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Peng X.,CAS Nanjing Institute of Soil Science | Hallett P.D.,James Hutton Institute | Zhang B.,Chinese Academy of Agricultural Sciences | Horn R.,Institute of Plant Nutrition and Soil Science
European Journal of Soil Science

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. © 2011 The Authors. Journal compilation © 2011 British Society of Soil Science. Source

Peng X.,CAS Nanjing Institute of Soil Science | Horn R.,Institute of Plant Nutrition and Soil Science
Soil Science Society of America Journal

A typical soil shrinkage curve is S-shaped and composed of four phases termed structural, proportional, residual, and zero shrinkage. However, many studies have not found all four soil shrinkage phases despite investigating the full spectrum of soil moisture content. The objectives of this paper were to determine different soil shrinkage types based on the presence of shrinkage phases and to define relationships between the parameters of different shrinkage types and soil properties. A total of 270 sets of shrinkage data were collected from published (N = 245) and our unpublished work (N = 25), covering a wide range of soil types, sample sizes, and measurement methods. According to the presence of different shrinkage phases, six types of soil shrinkage curves were classified using the shrinkage model proposed by Peng and Horn (2005). Soil shrinkage types generally depended on soil structure, but not on the measurement method. The coefficient of linear extensibility (COLE) had a positive relation with saturated soil bulk density (r = 0.50, P < 0.001), clay content (r = 0.20, P < 0.05), and soil organic carbon (SOC) content (r = 0.46, P < 0.001). This paper is the first to propose six soil shrinkage types that will improve our understanding of the relationship between soil structure and soil water content. Copyright © 2013 by the Soil Science Society of America, Inc. Source

Zhou H.,CAS Nanjing Institute of Soil Science | Peng X.,CAS Nanjing Institute of Soil Science | Peth S.,Institute of Plant Nutrition and Soil Science | Xiao T.Q.,CAS Shanghai Institute of Applied Physics
Soil and Tillage Research

Vegetation restoration is expected to improve soil microstructure and therefore enhance soil stability and reduce soil erosion. The objective of this study was to evaluate the effect of long-term vegetation restoration on the modification of aggregate microstructure with synchrotron-based high resolution X-ray micro-computed tomography (SR-μCT). Triplicate aggregates (5-mm diameter) from (a) severely eroded bare land (BL) and (b) two decades of vegetation restoration land (RL) from Ultisol, Southern China, were collected and scanned with 9μm voxel-resolution at SSRF (Shanghai Synchrotron Radiation Facility). ImageJ software and multifractal theory were used to analyze and quantify aggregate pore structure. Aggregate water stability, mechanical stability, and basic soil properties were also evaluated. Results showed that aggregate water stability and SOM content significantly increased in the RL treatment, while aggregate mechanical stability showed an inverse trend. The microstructure of aggregates had evolved from a very dense massive microstructure to a more porous hierarchical microstructure after two decades of vegetation restoration. Porosity, macro-porosity, fraction of elongated pores, and specific surface area were significantly higher in the RL aggregates as compared to the BL aggregates. Multifractal scaling was observed for the pore structure of the studied aggregates. Generalized dimensions (D q) were significantly higher in the RL treatment as compared to BL treatment, indicating improved pore system after vegetation restoration. This improved microstructure of RL aggregates was attributed to the increased SOM that prompted soil aggregation. This study showed the positive effects of vegetation restoration on soil microstructure and water stability, which was beneficial to the reduction of soil erosion and to the improvement of soil quality in this region. © 2012 Elsevier B.V. Source

Fan X.,CAS Nanjing Institute of Soil Science | Fan X.,Institute of Plant Nutrition and Soil Science | Fan X.,University of Florida | Schnug E.,Institute of Plant Nutrition and Soil Science | And 2 more authors.
Communications in Soil Science and Plant Analysis

Sustainable phosphorous (P) management is a key problem in organic farming. In situ digestion of naturally occurring rock phosphates (RPs) may be a solution. This would require the application of fertilizers consisting primarily of RP mixed with elemental sulfur (S). Through microbial action, the S is oxidized into sulfuric acid, which then transforms the RP into soluble, plant-available forms. By means of an incubation experiment, this study characterized the in situ digestion of RP and revealed how it is influenced by temperature and microbial action. When either S alone or S together with RP (SP) was added to soil that had been inoculated with S-oxidizing microorganisms, the soil pH decreased rapidly from about 7.3 to 3.2 over 12 weeks of incubation. In soil that had not been inoculated with of S-oxidizing microorganisms, the pH of the soil treated either with S or with SP decreased only slightly. The pH of the inoculated soil to which either S or SP had been added decreased more rapidly at 30 °C than at 23.8 °C during the first 4 weeks. The oxidation rate in inoculated soil was much greater than in noninoculated soil and greater at 30 °C than at 23.8 °C. The S oxidation rate in inoculated soil was significantly greater in the SP treatment than in the S treatment both at 23.8 °C and at 30 °C. After incubation, the amounts of water-soluble P and of CAL P (calcium-acetate-lactate-extracted P) were large only in the SP treatment in inoculated soil. © 2012 Copyright Taylor and Francis Group, LLC. Source

Mordhorst A.,Institute of Plant Nutrition and Soil Science | Peth S.,University of Kassel | Horn R.,Institute of Plant Nutrition and Soil Science

Mechanical disturbance of soil structure is commonly related to altered physical changes in pore systems, which control CO2 effluxes e.g. by changes in gas transport properties and in microbial activity. Soil compaction mostly leads to reduced CO2 fluxes. In contrast, structured soils can also release physically entrapped CO2 or give access to protected carbon sources inside aggregates due to aggregate breakdown by disruptive forces. In this study it was investigated how far arable soil management affects structure- and compaction-related CO2-releases using incubation experiments and CO2 gas analysis under standard matric potentials (-6kPa). CO2 efflux was analyzed before, during and after mechanical loading using the alkali trap method (static efflux) and a gas flow compaction device (GaFloCoD, dynamic efflux). Intact soil cores (236 and 471cm3) were collected from a Stagnic Luvisol with loamy sand (conservation and conventional tillage systems) and a Haplic Luvisol with clayey silt (under different fodder crops) from the topsoil (10-15cm) and subsoil (35-45cm). Mechanical stability was reflected by the pre-compression stress value (Pc) and by the tensile strength of aggregates (12-20mm). Changes in pore systems were described by air conductivity as well as air capacity and total porosity. While CO2-releases varied highly during the compaction process (GaFloCoD) for different stress magnitudes, soil depths and management systems, basal respiration rates were generally reduced after mechanical loading by almost half of the initial rates irrespective of soil management. For both methods (dynamic and static efflux) restriction in gas transport functionality was proved to have major influence on inhibition of CO2 efflux due to mechanical loading. GaFloCoD experiments demonstrated that decreases in CO2 efflux were linked to structural degradation of pore systems by exceeding internal soil strength (Pc). Otherwise, re-equilibrating matric potentials to -6kPa and re-incubating offset inhibition of soil respiration suggest a re-enhancement of microbial activity. At this state, physical influences were apparently overlapped by biological effects due to higher energy supply to microbes, which could be offered by spatial distribution changes of microorganisms and organic substrates within a given soil structure. This implies the susceptibility of physical protection mechanism for carbon by disruption of soil structure. In future, special focus should be given on a clear distinction between physical and microbiological effects controlling CO2 fluxes in structured soils. © 2013 Elsevier B.V. Source

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