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.     

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.     

The effect of soil water content and microbial activity on restoring the structure of a Vertisol

Soils with high clay content are susceptible to structural damage, if they are intensively cultivated. The structure of soils of the Vertisol group has the tendency to restore as a result of wetting and drying. Samples of a remoulded clayey soil were exposed to seven successive wetting/drying (w/d) cycles, in order to study the change of some structural features of the newly formed aggregates. To assess the effect of soil water content and microbial activity, two different matric water potential ranges were used under sterile and non-sterile conditions.

Aggregate size distribution depended on both the water potential range and microbial activity and approached to a steady state with increasing cycle number. The water stability of the 2–1 mm aggregates was affected by the activity of soil micro-organisms under wet conditions and by forces of mechanical nature when the soil was reaching dryness. All cases resulted in aggregates of reduced water stability with respect to the natural aggregates. The <50 μm aggregated particles initially preserved stability, but after a number of w/d cycles they collapsed at a rate, depending mainly on the water potential range.     

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.     

冻融循环作用对黑土团聚体破碎机制的影响

为探究冻融循环作用对黑土团聚体抵抗不同破坏作用能力的影响,以黑龙江省典型黑土耕作层土壤为研究对象,通过LB法的3种不同破碎处理,分析冻融循环次数、初始含水量对不同初始粒级土壤团聚体稳定性的作用机制。结果表明:冻融循环作用对1～2,2～3,3～5 mm不同粒级团聚体稳定性有显著影响(P0.05),其稳定性均呈现为湿润振荡(WS)慢速湿润(SW)快速湿润(FW),快速湿润(FW)处理对黑土团聚体稳定性的破坏作用最大,而慢速湿润(SW)和湿润振荡(WS)破坏作用较小。同时,初始含水量和冻融循环次数对3种不同粒级团聚体的快速湿润平均重量直径(MWD_(FW))、慢速湿润平均重量直径(MWD_(SW))、湿润振荡平均重量直径(MWD_(WS))均具有极显著影响(P0.01)。随着冻融循环交替次数的增加,0.2 mm的大团聚体破碎而小团聚体(0.2 mm)含量增加。初始含水量对于2～3,3～5 mm粒级团聚体具有破坏作用,而对1～2 mm粒级团聚体有助于改良团聚体结构,增大团聚体稳定性。     

Investigating the effects of wettability and pore size distribution on aggregate stability: the role of soil organic matter and the humic fraction

Soil organic matter (SOM) is an important factor influencing aggregate stability. Interactions between SOM and soil structure are widely studied, although the subtle relationship between SOM content, pore size distribution and aggregate stability is not fully known. Here we investigate such a relationship by means of a long‐term experiment established in 1962 in northeastern Italy, which considers different fertilizer practices (organic, mineral and mixed) applied to a continuous maize crop rotation. We measured wet stability of 1–2 mm aggregates subjected to different pretreatments. Both soil physical properties (such as pore size distribution and hydrophobicity) and chemical properties (soil organic and humic carbon content) affecting aggregate stability were considered. The chemical structure of humic substances was characterized by thermal and spectroscopic analyses (TG‐DTA, DRIFT and ¹H HR MAS NMR). The Pore‐Cor network model was then applied to evaluate the contribution of hydrophobicity and porosity to aggregate wetting. Our study suggests that SOM and its humic fraction can affect aggregate wetting and consequently slaking by modifying the pore size distribution with a shift from micropores (5–30 µm) and mesopores (30–75 µm) to ultramicropores (0.1–5 µm); hydrophobicity was also increased as a result of different humic composition. Spectroscopic analysis showed that hydrophobic compounds were mostly associated with complex humic molecules. Models of fast wetting dynamics, however, suggest that the contribution that hydrophobicity makes to aggregate stability, especially to soils with large carbon inputs, may not be the most significant factor.     

Mechanisms of aggregate stabilization in Ultisols from subtropical China

The quantification and interpretation of aggregate stability depend on internal soil properties and external factors such as measurement method and aggregate size. The objectives of this study were to: (i) determine the aggregate stability in Ultisols from subtropical China applying the Le Bisssonais Method; (ii) determine the effect of initial aggregate size on its stability, and (iii) interpret mechanisms of aggregate stabilization in the soils. Three aggregate-size ranges (5–3, 3–2 and 2–1 mm) were obtained by dry sieving. After the wetting treatments, the dominant fraction of fragments for each soil was 2–1 mm or 0.63–0.2 mm. The mechanisms of aggregate breakdown was in the order, slaking>mechanical breakdown>micro-cracking. They differed with soil type and composition. The normalized mean weight diameter (NMWD) of the aggregates after fast wetting and wet stirring were more correlated with soil properties, such as degree of micro-aggregation (DOA), cation exchange capacity (CEC), K₂O, Fe₂O₃ or Al₂O₃ rather than clay and soil organic carbon (SOC) content. The binding force by soil organic matter was smaller than the force caused by entrapped air or the force of combination of mechanical stress by stirring and differential swelling of minerals.The smaller the aggregate, the larger was the aggregate stability according to NMWD. The rankings of the soils differed with the soil aggregate sizes and the wetting treatments. Sandy loams from sandstone (Sc and Sw) were the weakest soils while the purple mudstone (Pp) was the strongest. All the cultivated soils decreased in aggregate stability compared with the comparable uncultivated soils or parent materials irrespective of the cultivation time and the changes in SOC content after cultivation.     

鄂南第四纪粘土红壤团聚体的稳定性及其稳定机制初探

用湿筛法和LeBissonnais法研究了鄂南第四纪红粘土母质发育的两种侵蚀程度的红壤团聚体的稳定性,并且分析了影响供试土壤团聚体稳定性的土壤性质。结果表明,轻度侵蚀的耕作土壤团聚体的稳定性较低,在水的作用下易崩解成较小粒径的水稳性团聚体;强度侵蚀的土壤表层团聚体的稳定性较高,崩解后产生较多的水稳性大团聚体。引起土壤团聚体破坏的主要作用机制是土壤团聚体中的闭蓄空气爆破引起的消散作用;研究区第四纪红壤团聚体的主要胶结物质是土壤中的粘粒、游离氧化铁铝和无定形铁。由于供试土壤中有机质含量很低,在本研究中,有机质含量与土壤团聚体稳定性之间没有显著正相关关系。     

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.     

干湿交替作用对西南地区黄壤团聚体稳定性的影响

为了探究我国西南地区干湿交替作用对土壤团聚体稳定性的影响,选取重庆缙云山典型黄壤为研究对象,在4个前期含水率(风干,10%,15%,20%)水平下,对4组不同粒级团聚体(1~2,2~3,3~5,5~7mm)分别进行7个不同干湿交替过程(1,2,3,5,7,10,15次)模拟。采用Le Bissonnais法,对干湿交替作用后的团聚体在不同破碎机制下的稳定性特征进行了探讨。结果表明:(1)快速湿润(FW)对团聚体稳定性的破坏程度最大,且与机械扰动(ST)和慢速湿润(SW)存在显著差异;(2)小粒径团聚体相较于大粒径团聚体稳定性更高;(3)干湿交替过程对团聚体存在明显的破坏作用,且在不同含水率变化范围下其破坏程度不同;(4)干湿交替作用主要通过影响团聚体破碎后2mm团聚体的百分比含量来影响团聚体的稳定性。该结果对于研究西南地区土壤侵蚀的发生机理具有一定的参考价值。     

Particulate organic matter in water stable aggregates formed after the addition of C-labeled maize residues and wetting and drying cycles in vertisols

Development of soil structure and the dynamics of water stable aggregates (WSA) in many soils are known to be closely related to the cycling of soil organic matter. In some fine and medium textured soils particulate organic matter (POM) has been found to act as a nucleus for macroaggregate formation. However, this role of POM in aggregate formation has not been demonstrated in soils dominated by smectitic clay minerals. This study explored aggregation processes in a Vertisol from a semi-arid region in Northeastern Mexico in relation to the addition of ¹⁴C-labeled maize residues and application of wetting and drying cycles during 105 days of incubation. Fractionation of the WSA formed showed that labeled residues were preferentially accumulated in large macroaggregates (>2000 μm). Treatments with addition of organic residues had three to four times more intra-aggregate particulate organic matter (iPOM) in large macroaggregates than the control after 14 days of incubation. Residue-derived carbon accounted for 53% and 41% of the total carbon stored in the iPOM fraction in amended treatments with and without wetting and drying cycles, respectively. Conversely, residue-derived carbon represented <20% of the total carbon in the iPOM fraction from small macroaggregates (250-2000 μm) and microaggregates (53-250 μm). Results also showed that the amount and concentration of carbon per large macroaggregate did not differ between the large macroaggregates formed under wetting and drying and those formed in continuous moist conditions. However, due to formation of higher number of large macroaggregates per kg of soil, more carbon could be stored in amended soils under wetting and drying than in constantly wet soil: 1.4, 1.8 and 2.7 times more ¹⁴C kg⁻¹ soil after 14, 58 and 105 incubation days, respectively. The results in this study suggest that wetting and drying enhanced protection of the added maize residues inside large macroaggregates by forming more aggregates, rather than by increasing the amount of POM entrapped per aggregate. Therefore, after the addition of organic residues, this soil could accumulate more C than continuous moist soil through the influence that wetting and drying has on soil aggregation.     

Physical response of rigid and non‐rigid soils to analogues of biological exudates

Exudates produced by plants and microorganisms can alter greatly the physical behaviour of soil. There is limited research that quantifies directly the underlying hydrological and mechanical mechanisms concerned, and so in this study we amended soils with a range of analogue biological exudate compounds with different physical and chemical properties: polygalacturonic acid (PGA), dextran, xanthan and lecithin. These were added to a structurally rigid soil (Plinthosol) and a non‐rigid soil (Gleysol) that were formed as repacked cores and exposed to five cycles of wetting and drying (WD). Aggregate stability, tensile strength, water sorptivity and water repellency were measured initially and after the first, third and fifth WD cycle. Improved aggregate stability was only found for some exudates and differed between the soils. Xanthan had the greatest impact on aggregate stability, causing a 95% increase in the Plinthosol and 75% increase in the Gleysol. Xanthan also caused the greatest increase in tensile strength (50% in the Plinthosol and 148% in the Gleysol) but had minimal impact on water repellency in both soils, indicating mechanical stabilization. Lecithin reduced tensile strength but caused the greatest increase in water repellency, indicating hydrological stabilization. Both PGA and dextran had clear positive impacts on soil stability, but the underlying processes were not detected in the hydrological and mechanical tests. Increasing the number of WD cycles diminished aggregate stability, tensile strength and water repellency more rapidly in the non‐rigid Gleysol than in the rigid Plinthosol. This study demonstrated that the effects of analogous biological exudates on aggregation and stabilization depend on the nature of exudate, the rigidity of soil structure and the number of WD cycles.     

Changes of structure and tilth mellowing in a Vertisol due to wet/dry cycles in the liquid and vapour phases

Development of a fine tilth in Vertisols increases infiltration, plant-available water and ease of cultivation and produces a fine seed bed. The tilth-mellowing properties of a strongly self-mulching Vertisol from Zimbabwe were investigated by applying different types of wetting to a worked soil and examining macromorphological features, size, density, strength and friability of the resulting clods/aggregates, developed through successive wet/dry cycles. Wetting regimes were chosen to simulate likely field conditions and included rapid flood-, slow and fast capillary-, simulated rainfall- and vapour-wetting. Tilth development was compared to that of field soils. All wetting treatments in the liquid phase resulted in decreases in aggregate density. Fast capillary wetting rapidly reduced size and strength of aggregates to below that of field soils whereas slow capillary wetting similarly rapidly decreased size but reduced strength more slowly. Flood wetting caused little change in size but aggregates showed a small decrease in strength. Rainfall wetting resulted in changes intermediate between these extremes. There was a significant linear relationship between strength and porosity of aggregates! For rainfall- and flood-wetting, friabilities were at a maximum after one wet/dry cycle but subsequently decreased. Vapour wet/dry cycles reduced strength but not density of worked soils, implying changes in internal microstructure without measurable porosity change. Hypotheses to explain these changes are put forward.     

Biological and physical resilience of soil amended with heavy metal-contaminated sewage sludge

Stability and resilience of a variety of soil properties and processes are emerging as key components of soil quality. We applied recently developed measures of biological and physical resilience to soils from an experimental site treated with metal‐contaminated sewage sludge. Soils treated with cadmium‐, copper‐ or zinc‐contaminated, digested or undigested sewage sludge were studied. Biological stability and resilience indices were: (i) the time‐dependent effects of either a transient stress (heating to 40°C for 18 hours) or a persistent stress (amendment with CuSO₄) on decomposition, and (ii) the mineralization of dissolved organic carbon (DOC) released by drying–rewetting cycles. Physical stability and resilience measures were: (i) compression and expansion indices of the soils, and (ii) resistance to prolonged wetting and structural regeneration through drying–rewetting cycles. Soil total carbon and DOC levels were greater in the sludge‐amended soils, but there were no differential effects due to metal contamination of the sewage sludge. Effects of metals on physical resilience were greater than effects on soil C, there being marked reductions in the expansion indices with Cd‐ and Cu‐contaminated sludge, and pointed to changes in soil aggregation. The rate of mineralization of DOC released by drying and wetting was reduced by Zn contamination, while biological resilience was increased in the Zn‐contaminated soil and reduced by Cd contamination. We argue that physical and biological resilience are potentially coupled through the microbial community. This needs to be tested in a wider range of soils, but demonstrates the benefits from a combined approach to the biological and physical resilience of soils.     

Changes in soil aggregate water stability induced by wetting and drying cycles in non-saturated soil

Wetting and drying of remoulded soil resulted in water stable aggregation. The greatest proportions of water stable aggregates arose from wetting and drying in the ?1 to ?100 kPa range of matric water potential. The effect occurred with sterile and non-sterile soil. but the proportion of water stable aggregates was less with sterile soil. The application of wetting and drying cycles in the laboratory to non-tilled soil resulted in a steady decrease in the proportion of water stable aggregates. With tilled soil, the proportion of water stable aggregates first increased to a maximum and then decreased steadily with further wetting and drying cycles. However. with sterilized, tilled soil, only a steady decrease in the proportion of water stable aggregates was observed. Natural water content fluctuations in the field after tillage gave an increase in water stability to a maximum after a few days followed by a steady decrease. The similarity of this result to that obtained in the laboratory for tilled. non-sterilized soil indicates that micro-organisms were probably contributing to the observed short-term changes in the water stability of aggregates in the field.     

The influence of organic matter on aggregate stability in some British soils

The stability of aggregates from 26 soils selected from agricultural areas was measured by wet-sieving and the results correlated with sand, silt, clay, nitrogen, organic matter and iron contents and with cation exchange capacity. Highly significant correlations were obtained for the relationships between aggregate stability and organic matter and some properties associated with it. No other soil constituent investigated had a significant relationship with aggregate stability, indicating that organic matter is mainly responsible for the stabilization of aggregates in these soils. The relationships between aggregate stability, and organic matter content plus some of its component fractions were examined in more detail using 120 soils. Total organic matter, total carbohydrate and humic material extracted by various reagents each gave highly or very highly significant correlations with aggregate stability. However, whilst it was not possible to distinguish whether any one organic component was more important than another, the results indicate that soil organic matter levels can be used diagnostically to identify soils which may show problems of structural instability.     

土壤理化性质异质性研究及其影响

Structured soils are characterized by the presence of inter- and intra-aggregate pore systems and aggregates, which show varying chemical, physical, and biological properties depending on the aggregate type and land use system. How far these aspects also affect the ion exchange processes and to what extent the interaction between the carbon distribution and kind of organic substances affect the internal soil strength as well as hydraulic properties like wettability are still under discussion. Thus, the objective of this research was to clarify the effect of soil aggregation on physical and chemical properties of structured soils at two scales： homogenized material and single aggregates. Data obtained by sequentially peeling off soil aggregates layers revealed gradients in the chemical composition from the aggregate surface to the aggregate core. In aggregates from long term untreated forest soils we found lower amounts of carbon in the external layer, while in arable soils the differentiation was not pronounced. However, soil aggregates originating from these sites exhibited a higher concentration of microbial activity in the outer aggregate layer and declined towards the interior. Furthermore, soil depth and the vegetation type affected the wettability. Aggregate strength depended on water suction and differences in tillage treatments.     

湿润速率和粘粒含量对红壤沟间侵蚀的影响

An aggregate stability test and a simulated rainfall test were conducted on four representative Ultisols from southeastern China. The soils selected, with clay contents ranging between 117 and 580 g kg-1 , were derived from shale and Quaternary red clay. The stability of aggregates (2–5 mm in diameter) obtained from the soil samples were determined by the Le Bissonnais method. For determination of infiltration, runoff, and erosion, the soil samples were packed in 30 cm × 60 cm trays, wetted at rates of 2, 10, and 60 mm h-1 , and then exposed to simulated rainfall at 60 mm h-1 for 1 h. The results indicated that both aggregate stability and slaking caused by fast wetting increased with increasing clay content. The effect of wetting rate (WR) on infiltration and seal formation varied with clay contents. In the soil with low clay content (sandy loam), the infiltration rate was affected slightly by WR due to low aggregate stability and slaking. In the soils with medium clay content (silt clay loam and clay), WR affected infiltration significantly due to the high aggregate slaking force. In the soil with high clay content, the effect of WR on infiltration was significant, but not as evident as in the soils with medium clay content, which may be related to high aggregate stability by wetting partially compensating for slaking force. The effect of WR on soil loss was similar to that of runoff, but more pronounced. The findings from this study indicated that the relationship between wetting rate and clay content should be considered when predicting interrill erosion in Ultisols.     

The effects of wetting and drying cycles,temperature and extracting solutions on measured potassium fixation in soils of two regions of morocco

Abstract

Five representative soils with contrasting physical, chemical, and mineralogical characteristics from the Chaouia and Gharb regions of northwestern Morocco were selected for a study of the effects of wetting and drying cycles (W‐D), temperature, amount of K⁺ added, and extracting solution on the determination of K‐fixation. With drying at 40°C and the use of of 0.2 M CaCl₂ for K⁺ extractions, the measured amount of K⁺ fixed increased with increased number of W‐D cycles when high quantities of K were added. The drying, however, caused release of K⁺ when no or small amounts of K⁺ were added even for soils not containing mica (illite). With 2 W‐D cycles, measured K⁺ fixation decreased with increasing drying temperature from 40^oC to 100^oC regardless of the extracting solution when the calculated fixation was based on the initial extractable K⁺ rather than the quantity extracted from the zero treatment after wetting and drying. Also, significant differences in K⁺ fixation existed between extracting solutions at any given temperature. Because of the different drying temperatures and extracting salts used it is difficult to compare results of K⁺ fixation reported in different studies. Frequent changes in temperature and soil water content during the growing season in Mediterranean climates may have an important influence on K⁺ availability.     

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.