm that aggregate stability is a relevant indicator of soil susceptibility to runoff and erosion, especially in Mediterranean and tropical areas where intense rainfall is frequent. They also confirm that simple laboratory determination can provide data closely correlated with those resulting from field investigations.     

Aggregate stability of a silt loam soil as affected by roots of corn,soybeans and wheat

Abstract

A greenhouse experiment was conducted to investigate the effect of root growth and exudation of 3 crop species on soil aggregation. Two plant populations for each of 3 crops (corn, soybeans, and wheat) were grown in a Fincastle silt loam for 5 time periods (7, 14, 21, 28, and 41 days) and compared with fallow controls. Aggregate stability was estimated by the wet‐sieve method on both initially moist and air‐dry samples.

Soil water content of initially moist soil samples varied widely among replicates, crops, and sampling dates. Wet‐sieving using initially moist soil showed that samples with higher initial soil water content had greater aggregate stability. Wet‐sieving performed on initially air‐dry soil samples was used for subsequent interpretation because the water content variable was removed.

The presence of any crop and its roots in the planted soils versus the fallow controls was associated with increases in aggregate stability. No differences in aggregate stability were found among the different crops or over the established range of root length densities. Aggregate stability decreased from the original level during the first 14 to 21 days of the experiment, possibly due to daily watering. After 21 days, as root growth continued to increase, restabilization occurred until the original aggregate stability of the soil was exceeded for all crops. The observed increase in aggregate stability may be due in part to the physical entanglement of aggregates by roots and to the increased production of root exudates resulting from increased root growth.     

Aggregate stability in organically and conventionally farmed soils

A range of factors that influence aggregate stability and soil erodibility were analysed for soils sampled from land managed under contrasting agricultural methods. These included: an organic farm; a conventional farm that incorporated organic fertilizers; a conventional farm that only used inorganic fertilizers; and a non-cultivated control site. The stability of aggregates that compose the bulk soil structure (macroaggregates), and aggregates that were mobilized from the soil by simulated rainfall and surface runoff (microaggregates), were evaluated in terms of the soil fragmentation fractal dimension, organic carbon content and ATP (adenosine 5'-triphosphate; a signature of live biomass) concentration. The results were used to interpret the existing physical condition of the soils, the (microbial) processes that contribute to that physical structure, and how both pedogenic processes and existing soil quality are influenced by agricultural methods. The soils sampled for this study were demonstrated to be multi-fractal in nature: soils with greater bulk density were composed of more stable macro-aggregates, which, in turn, fragmented into larger, more stable micro-aggregates, rendering the entire soil structure less erodible. Soil erodibility and sustainable soil management should therefore be approached at multiple scales. The primary control on both macro- and micro-aggregate stability was determined to be the organic matter input to the soil, as represented by measurements of organic carbon and ATP. Organic content was greatest for the non-cultivated soil, which reflects the degradation of organic reserves in cultivated soils. For cultivated soils, it was not possible to differentiate aggregate stability for soils managed under organic or conventional (i.e. using biological and inorganic fertilizers) farming practices, but aggregates of soils that only received artificial fertilizers consistently exhibited less stability.     

A soil fauna index for assessing soil quality

The characteristics of the faunal community in soil are closely related to soil quality. Using the soil faunal community as an indicator of soil quality is not cost-effective because of the complex taxonomic distinction and identification required, moreover; the power of the assessment is weak in relation to soil function. Recently, a functional method incorporating eco-morphological traits has been proposed, but it depends upon presence/absence data of soil arthropod community exclusively. To overcome the limitation, we designed a novel index using the diversity of the soil faunal community along with its functional traits, and the abundance of its members (Abundance-based Fauna Index, referred as FAI). The FAI method supports both the presence/absence data and its abundance. Using real and simulated data sets with eco-morphological and niche breadth traits, two taxonomic independent functional traits, we tested associations of FAI values with soil quality and found a good relationship. In addition to its usefulness for measuring soil quality, FAI may provide valuable information for linking functional traits of soil fauna and the belowground environmental condition.     

Microbial indicators for soil quality

The living soil is instrumental to key life support functions (LSF) that safeguard life on Earth. The soil microbiome has a main role as a driver of these LSF. Current global developments, like anthropogenic threats to soil (e.g., via intensive agriculture) and climate change, pose a burden on soil functioning. Therefore, it is important to dispose of robust indicators that report on the nature of deleterious changes and thus soil quality. There has been a long debate on the best selection of biological indicators (bioindicators) that report on soil quality. Such indicators should ideally describe organisms with key functions in the system, or with key regulatory/connecting roles (so-called keystone species). However, in the light of the huge functional redundancy in most soil microbiomes, finding specific keystone markers is not a trivial task. The current rapid development of molecular (DNA-based) methods that facilitate deciphering microbiomes with respect to key functions will enable the development of improved criteria by which molecular information can be tuned to yield molecular markers of soil LSF. This review critically examines the current state-of-the-art in molecular marker development and recommends avenues to come to improved future marker systems.     

Aggregate characterization as compared to soil bulk properties

The aim of this paper is to clarify the effect of soil aggregation on the physical and chemical properties of structured soils and as compared with the homogenized material. Aggregation and aggregate strength do not only depend on biological activity and organic exudates, but also on the intensity, number and time of swelling, and drying events. Such aggregates are not only more dense than the structured bulk soil, the intra-aggregate pore distribution consists not only of finer pores, but they are also more tortuous. Thus, water and ion fluxes by mass flow as well as ion transportation by diffusion are delayed, whereby the length of the flow path in such tortuous finer pores further retards chemical exchange processes. Futhermore, the chemical composition of the percolating soil solution differs more from that of the corresponding homogenized material the stronger and denser the aggregates are. From the mechanical point of view, the strength of single aggregates, determined as the angle of internal friction and cohesion, depends on the number of contact points or the forces, which can be transmitted at each single contact point. However, internal soil parameters, like grain size distribution or chemical composition, further affect the strength. The more structured the soils are, the higher is the proportion of the effective stress on total stress, but even in single aggregates neutral stresses can be revealed. This is true because of the relationship to the smaller value of the hydraulic conductivity and higher tortuosity. Finally, some dynamic effects on aggregation and aggregate deterioration are discussed.     

Aggregate stability and glomalin in alternative crop rotations for the central Great Plains

 Land productivity, along with improvement or maintenance of soil health, must be evaluated together to achieve sustainable agricultural practices. Winter wheat-fallow (W-F) has been the prevalent cropping system in the central Great Plains for 60 years where moisture is a limitation to crop production. Alternative cropping systems show that producers can crop more frequently if residue management and minimum tillage are used. The impact of different crops, crop rotations and tillage management practices on soil quality was assessed by measuring aggregate stability and glomalin production by arbuscular mycorrhizal (AM) fungi in soil from cropping trials established in 1990. Crops were wheat (W), corn (C), proso millet (M), and sunflower (S). Rotations sampled were W-F, W-C-M, W-C-M-F, W-C-F, and W-S-F. In the same area as the cropping trials, soils were taken from a perennial grass (crested wheatgrass) and from a buffer area that had been planted to Triticale for the past 2 years but prior to that had been extensively plowed for weed control. We found that aggregate stability and glomalin were linearly correlated (r=0.73, n=54, P<0.001) across all treatments sampled. Highest and lowest aggregate stability and glomalin values were seen in perennial grass and Triticale soils, respectively. Aggregate stability in W-S-F was significantly lower than in the other crop rotations (P≤0.03), while W-C-M had significantly higher glomalin than the other rotations (P<0.05). Differences between crop rotations and the perennial grass indicate that selected comparisons should be studied in greater detail to determine ways to manage AM fungi to increase glomalin and aggregate stability in these soils. Received: 16 March 1999     

Aggregate breakdown during tillage in a Mediterranean loamy soil

Long-term tillage negatively affects soil aggregation, but little is known about the short-term effects of tillage. We investigated the effects of intensive tillage (moldboard plowing) and conservation tillage (chisel plowing) on aggregate breakage during tillage in a long-term study located in the semiarid Ebro river valley (NE Spain). The type of tillage resulted in different soil aggregate distributions. In the 0–5-cm and 5–10-cm soil layers, chisel plowing decreased dry mean weight diameter (DMWD) 29% and 35%, respectively, while moldboard plowing decreased DMWD by only 2% and 16%, respectively. The decrease in DMWD was mainly due to breaking of large aggregates ranging (2–8 mm) into small aggregates (<0.5 mm). Tillage method had no effect on water stability of 1–2 mm aggregates. The differences in DMWD demonstrate that the choice of the tillage implement can be a key factor in improving soil management and productivity. The surprising result that aggregate breakdown was greater with chisel than moldboard plowing needs further research to determine the mechanisms controlling aggregate breaking during tillage.     

Aggregate breakdown dynamics under rainfall compared with aggregate stability measurements

Aggregate breakdown due to rainfall action causes crusting and interrill erosion. Erodibility is seemingly determined by the capacity of surface aggregates to resist the effects of rainfall. In this paper, we evaluated the relevance of an aggregate stability measurement, which comprises three treatments, in order to characterize aggregate breakdown dynamics. Two cultivated soils were studied: a clay loam slightly sensitive to erosion and a more susceptible silt loam. We compared the size distributions of microaggregates produced by the three aggregate stability treatments with the results from a rainfall simulation. The behaviour of four initial aggregate size classes (< 3 mm, 3–5 mm, 5–10 mm and 10–20 mm) was also compared to study the influence of the initial aggregate size on the nature of resulting aggregates. The mean weight diameter was from 200 to 1400 µm for the silt loam and from 600 to 7000 µm for the clay loam. The two experiments – aggregate stability measurements and aggregate breakdown dynamics under rainfall – yielded similar results. Qualitative analysis showed that for both soils the sizes of fragments produced by breakdown with the aggregate stability tests and under rainfall were similar and seemed to be qualitatively independent of the size of initial aggregates. We first schematized the structural organization of aggregates in cultivated horizons with a simple hierarchical model at two levels: (i) < 250 µm microaggregates and (ii) > 250 µm macroaggregates made by the binding together of microaggregates. We then developed a model of aggregate breakdown dynamics under rainfall which gives, for various rainfall durations, the size distributions of resulting fragments on the basis of aggregate stability measurements. We obtained a correlation coefficient, r, of 0.87 for the silt loam and of 0.91 for the clay loam, showing that the experimental and predicted mass percentages were linearly related for each size fraction.     

土壤质量指标和评价方法

土壤质量是反映土壤保持生物生产力、环境质量以及动植物健康能力的土壤的内在属性。土壤质量评价是根据土壤内部属性对土壤综合状况进行表达。评价必须确定合适的时间和空间尺度,综合量化分析指标和不可量化的描述性指标。评价方法包括定性评价和定量评价,系统化的定性评价仍有其现实意义。定量评价要选择指标的最小数据集(MDS),采用最小数据集(MDS) 土壤变换函数(PTF)的方法将土壤质量指标和土壤功能相联系,对土壤的各个功能因子进行评价,在此基础上利用各种数学评价方法得到综合的量化土壤质量评价结果。     

Application of soil enzyme activity test kit in a field experiment

We developed a test kit utilizing fluorogenic substrate analogues for the measurement of several enzyme activities in soil samples. Our hypothesis was that the pattern of different enzyme activities reflects changes in microbial structure and activity in a more sensitive way than total microbial biomass or activities of individual enzymes. In the test kit activities of 11 soil enzymes were measured simultaneously from diluted soil slurries on multiwell plates. After incubation for 3 h the fluorescence was measured quantitatively using an automated plate reader. The test kit was evaluated in a field experiment designed to investigate various management practices. During the years 1981–1993 the experimental field was divided into four sites. One of the plots received full mineral fertilization on the basis of chemical soil analysis, another received 50% of the recommended mineral fertilization, the third plot received green manure (harvested crop residues) and the fourth plot received composted crop residues. Since 1994 the sampling area included sites in a transitional stage to organic cultivation and the minerally fertilized site. Season, crop plant and management practice all gave rise to differences in the enzyme activity profile. The fluorogenic analysis was sensitive and allowed quantification of all the enzyme activities in all the samples. When four replicate measurements were used the standard deviation was usually less than 10%, depending on the enzyme.     

Combining pitfall traps and soil samples to collect Collembola for site scale biodiversity assessments

Collembola are rarely included in landscape-level biodiversity assessments, large-scale surveys and monitoring projects because huge numbers of specimens would accumulate even in moderately sized programmes. Budgets are always limited, so sampling methods and identification need to be optimized. As no single sampling method collects all collembolan species equally well, we tested the efficiency of a combination of six pitfall traps and five soil subsamples in 30 oil seed rape fields in Eastern Austria. Work effort in man hours for sampling, sorting and identification was quantified for each method and related to the species richness of the collected fauna. Total identification effort was four times higher for the soil subsamples than the pitfall traps, however, soil samples also yielded more species (53 and 34, respectively). Out of the 70 species collected in total, an average of thirteen species per site was found in the pitfall samples, seventeen in the soil subsamples and 25 when combining the two methods. Using more than six pitfall samples alone would not have collected considerably more species. For the soil subsamples, still more species can be expected with the processing of more than five subsamples, but this would also result in higher costs. When including Collembola in large scale biodiversity assessments, surveys or monitoring projects, we therefore recommend combining the two methods. In combination, the identification of the catch from only two pitfalls and two soil subsamples already collected more than the average number of species in five soil subsamples or six pitfalls, respectively. Thus, combining the methods yielded a more complete picture of the collembolan community of a site than either method alone.