Organic compounds of different extractability in total solvent extracts from soils of contrasting water repellency

Previous studies examining organic compounds that may cause water‐repellent behaviour of soils have typically focussed on analysing only the lipophilic fraction of extracted material. This study aimed to provide a more comprehensive examination by applying single‐ and sequential‐accelerated solvent extraction (ASE), separation and analysis by GC/MS of the total solvent extracts of three soils taken from under eucalypt vegetation with different degrees of water repellency. Water repellency increased in all the soils after extraction with DCM/MeOH (95:5), but was eliminated with iso‐propanol/ammonia (95:5). Quantities of major lipid compound classes varied between solvents and soils. Iso‐propanol/ammonia (95:5) solvent released saccharides, glycerol, aromatic acids and other polar organic compounds, which were more abundant in fractionated extracts from the single extraction and the third step sequential ASE extraction, than in the extracts from the DCM/MeOH ASE solvent. Dominant compounds extracted from all soils were long‐chain alkanols (>C₂₂), palmitic acid, C₂₉ alkane, β‐sitosterol, terpenes, terpenoids and other polar compounds. The soil with the lowest repellency lacked >C₁₈ fatty acids and had the lowest concentrations of alkanols (C_26,C₂₈ and C₃₀) and alkanes (C₂₉, C₃₁), but a greater abundance of more complex polar compounds than the more repellent soils. We therefore speculate that the above compounds play an important role in determining the water repellency of the soils tested. The results suggest that one‐stage and sequential ASE extractions with iso‐propanol/ammonia and subsequent fractionation of extracts are a useful approach in providing a comprehensive assessment of the potential compounds involved in causing soil water repellency.     

Changes in soil organic compound composition associated with heat‐induced increases in soil water repellency

Soil heating, as for example experienced during vegetation fires, often increases soil water repellency; however, no detailed analysis of the soil chemical changes associated with this increase has been conducted to date. Here we characterize the changes in organic compound composition associated with heat‐induced increases in water repellency for three Australian eucalypt‐forest soils (one sandy loam, two sands). Laboratory heating (300°C) strongly increased water drop penetration times (WDPTs) in all soils. Soils were extracted by accelerated solvent extraction (ASE) with an iso‐propanol/ammonia mixture (IPA/NH₃ 95:5) and pure iso‐propanol (IPA). Extracts were fractionated into less and more polar fractions and analysed by GC‐MS. Water repellency was eliminated in unheated and heated soils by IPA/NH₃, but not by pure IPA. Before heating, total solvent extracts were dominated by n‐alkanols, terpenoids, C₁₆ acid, C₂₉ alkane, β‐sitosterol and polar compounds. After heating, dominant compounds were aromatic acids, aldehydes, levoglucosan, simple sugars and glycosides. Heating resulted in a sharp absolute decrease of homologous aliphatic series of alkanols and alkanes, a shift of fatty acid signature to members <C₂₀ and an increase in total content of aromatic compounds. Heating also caused the formation of complex high‐molecular‐weight compounds detected in the more polar fractionated extracts and low‐molecular‐weight oxo‐ and hydroxyacids and aromatics in the IPA/NH₃ solvent. We speculate that these compounds in conjunction with fatty acids of <C₁₂ interact with organic and mineral soil surfaces and cause the observed strong increases in soil water repellency following heating.     

Water repellency of volcanic ash soils from Ecuadorian páramo: effect of water content and characteristics of hydrophobic organic matter

Water repellency of volcanic ash soils from the Ecuadorian páramo was studied by a combination of extraction and analysis of water‐repellent products, Molarity Ethanol Droplet values, water contact‐angle measurements by capillary rise, and N₂ adsorption isotherms. The undried samples studied are hydrophilic, but exhibit water repellency after moderate drying (48 hours at 30°C). The advancing water contact‐angle measured by capillary rise varies from 78° to 89°. These water contact‐angles decrease strongly after extraction of organic materials by an isopropanol–water mixture. Elemental analysis, infrared spectra and gas chromatography‐mass spectrometry analyses were used to characterize the extracts. The results show that long‐chain fatty acids and more complex non‐polar alkyl components (waxes) are the main water‐repellent materials. The deposition of such extracted materials onto hydrophilic sand leads to the rapid increase of water contact‐angle until values close to those measured on the soil samples are achieved. Assuming a coating of the mineral surface by organic hydrophobic products and using Cassie's law, the water contact‐angle of extracted materials was computed. The values ranged from 100° to 157°. Nitrogen specific surface areas of the soils studied were very small, indicating a low adsorbent–adsorbate interaction on hydrophobic surfaces. These results partially validate the hypothesis of water‐repellent materials that occur as coatings at least after a drying process.     

Do surface active substances from water repellent soils aid wetting?

Soil water repellency is usually unstable, as exemplified by the common method of quantifying repellency degree – the water drop penetration time (WDPT) test. Dynamic penetration and infiltration of water into repellent soils is generally attributed to either reduction of the solid‐liquid interfacial energy (γ_SL) or reduction of the liquid‐vapour interfacial energy (γ_LV), or both. The reduction of γ_SL can result from conformation changes, hydration, or rearrangement of organic molecules coating soil particle surfaces as a result of contact with water, while the reduction of γ_LV can result from dissolution of soil‐borne surface active organic compounds into the water drop. The purpose of this study was to explicitly test the role of the second mechanism in dynamic wetting processes in unstably repellent soils, by examining the drop penetration time (DPT) of water extracts from repellent soils obtained after varying extraction times and at different soil : water ratios. It was indeed found that soil extracts had lower surface tensions (γ_LV approx. 51–54 mN m⁻¹) than distilled water. However, DPT of the soil extracts in water repellent soils was generally the same or greater than that of water. Salt solutions with the same electrical conductivity and monovalent/divalent cation ratio as the soil extracts, but lacking surface active organic substances, had the same DPT as did the extracts. In contrast, DPT of ethanol solutions prepared with the same γ_LV, electrical conductivity, and monovalent/divalent cation ratio as the soil extracts, was much faster. Ethanol solutions are usually used as an agent to reduce γ_LV and as such, to reduce DPT. It is concluded that the surface‐active, soil‐derived organic substances in aqueous soil extracts do not contribute to wetting dynamics, and as such, this mechanism for explaining kinetics of water penetration into water repellent soils is rejected. It is also concluded that the rapid penetration of ethanol solutions must be due not only to changes in γ_LV, but to also to changes in either or both γ_SL and the solid‐vapour interfacial energy (γ_SV). These results stand in sharp contrast to well‐accepted logical paradigms.     

斥水程度对脱水土壤水分特征曲线的影响

为研究斥水程度对土壤水分特征曲线的影响,该文基于滴水穿透时间法,人工配置7种斥水程度的黏壤土(L0~L6)和6种斥水程度的砂土(S0~S5),用高速离心机测定其土壤水分特征曲线,应用van Genuchten-Mualem(VG)模型进行拟合,得出VG模型水力参数。结果表明:在同一吸力条件下,斥水黏壤土的含水率比斥水砂土的高;随着斥水程度增加,在相同吸力情况下,土壤含水率随斥水程度增大而减小;斥水黏壤土的残余含水率随着斥水程度增加而减小,斥水砂土S0~S3的残余含水率没有差异,S4、S5的残余含水率显著减小;斥水黏壤土和砂土进气值的对数与斥水剂添加量呈负线性相关;随着斥水程度增加,田间持水率减小,凋萎系数没有明显差异,重力水增加,有效水和易有效水减小,易利用水比例随着斥水程度增加而减小;对于斥水黏壤土,微孔隙(0.3~5μm)和小孔隙(5~30μm)含量随着斥水程度的增加明显减小,土壤空隙(≥100μm)随着斥水程度的增加急剧增加;对于斥水砂土,中等孔隙(30~75μm)的含量随斥水程度的增加明显增加。该研究成果可为斥水土壤的入渗、蒸发和数值模拟提供理论支持。     

Effects of olive oil mill wastewater on chemical,microbiological, and physical properties of soil incubated under four different climatic conditions

The objective of this study was to understand the degradation of the organic matter of olive mill wastewater (OMW) and its phytotoxic and water repellent effects in dependence on four different climatic conditions. We hypothesized that warm conditions with sufficient soil moisture ensure optimal biological activity and thus minimize negative effects of the OMW treatment. Therefore, OMW-treated soil was incubated for 60 days under four climatic conditions. During incubation, we monitored pH, contents of nitrate, manganese and phenolic compounds, soil respiration, soil water repellency, and δ¹³C. Additionally, calorific value and thermal stability of the soil organic matter at the beginning and end of incubation were determined. Soil samples of the wet-cold and moist-warm incubation were tested for phytotoxicity using a seed germination bioassay with Lepidium sativum. As a function of climatic conditions, positive and negative effects, e.g., addition of nutrients, phytotoxicity, and soil water repellency, were observed. Under dry-hot conditions, the soil was still water repellent after 60 days of incubation whereas the wet-hot, moist-warm, and wet-cold incubation show that soil would stay wettable if soil moisture before OMW treatment would be sufficient. Thus, the impact of OMW treatment on soil quality strongly depends on the environmental conditions which should favor an enhancement of microbial activity to minimize negative effects.     

The effect of addition of a wettable biochar on soil water repellency

The potential of biochar to ameliorate soil water repellency has not been widely studied. Previous studies have focused on the potential for biochar to induce or exacerbate existing water repellency rather than alleviate it. This study investigates the effect of adding wettable biochar to water‐repellent soil by comparing the water drop penetration times (WDPTs) of a control and biochar‐amended soil. The potential of wettable biochar to act as a physical amendment to water‐repellent soil was evaluated by mixing coarsely‐ground biochar (CGB, particle size range 250–2000 µm) or finely‐ground biochar (FGB, particle size range < 250 µm) with one strongly and one severely naturally water‐repellent soil in various quantities, and then measuring the WDPT for each mixture. When biochar particles did not fall within the size range of existing soil particles, an initial increase in both mean WDPT (WDPT_M) and variation in WDPT was observed with small additions of biochar. These effects possibly result from increased surface roughness and inhibition of infiltration by the suspension of drops above the average soil–air interface at a few hydrophobic points. Both CGB and FGB reduced soil water repellency, FGB more effectively than CGB. The addition of 10% w/w FGB reduced soil WDPT by 50%, and 25% FGB eliminated repellency. Direct absorption of water by biochar and an increase in soil surface area in contact with water are the predominant physical mechanisms involved. This exploratory study suggests biochar has the potential to amend water‐repellent soil.     

Occurrence, prediction and hydrological effects of water repellency amongst major soil and land-use types in a humid temperate climate

Knowledge of soil water repellency distribution, of factors affecting its occurrence and of its hydrological effects stems primarily from regions with a distinct dry season, whereas comparatively little is known about its occurrence in humid temperate regions such as typified by the UK. To address this research gap, we have examined: (i) water repellency persistence (determined by the water drop penetration time method, WDPT) and degree (determined by the critical surface tension method, CST) for soil samples (0–5, 10–15 and 20–25 cm depth) taken from 41 common soil and land‐use types in the humid temperate climate of the UK; (ii) the supposed relationship of soil moisture, textural composition and organic matter content with sample repellency; and (iii) the bulk wetting behaviour of undisturbed surface core samples (0–5 cm depth) over a period of up to 1 week. Repellency was found in surface samples of all major soil textural types amongst most permanently vegetated sites, whereas tilled sites were virtually unaffected. Repellency levels reached those of the most severely affected areas elsewhere in the world, decreased in persistence and degree with depth and showed no consistent relationship with soil textural characteristics, organic matter or soil moisture contents, except that above a water content of c. 28% by volume, repellency was absent. Wetting rate assessments of 100 cm³ intact soil cores using continuous water contact (–20 mm pressure head) over a period of up to 7 days showed that across the whole sample range and irrespective of texture, severe to extreme repellency persistence consistently reduced the maximum water content at any given time to well below that of wettable soils. For slightly to moderately repellent soils the results were more variable and thus hydrological effects of such repellency levels are more difficult to predict. The results imply that: (i) repellency is common for many land‐use types with permanent vegetation cover in humid temperate climates irrespective of soil texture; (ii) supposedly influential parameters (texture, organic matter, specific water content) are poor general predictors of water repellency, whereas land use and the moisture content below which repellency can occur seem more reliable; and (iii) infiltration and water storage capacity of very repellent soils are considerably less than for comparable wettable soils.     

Ammonium fluoride extraction for determining inorganic sulphur in acid forest soils

Current methods for determining inorganic sulphur (S) in aerated mineral soil horizons often result in underestimates. To overcome this defect we developed a new method combining a batch extraction with 0.5 m NH₄F solution at a soil:solution ratio of 1:5 with a subsequent analysis of the mobilized SO₄^2– by ion chromatography. The ammonium fluoride extraction enables us to characterize inorganic sulphate in non‐calcareous forest soils. It is more efficient than conventional procedures in which inorganic S is extracted with phosphate or bicarbonate solution. In contrast to the extraction with strongly alkaline reagents (NaOH, KOH, LiOH), the NH₄⁺–NH₃ buffer system in NH₄F prevents the pH of the suspension from exceeding 9.0 and thus the undesired conversion of organic S into SO₄^2– by auto‐oxidation and hydrolysis of ester sulphate. In a comparison we demonstrated that the inorganic S in six German forest soils is underestimated by up to 50% or 200 kg S ha^?1 in the uppermost 60 cm, if it is assessed by extraction with 0.016 m KH₂PO₄ or 0.5 m NaHCO₃ instead of 0.5 m NH₄F. Conversely, the pool of ester sulphate is overestimated almost threefold.     

Fire-induced soil water repellency under different vegetation types along the Atlantic dune coast-line in SW Spain

The distribution and variation with soil depth of water repellency has been studied in fire-affected sand dunes under three different vegetation types (pine forest, shrubland and sparse herbaceous vegetation) in SW Spain. The persistence and intensity of water repellency at the exposed surface of soil was measured using the water drop penetration time test and the contact angle method, respectively, in surface samples (0–3 cm) collected at burned and unburned areas. The variation of water repellency with depth in burned areas was studied in soil profiles every 5 cm between 0 and 40 cm depth. None or slight soil water repellency was observed at unburned soil sites, whereas burned soil sites showed a high degree of repellency, especially under pines and shrubland. The spatial pattern of fire-induced soil water repellency was found to be associated to vegetation types, although it was modulated by soil acidity and the soil organic carbon content. Soil water repellency was generally higher at the soil surface, and decreased with depth. Dense pine forests and shrublands showed strong and/or severe water repellency in depth, but it was rare and limited to the first five centimeters under sparse herbaceous vegetation. The heterogeneity of moisture patterns under dense pine forests or shrublands showed the existence of wetting and water repellent three-dimensional soil patches.     

Thermal destruction of soil water repellency and associated changes to soil organic matter as observed by FTIR spectroscopy

The heat generated during wildfires often leads to increases in soil water repellency. Above a critical heating threshold, however, its destruction occurs. Although the temperature thresholds for repellency destruction are relatively well established, little is known about the specific changes in the soil organic matter that are responsible for repellency destruction. Here we report on the analysis of initially water repellent surface soil samples (Dystric Cambisol, 0–5 cm depth) by transmission Fourier Transform Infrared (FTIR) spectroscopy analysis before and after destruction of its water repellency by heating to 225 °C in order to investigate heating-induced changes in soil organic matter (SOM) composition. Although assignment of absorption bands is made difficult by overlapping of some bands, it was possible to distinguish bands relevant for hydrophobicity of SOM in the soil before heat treatment. The most significant decrease in absorbance following water repellency destruction took place in the frequency area corresponding to stretching vibrations of aliphatic structures within SOM. The results suggest that besides a general decrease of SOM content during heating, the loss of soil water repellence is primarily caused by the selective degradation of aliphatic structures.     

Effects of compaction on soil surface water repellency

Water repellency can reduce the infiltration capacity of soils over timescales similar to those of precipitation events. Compaction can also reduce infiltration capacity by decreasing soil hydraulic conductivity, but the effect of compaction on soil water repellency is unknown. This study explores the effect of compaction on the wettability of water repellent soil. Three air‐dry (water content ～4 g 100 g^?1) silt loam samples of contrasting wettability (non‐repellent, strongly and severely water repellent) were homogenized and subjected to various pressures in the range 0–1570 kPa in an odeometer for 24 h. Following removal, sample surface water repellency was reassessed using the water drop penetration time method and surface roughness using white light interferometry. An increase in compaction pressure caused a significant reduction in soil surface water repellency, which in turn increases the soil's initial infiltration capacity. The difference in surface roughness of soils compacted at the lowest and highest pressures was significant (at P > 0.2) suggesting an increase in the contact area between sessile water drops and soil surfaces was providing increased opportunities for surface wetting mechanisms to proceed. This suggests that compaction of a water repellent soil may lead to an increased rate of surface wetting, which is a precursor to successful infiltration of water into bulk soil. Although there may be a reduction in soil conductivity upon compaction, the more rapid initiation of infiltration may, in some circumstances, lead to an overall increase in the proportion of rain or irrigation water infiltrating water repellent soil, rather than contributing to surface run‐off or evaporation.     

Soil water repellency and infiltration in coarse-textured soils of burned and unburned sagebrush ecosystems

Millions of dollars are spent each year in the United States to mitigate the effects of wildfires and reduce the risk of flash floods and debris flows. Research from forested, chaparral, and rangeland communities indicate that severe wildfires can cause significant increases in soil water repellency resulting in increased runoff and erosion. Few data are available to document the effects of fire on the spatial and temporal variability in soil water repellency and potential impacts on infiltration and runoff on sagebrush-dominated landscapes. Soil water repellency, infiltration and runoff were assessed after two wildfires and one prescribed fire in three steep, sagebrush-dominated watersheds with coarse-textured soils. Water repellency was generally greater on unburned hillslopes and annual variability in water repellency had a greater impact on infiltration capacity than fire effects. The most significant impact of fire was canopy and ground cover removal on coppice microsites. Infiltration rates decreased on coppice microsites after fire even though soil water repellency was reduced. Fire-induced reduction in infiltration resulted from the combined effect of canopy and ground cover removal and the presence of naturally strong water repellent soils. Removal of ground cover likely increased the spatial connectivity of runoff areas from strongly water repellent soils. The results indicate that for coarse-textured sagebrush landscapes with high pre-fire soil water repellency, post-fire increases in runoff are more influenced by fire removal of ground and canopy cover than fire effects on soil water repellency and that the degree of these impacts may be significantly influenced by short-term fluctuations in water repellent soil conditions.     

岷江上游干旱河谷旱地土壤斥水性特征初步研究

土壤斥水性是土壤颗粒不易被水滴浸润的现象,对土壤水分特征曲线、土壤溶质运移、土壤优先流、土壤导水率以及地表径流和土壤侵蚀等具有重要影响。研究结果表明,3月份岷江上游干旱河谷0-5cm土层具斥水性的土壤在空间上的分布概率约为34%,其中强度斥水性土壤分布比例为5%;在时间分布上,土壤斥水性主要表现在7月,轻度以下斥水性概率为91%,强度以上斥水性概率为58%;从各粒级土壤斥水性的研究结果来看,斥水性与土壤粒级呈显著负相关,粒级越小,土壤斥水性越高。因此,岷江上游干旱河谷旱地土壤斥水性具有明显的时空分布差异,并且粒级越小土壤斥水性越强,7月份土壤表层的土壤斥水性强度与分布比例高。这可能是导致干旱河谷严重水土流失、土壤砂砾化的一个重要原因。     

Amino acids and amino sugars extracted by EUF from a sandy soil incubated with green manure,bacterial biomass or cellulose

The extraction of soils by the electro-ultrafiltration (EUF) method yields organic N which has been used as an index for mineralisable N in soils. This EUF extractable organic fraction contains a mixture of various N compounds not yet completely identified. It has been proposed that the amino N compounds are more indicative for the potentially mineralisable N in soils than the total organic N extracted (Mengel et al., 1999). An amendment of soils with easily mineralisable organic matter may, therefore, alter the amino N concentrations of the organic N extracted. Our determination of the amino N compounds aimed to prove this hypothesis. The principle of our experiment was to mix soil with green manure, bacterial biomass and cellulose, respectively, and to incubate the treated soil aerobically for 80 days at 20°C in the laboratory. Control treatments without organic amendment were also incubated. Soil samples were taken several times during the incubation period and analysed for the inorganic N (NO₃⁻-N and NH₄⁺-N) and for the EUF extractable organic N. Amino acids and amino sugars were determined in the hydrolysed EUF extracts. The concentrations of amino acids and amino sugars in the organic N extracted varied with time and differed between the treatments. Glutamic acid has been found to be the most relevant amino acid in the EUF extracts and was particularly indicative for the existence of mineralisable green manure in the soil. Glucosamine was the most relevant amino sugar in the EUF extracts and this amino sugar appears to be indicative for the easily mineralisable relics of microbial cells in the soil.     

再生水灌溉对农田土壤水流运动影响的研究进展

再生水灌溉农田既可节约宝贵的水资源、缓解农业用水紧缺,同时再生水中的多种营养元素和微量元素可促进作物生长、提高粮食产量。但再生水中的物质进入农田后将引起土壤孔隙结构、团聚体结构、黏粒分散特征和水土作用关系等一系列的变化,进而引起土壤入渗性能和导水性能的改变,增大环境污染风险。该文综述了再生水中的悬浮无机固体、大分子有机质、油脂、表面活性剂和盐分对农田土壤水流运动的影响及其作用机理,指出受灌农田土壤结构性质演化过程与驱动机制、受灌农田土壤与灌溉入渗水流之间的相互作用关系为该领域亟需开展的2个研究方向。文章对再生水农田灌溉制度制定、污染风险控制和生态环境保护均有参考价值。     

Influence of different plant species on water repellency in Mediterranean heathland soils

It is established that soil hydrophobicity reduces soil infiltration rates, and enhances runoff flow and soil erosion. Water repellency has been studied with special interest in coniferous and eucalyptus forests, particularly after burning, but the number of studies concerning Mediterranean heathlands is still very low. In this paper, we study the occurrence and persistence of water repellency in soil samples collected under different plant species susceptible to induce soil hydrophobicity (Erica arborea, Erica australis, Calluna vulgaris, Quercus lusitanica and Rhododendron ponticum) in a natural protected area in southern Spain. Great attention has been paid to the relationships between soil water repellency and environmental factors as organic matter content and soil acidity. The largest hydrophobicity was measured in soil samples collected under E. australis, E. arborea and C. vulgaris. For these species, the organic matter content and pH showed positive and negative correlations with the persistence of water repellency, respectively. The hydrophobicity originated by humic substances in the soil seems to be the only explanation for slight soil water repellency under Q. lusitanica or R. ponticum. The patchy patterns of occurrence and persistence of soil water repellency is governed by the spatial distribution of the studied species and modulated by other factors. Soil surface water repellent layers reduce the infiltration rates and limit the water storage capacity. However, the macropore flow can be enhanced on non-repellent layers, cracks or roots such us the wetting's front shown. The vegetation effects on soil hydrology should be considered for afforestation works and flooding control.     

Soil surfactant stops water repellency and preferential flow paths

This study reports the effects of a soil surfactant on reduction and prevention of water repellency and preferential flow paths in a sandy soil of a golf course fairway, located at Bosch en Duin near Utrecht, the Netherlands. The golf course is constructed on inland dunes composed of fine sand with low organic matter content. The topsoil (0–25 cm) of the fairways exhibits an extremely water repellent behaviour resulting in the development of numerous localized dry spots during dry periods in spring and summer. The influence of surfactant treatments on the wetting of the soil was studied by measuring the volumetric water content with a hand‐held Time Domain Reflectometry (TDR) device. Actual water repellency was assessed by placing water drops at regular distances on soil cores taken to a depth of 25 cm with a small (1.5 cm diameter) auger at intervals of 25 cm over a distance of 25 m across the untreated and treated parts of the fairway. Surfactant applications resulted in more homogeneous wetting of the soil profile and elimination of actual water repellency in the fairway soil. Treatments significantly increased water uptake and moisture levels of the soil and prevented the development of preferential flow paths. A visible improvement in turf quality and density was evident on the treated part of the fairway.     

Water-repellent soil and its relationship to granularity, surface roughness and hydrophobicity: a materials science view

Considerable soil water repellency has been observed at a wide range of locations worldwide. The soil exhibiting water repellency is found within the upper part of the soil profile. The reduced rate of water infiltration into these soils leads to severe runoff erosion, and reduction of plant growth. Soil water repellency is promoted by drying of soil, and can be induced by fire or intense heating of soil containing hydrophobic organic matter. Recent studies outside soil science have shown how enhancement of the natural water repellency of materials, both porous and granular, by surface texture (i.e. surface roughness, pattern and morphology) into super‐hydrophobicity is possible. The similarities between these super‐hydrophobic materials and observed properties of water‐repellent soil are discussed from a non‐soil scientist, materials‐based perspective. A simple model is developed for a hydrophobic granular surface and it is shown that this can provide a mechanism for enhancement of soil water repellency through the relative size and spacing of grains and pores. The model provides a possible explanation for why soil water repellency should be more prevalent under dry conditions than wet. Consequences for water runoff, raindrop splash and soil erosion are discussed.     

Dynamics of soil extractable carbon and nitrogen under different cover crop residues

Purpose

Cover crop residue is generally applied to improve soil quality and crop productivity. Improved understanding of dynamics of soil extractable organic carbon (EOC) and nitrogen (EON) under cover crops is useful for developing effective agronomic management and nitrogen (N) fertilization strategies.

Materials and methods

Dynamics of soil extractable inorganic and organic carbon (C) and N pools were investigated under six cover crop treatments, which included two legume crops (capello woolly pod vetch and field pea), three non-legume crops (wheat, Saia oat and Indian mustard), and a nil-crop control (CK) in southeastern Australia. Cover crops at anthesis were crimp-rolled onto the soil surface in October 2009. Soil and crop residue samples were taken over the periods October?CDecember (2009) and March?CMay (2010), respectively, to examine remaining crop residue biomass, soil NH₄ ⁺?N and NO₃ ^??CN as well as EOC and EON concentrations using extraction methods of 2?M KCl and hot water. Additionally, soil net N mineralization rates were measured for soil samples collected in May 2010.

Results and discussion

The CK treatment had the highest soil inorganic N (NH₄ ⁺?N?+?NO₃ ^??CN) at the sampling time in December 2009 but decreased greatly with sampling time. The cover crop treatments had greater soil EOC and EON concentrations than the CK treatment. However, no significant differences in soil NH₄ ⁺?N, NO₃ ^??CN, EOC, EON, and ratios of EOC to EON were found between the legume and non-legume cover crop treatments across the sampling times, which were supported by the similar results of soil net N mineralization rates among the treatments. Stepwise multiple regression analyses indicated that soil EOC in the hot water extracts was mainly affected by soil total C (R ²?=?0.654, P?<?0.001), while the crop residue biomass determined soil EON in the hot water extracts (R ²?=?0.591, P?<?0.001).

Conclusions

The cover crop treatments had lower loss of soil inorganic N compared with the CK treatment across the sampling times. The legume and non-legume cover crop treatments did not significantly differ in soil EOC and EON pools across the sampling times. In addition, the decomposition of cover crop residues had more influence on soil EON than the decomposition of soil organic matter (SOM), which indicated less N fertilization under cover crop residues. On the other hand, the decomposition of SOM exerted more influence on soil EOC across the sampling times among the treatments, implying different C and N cycling under cover crops.