Water-dispersible clay after wetting and drying cycles in four Brazilian oxisols

Widespread intensive land use in the seasonal tropics can damage the physical stability of aggregates. Similar damage can be expected from wetting and drying cycles causing aggregate fragmentation and, consequently, leading to an increase in their specific area and exposure of internal electric charges. Thus, we hypothetised that the influence of wetting and drying cycles is dependent on the mineralogical composition of oxisols (latosols) and it is higher in soils with low aggregate stability. A greenhouse experiment was carried out to test this hypothesis in highly weathered soils from Brazil, all with variable-charge clays and highly stable aggregates. Wetting and drying cycles were defined from the quantity of water available between field capacity and the permanent wilting point. Soil columns were submitted to 0, 2, 6, 9, 12, 15 and 18 wetting and drying cycles. After each number of wetting and drying defined physical and chemical properties were determined. Statistical analysis, such as simple and multiple linear regression and Pearson's correlation were performed, showing significantly correlated WDC contents with wetting and drying cycles. The obtained results led to the conclusion that there was a close interdependence among mineralogical composition, aggregate stability and WDC influenced by wetting and drying cycles. Soils of reduced aggregate stability like kaolinitcs made them more susceptible to the action of wetting and drying on the WDC. Changes in the WDC with wetting and drying cycles showed correlated with eletrochemical properties.     

Nematode communities of grazed and ungrazed semi-natural steppe grasslands in Eastern Austria

Soil nematode communities were investigated at eight semi-natural steppe grasslands in the National Park Seewinkel, eastern Austria. Four sites were moderately grazed by horses, cattle and donkeys, four were ungrazed. Nematodes were sampled on four occasions from mineral soil, and their total abundance, diversity of genera, trophic structure and functional guilds were determined. Altogether 58 nematode genera inhabited the grasslands, with Acrobeloides, Anaplectus, Heterocephalobus, Prismatolaimus, Aphelenchoides, Aphelenchus, Tylenchus and Pratylenchus dominating. Mean total abundance at sites was 185–590 individuals per 100 g soil. Diversity indices did not separate communities well, but cluster analysis showed distinct site effects on nematode generic structure. Within feeding groups the relative proportion of bacterial-feeding nematodes was the highest, followed by the fungal- and plant-feeding group. Omnivores and predators occurred in low abundance. The maturity indices and plant parasite indices were characteristic for temperate grasslands, but the abundance of early colonizers (c-p 1 nematodes) was low. A high density of fungal-feeding c-p 2 families (Aphelenchoidae, Aphelenchoididae) resulted in remarkably high channel index values, suggesting that decomposition pathways are driven by fungi. Nematode community indices of all sites pointed towards a structured, non-enriched soil food web. At most sites, grazing showed little or no effect on nematode community parameters, but total abundance was higher at ungrazed areas. Significant differences in the percentage of omnivorous nematodes, the sum of the maturity index, the number of genera and Simpson's index of diversity were found at one long-term grazed pasture, and this site was also separated by multi-dimensional scaling (MDS).     

Soil microbial activity and litter turnover in native grazed and ungrazed rangelands in a semiarid ecosystem

Long-term overgrazing is known to influence soil microbiological properties and C sequestration in soil organic matter. However, much remains to be known concerning overgrazing impacts on soil microbial activity and litter turnover in heavily grazed rangelands of Central Iran. Aboveground litter decomposition of three dominant species (Agropyron intermedium, Hordeum bulbosum, and Juncus sp.) were studied using a litter bag experiment under field conditions in three range sites of Central Iran, a site with continuous grazing, a site ungrazed for 17 years with dominant woody species (80% cover), and a site ungrazed for 17 years with dominant pasture species (70% cover). Soil samples were taken from 0 to 30 cm depth and analyzed for their chemical and microbiological properties. Results demonstrate that soil organic C and total N contents and C/N ratios were similar for both ungrazed and grazed sites, while available P and K concentrations significantly decreased under grazed conditions. It was also evident that range grazing decreases soil respiration and microbial biomass C, suggesting a lower recent annual input of decomposable organic C. Nevertheless, grazing conditions had no significant effect on litter decomposition indicating soil microclimate is not affected by grazing animals in this ecosystem. It is concluded that overgrazing may presumably depress microbial activity through either reduced input of fresh plant residue into the surface soil or lack of living roots and exudates for stimulating microbial activity. This study also suggests that 17 years of livestock exclusion might be insufficient time for expected C accumulation in soil.     

Effects of wetting and drying cycles on in situ soil particle mobilization

Understanding particle mobilization and transport in soils is a major concern for environmental protection and water resource management as they can act as vectors for sorbing pollutants. In natural soils, the existence of a finite size and renewable pool of dispersible particles has been hypothesized. Even though freeze‐thaw and wetting‐drying cycles have been identified as possible mechanisms of pool replenishment between rainfall events, to date the underlying phenomena ruling the renewal of particle pools are still largely unexplored. We carried out a series of infiltration‐drainage experiments to study systematically the effects of periods without rain (pauses) on in situ particle mobilization in undisturbed soil columns. We found that, for a given column, pause duration between two rainfall events has a major influence on subsequent particle mobilization: the mass of leached particles increases with pause duration until it reaches a maximum (mass for a 200‐hours pause is 15 time greater than for a 1‐hour pause), and then it decreases for even longer pauses. This behaviour was correlated with soil water content, and can be explained by soil matrix weakening due to differential capillary stresses during drying. The consequences of this finding are important because the 15‐fold increase in mass of leached particles, when pause duration is changed from 1 hour to 4 days, might overwhelm variations caused by changes in other parameters such as the ionic strength of the incoming solution or the rainfall intensity.     

Effect of soil wetting and drying cycles on metolachlor fate in soil applied as a commercial or controlled-release formulation

A controlled-release formulation (CRF) has been developed for metolachlor, which reduced its leaching in a sandy soil and improved weed control in comparison with the commercial formulation. The current study tested the effect of soil wetting and drying cycles (WDCs) on metolachlor fate (desorption, leaching, and weed control) applied as the CRF and as the commercial formulation. Metolachlor adsorption to a heavy soil (Terra-Rosa) was predominately to the clay minerals and oxides. Metolachlor release from a heavy soil subjected to WDCs was higher than its release from the soil not subjected to WDCs. Consequently, a bioassay in soil columns treated with the commercial formulation indicated enhanced metolachlor leaching in heavy soils under WDCs. In contrast, when metolachlor was applied as the CRF, leaching was suppressed and not affected by WDCs. These results emphasize the advantages of the CRF also in heavy soils subjected to WDCs.     

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.     

Experimental study of soil microfabrics induced by isotropic stresses of wetting and drying

This experimental study examines the relationship between texture and microfabric under isotropic stresses generated by wetting and drying. This can be achieved by observing and measuring the reorganization of microfabrics (plasmic fabrics and related distribution pattern, RDP). Microfabrics are produced in the laboratory by mixing montmorillonite with various proportions of sand and silt, and subjecting them to wetting and drying without confinement. Thin sections are cut to study their microfabrics. Results show broad and systematic variations in fabric with changes in texture. Fine matrix (silt plus clay in physical continuity) is developed in samples with low sand:silt ratio, and regardless of this ratio at high clay levels. Coarse porphyric and fine porphyric RDP are formed at low and high clay contents, respectively. Plasmic fabrics are represented mainly by insepic and weakly developed mosepic and masepic fabrics in the matrix in the form of small plasma separations with random orientation, and weak skelsepic plasma as embedded grain argillans. On the other hand, coarse matrix (sand grains in physical continuity) dominates the sandy samples which are associated with relatively thick and strongly birefringent free grain argillans giving well-developed skelsepic fabric. The RDP follows two major paths: beginning with orthogranic (very low clay content) along the iunctic sequence at low clay:silt ratio and along the chlamydic sequence at high ratio. The regularities in fabric development are interpreted in the light of the dynamic properties and behaviour of the active clay fraction in a clay-water system. The pedogenetic implications of the results are explored, and the term “erdic” fabric defined.     

Soil biophysical controls over rice straw decomposition and sequestration in soil: The effects of drying intensity and frequency of drying and wetting cycles

Although it is well known that fluctuations in soil moisture affect the decomposition of organic matter, few studies have provided direct evidence of the underlying biophysical mechanisms. Cycles of wetting and drying (W/D) may not only alter soil pore structure, but also stimulate a proliferation of fungi, since these organisms are typically less affected by drought stress than bacteria, and hence the development of fungal-induced soil water repellency. The biophysical interaction between these processes is likely to influence the decomposition of organic matter amendments to soil and carbon sequestration. By using soil cores amended with rice straw, the objectives of this study were to determine the effects of drying intensity and frequency of W/D cycles on decomposition rate after rewetting, soil pore-size distribution, soil microbial biomass (SMB) and soil water repellency, and to assess their biophysical interaction. One W/D cycle consisted of wetting a soil core from the bottom for 1.5-days at −0.03 kPa followed by 1.5, 3.5 or 6.5 days of drying in open air at 25 ± 2.5 °C. This resulted in different intensities of drying and frequencies of W/D cycles over a 120-d incubation period. The decomposition rate decreased with repeated W/D cycles and increasing drying intensity, particularly between the 3rd and 9th W/D cycles. The SMB-C concentration and soil water repellency peaked at the 3rd W/D cycle. The peak size of the SMB-C concentration was larger in the drier soils and soil water repellency was significantly related to SMB-C concentration (R = 0.57, P = 0.025). The soil with the strongest drying treatment had a greater concentration of particulate organic carbon (POC) and the lowest C:N ratio in POC. Although the decomposition rate was significantly correlated to the concentration of soil organic carbon (SOC) (P < 0.01), POC (P < 0.01) and SMB-C (P < 0.05), stepwise regression analysis further identified that it was largely correlated to soil pore characteristics. The decrease in the decomposition rate in the drier soil was largely explained by the increase in macropores >300 μm in diameter (R = 0.98). The results suggest that an increased drying intensity or a longer duration of drying after rainfall or irrigation may favour SOC sequestration through inhibiting decomposition of amended residue. This may be due to the formation of macropores and their subsequent stabilization via fungal growth and fungal-induced soil water repellency.     

温度和干湿交替对施用有机肥的黄土的铵固定的影响

Effects of temperature and drying and wetting alternation (DWA) on ammonium fixation in manured loessial soil were studied by means of Batch Equilibrium with varying concentration solutions of ammonium chloride.ammonium fixation time,and soil clay contents.The purpose of the research was to find out the pattern of ammonium fixation affected by the varying factors.The results showed a remarkable variation in ammonium fixation.Fixed ammonium increased with temperature and treatments of DWA.The ammonium fixation in manured loessial soil was characterized by the effect of temperature and DWA.     

Organic matter quality of a forest soil subjected to repeated drying and different re‐wetting intensities

Extended drought periods followed by heavy rainfall may increase in many regions of the Earth, but the consequences for the quality of soil organic matter and soil microbial communities are poorly understood. Here, we investigated the effect of repeated drying and re‐wetting on microbial communities and the quality of particulate and dissolved organic matter in a Haplic Podzol from a Norway spruce stand. After air‐drying, undisturbed soil columns were re‐wetted at different intensities (8, 20 and 50 mm per day) and time intervals, so that all treatments received the same amount of water per cycle (100 mm). After the third cycle, SOM pools of the treatments were compared with those of non‐dried control columns. Lignin phenols were not systematically affected in the O horizons by the treatments whereas fewer lignin phenols were found in the A horizon of the 20‐ and 50‐mm treatments. Microbial biomass and the ratio of fungi to bacteria were generally not altered, suggesting that most soil microorganisms were well adapted to drying and re‐wetting in this soil. However, gram‐positive bacteria and actinomycetes were reduced whereas gram‐negative bacteria and protozoa were stimulated by the treatments. The increase in the (cy 17: 0 + cy 19: 0)/(16:1ω7c + 18:1ω7c) ratio indicates physiological or nutritional stress for the bacterial communities in the O, A and B horizons with increasing re‐wetting intensity. Drying and re‐wetting reduced the amount of hydrolysable plant and microbial sugars in all soil horizons. However, CO₂ and dissolved organic carbon fluxes could not explain these losses. We postulate that drying and re‐wetting triggered chemical alterations of hydrolysable sugar molecules in organic and mineral soil horizons.     

Micromorphological analysis to characterize structure modifications of soil samples submitted to wetting and drying cycles

The physical characteristics of the soil surface are of extreme importance in relation to energy and matter transfer processes between the atmosphere and the soil. Soil internal structure changes can be due to natural or artificial causes and one important natural process is the alternation of wetting and drying (W–D) processes, which induce swelling and shrinking of soil particles, causing modifications in pore size and shape. To study the consequence of these W–D events on possible modifications in pore size distribution, pore number, and pore shape of soil samples collected in metal rings pore image analysis was used. Samples were taken from profiles of three soils of different characteristics, named as Geric Ferralsol (GF), Eutric Nitosol (EN), and Rhodic Ferralsol (RF). Confined volumetric samples (50 cm³) were submitted to none (T₀), three (T₁), and nine (T₂) subsequent W–D cycles. Image cross sections of resin impregnated soil permitted the micrometric and macrometric characterization of changes in soil structure induced by sequences of W–D cycles. Duncan's statistical test indicated that there were significant differences (α = 0.05) among treatments for all soil samples. General conclusions indicate that total pore area increased for all soils after repeated W–D processes, specifically 19.0 to 28.9% for GF, 5.9 to 11.7% for EN, and 13.0 to 17.2% for RF. Main changes of pore diameter occurred in pores larger than 500 μm, and minor changes were observed in the total number of these pores. It is demonstrated that soil samples undergo important changes in their structures after repeated W–D cycles. The information presented here is very important for the evaluation of soil water retention curves and other soil hydric properties, because soil samples used in these procedures are collected in rings and frequently submitted to several W–D cycles.     

Structural change induced by wetting and drying in seedbeds of a hardsetting soil with contrasting aggregate size distribution

Hardsetting has been related to two main processes: (i) development of strength when the soil is still moist (–100 kPa) due to matric suction acting within interparticle and interaggregate bridges and (ii) temporary cementation of dry soil by poorly ordered silica and aluminosilicates. In both cases, hardsetting of a seedbed should depend on geometric aspects of macro- and micro-structure. This study deals with aggregate breakdown and/or deformation on wetting and with the structural changes which follow on drying. Repacked seedbeds of a hardsetting red-brown earth were wetted by capillary rise. Seedbeds with a coarse and a fine aggregate size distribution were examined. Before and after wetting, the bulk density of the seedbeds was measured at 5-mm increments using a gamma probe. Replicate samples were air dried, gamma scanned and impregnated. Binary images of pore space (> 107 μm) of vertical faces were used to generate depth functions of structure attributes, including macroporosity. Bulk density measurements combined with image analysis resulted in successful structure characterization. Thin-section observations were used to interpret the quantified changes in terms of physical processes. In both coarse and fine seedbeds the physical processes which determined structural change occurred upon wetting rather than on drying. Coalescence of aggregates under plastic conditions, partly due to overburden pressure and enhanced by microcracking and partial slaking, occurred at the bottom of the coarse seedbed. In the fine seedbed, the fine material agglomerated due to matric suction. These different processes led to rather similar, microporous microstructures which exhibited similar strength properties after drying.     

干湿交替条件下稻田土壤氧气和水分变化规律研究

在水稻高产栽培技术中,为合理利用水资源,改良土壤通气性,干湿交替灌溉技术已广泛应用。干湿交替灌溉技术的核心是协调土壤水分与氧气之间的平衡。本研究采用大田试验,在水稻生育后期进行干湿交替处理,同步监测稻田5 cm土层土壤氧气、水分及温度的变化,以探讨稻田土壤氧气和水分运移变化规律及其互作关系,为进一步揭示干湿交替灌溉技术的内在生理机制奠定基础。研究结果表明,干湿交替条件下,稻田土壤含水量处于饱和状态时,基本监测不到土壤含氧量;而稻田土壤在慢慢变干的过程中,5 cm土层土壤体积水分含量逐渐降低,土壤氧气含量逐渐升高。在花后19 d、24 d、29 d,当5 cm土层土壤体积含水量分别下降为25.4%、25.1%、24.7%时,土壤含氧量则分别上升到17.5%、17.4%、17.4%。在水稻生育后期不同阶段,土壤氧气含量的日变化呈现先降低再升高的趋势,谷值一般出现在14:00—15:00之间;土壤含水量随时间呈波动式逐渐降低的变化趋势;土壤温度呈现先升高后降低的趋势,峰值一般出现在15:00—16:00之间。从上午8:00到下午17:59,当土壤温度升至峰值时,土壤水分含量较低,而土壤含氧量开始升高。水稻开花期、灌浆期和成熟期的土壤氧气含量与土壤含水量均表现出极显著负相关关系,土壤温度与土壤水分间呈显著负相关关系,而土壤氧气与土壤温度之间无显著相关关系。说明干湿交替条件下,水稻生育后期稻田土壤中的水分和氧气含量存在一定的此消彼长的关系。因此,通过适度的干湿交替管理措施,可在一定程度上调节水稻根系周围的土壤水分和氧气的平衡。     

Effect of wetting and drying of soils on phosphate adsorption and resin extraction of soil phosphate

Sixteen topsoils from Denmark and the UK were subjected to two wetting and drying treatments: (i) moist incubation (wet), (ii) eleven wetting and drying cycles (W/D). The W/D treatment resulted in larger P adsorption and resin extraction of soil P than the wet treatment. The differences in P adsorption at the final concentration of 800 μM P were mainly above 20 per cent, whereas the differences in amount of resin-extracted P were mainly less than 20 per cent. The effects were positively correlated with the cation exchange capacity of inorganic components. Furthermore, the increase in rapidly released P was positively correlated with pH. It is suggested that wetting and drying effects on P adsorption and desorption are associated with changes in soil structure caused by rewetting of dry samples.     

青海共和盆地封育季节草场生物量、种群与生境变化

试验研究青海共和盆地封育季节草场生物量、种群与生境变化结果表明,封育草地生物量和株高均高于未封育草地,且依据未封育与封育草地生物量之比,共和盆地草场退化等级为中度和重度;由于沙生和荒漠植被入侵并逐渐成为优势种或伴生优势种,退化草地优势物种生物量降低,依据其他物种与优势物种生物量之比,确定共和盆地草场退化等级<0.1为弱度,0.1~0.5为轻度,0.5~1.0为中度,1.0~5.0为重度,>5.0为严重,芨芨草草原处于中度、重度退化,针茅草原处于重度和严重退化;共和盆地封育草场存在两极恢复(原草场和新退化草场类型),而未封育草场只有单极退化(荒漠),故过牧不仅使共和盆地草地退化,且使草地生境不断恶化,荒漠化程度加剧;相反封育有利于草地恢复和生境改善。     

Effect of air‐drying on phosphorus fractions in clay soil

Despite the publication of a number of papers dealing with the effect of drying on the soil labile P pool, less attention has been paid to the possible drying‐evoked changes in the more stable P pools. We applied Hedley's sequential fractionation procedure that aims at quantifying soil P reserves according to their decreasing plant availability to examine the effects of drying on soil P fractions in clayey soil samples of different cultivation history. To further investigate the contribution of organic matter disruption to the solubility of soil P, the P extracted in each fractionation step was divided into two size classes by filtering the suspension through a 0.2 μm membrane filter. There were no air‐drying‐induced changes in the total amount of P extracted in each fractionation step. However, air‐drying changed the distribution of water‐extractable P in size fractions; increase in the small‐sized P took place at the expense of large‐sized P. Air‐drying increased also small‐sized molybdate‐unreactive P (MUP) in the NaOH fraction giving evidence that drying‐induced alterations take place also in less labile P forms. The results revealed that air‐drying alters the extractability and distribution of P in various pools rather than the total amount of extracted P and that a large proportion of H₂O‐ and NaOH‐extractable large‐sized MUP may remain undetected if only filtered samples are analyzed.     

Amidase and urease activity in soil as affected by sludge,salinity and wetting and drying cycles

Three incubation experiments were carried out to assess the effect of different levels of sludge, salinity and of different wetting and drying cycles on amidase and urease activity in a calcareous silt loam soil. Both amidase and urease activities were largely increased with the increase in the amount of sludge applied. The addition of NH₄⁺ - N with sludge slightly reduced both activities. Addition of increasing amounts of a 1:1 NaCl: CaCl₂ salt mixture increasingly reduced the soil activities of amidase and urease. This effect was less severe in sludge amended soil. The data also showed that drying of either unamended or sludge amended soil cause a marked drop in amidase and urease activity. Rewetting increased both enzyme activities.     

干湿效应下崩岗区岩土抗剪强度衰减非线性分析

发育于花岗岩的崩岗侵蚀区红土受干湿变化影响显著。通过室内直剪试验,研究了不同干湿效应对崩岗侵蚀区岩土抗剪强度衰减的影响。试验处理采用5种干湿效应水平(风干48h、风干24h、自然含水率、浸30s和浸60s)。结果显示:土壤黏聚力c和内摩擦角φ随干湿变化呈非线性衰减趋势,当土壤含水率13%左右时,对应的抗剪强度指标出现峰值;峰值强度前符合线性递增规律,峰值强度后符合一阶指数衰减规律。在风干阶段,抗剪强度主要受裂隙性影响,而在增湿阶段,基质吸力是影响抗剪强度的主要因素;探讨了干湿循环效应对崩岗侵蚀发育的影响。