Stephan Peth

Grazing changes topography-controlled topsoil properties and their interaction on different spatial scales in a semi-arid grassland of Inner Mongolia, P.R. China

Semiarid steppe ecosystems account for large terrestrial areas and are considered as large carbon (C) sinks. However, fundamental information on topsoil sensitivity to grazing is lacking across different spatial scales including the effects of topography. Our interdisciplinary approach considering soil chemical, physical, and vegetation properties included investigations on pit scale (square-metre scale), plot scale (hectare scale), and the scale of a landscape section (several hectares). Five different sites, representing a grazing intensity gradient, ranging from a long-term grazing exclosure to a heavily grazed site were used. On the pit scale, data about aggregate size distribution, quantity of different soil organic carbon (SOC) pools, SOC mineralisation, hydraulic conductivity and shear strength was available for topsoil samples from representative soil profiles. Spatial variability of topographical parameters, topsoil texture, bulk density, SOC, water repellency, and vegetation cover was analysed on the basis of regular, orthogonal grids in differently grazed treatments by using two different grid sizes on the plot scale and landscape section. On the pit scale, intensive grazing clearly decreased soil aggregation and the amount of fresh, litter-like particulate organic matter (POM). The weak aggregation in combination with animal trampling led to an enhanced mineralisation of SOC, higher topsoil bulk densities, lower infiltration rates, and subsequently to a higher risk of soil erosion. On the plot scale, the effects of soil structure disruption due to grazing are enhanced by the degradation of vegetation patches and resulted in a texture-controlled wettability of the soil surface. In contrast, topsoils of grazing exclosures were characterised by advantageous mechanical topsoil characteristics and SOC-controlled wettability due to higher POM contents. A combined geostatistical and General Linear Model approach identified topography as the fundamental factor creating the spatial distribution of texture fractions and related soil parameters on the scale of a landscape section. Grazing strongly interfered with the topography-controlled particle relocation processes in the landscape and showed strongest effects on the aboveground biomass production and biomass-related soil properties like SOC stocks. We conclude that interdisciplinary multi-scale analyses are essential (i) to differentiate between topography- and grazing-controlled spatial patterns of topsoil and vegetation properties, and (ii) to identify the main grazing-sensitive processes on small scales that are interacting with the spatial distribution and relocation processes on larger scales.

Soil microstructure as an under-explored feature of biological soil crust hydrological properties: case study from the NW Negev Desert

Biological soil crusts (BSCs) can play an important role in hydrological cycles, especially in dryland ecosystems where the availability of water is limited. Many factors influence the hydrological behavior of BSCs, one of which is the microstructure. In order to describe the influence of the soil microstructure of BSCs on water redistribution, we investigated the change of the pore system of three different successional stages of BSCs, as well as their respective subcrusts in the NW Negev desert, Israel, using 2-dimensional thin sections, as well as non-invasive X-ray 3D computed microtomography (XCMT) and mercury intrusion porosimetry. Our results show that the pore system undergoes significant changes during crust succession. Both the total porosity, as well as the pore sizes significantly increased from cyano- to lichen- to mosscrust and the pore geometry changed from tortuous to straight pore shapes. We introduce two new mechanisms that contribute to the hydrological properties of the BSCs in the NW Negev that impede infiltration: (i) vesicular pores and (ii) a discontinuous pore system with capillary barrier effects, caused by a rapid change of grain sizes due to sand burial. Since both of these mechanisms are present mostly in early stage cyanobacterial crusts and their abundance decreases strongly with succession, it is very likely that they influence BSC hydrology to different extents in the various crust types and that they are partly responsible for differences in runoff in the NW Negev.

Challenges in imaging and predictive modeling of rhizosphere processes

BackgroundPlant-soil interaction is central to human food production and ecosystem function. Thus, it is essential to not only understand, but also to develop predictive mathematical models which can be used to assess how climate and soil management practices will affect these interactions.ScopeIn this paper we review the current developments in structural and chemical imaging of rhizosphere processes within the context of multiscale mathematical image based modeling. We outline areas that need more research and areas which would benefit from more detailed understanding.ConclusionsWe conclude that the combination of structural and chemical imaging with modeling is an incredibly powerful tool which is fundamental for understanding how plant roots interact with soil. We emphasize the need for more researchers to be attracted to this area that is so fertile for future discoveries. Finally, model building must go hand in hand with experiments. In particular, there is a real need to integrate rhizosphere structural and chemical imaging with modeling for better understanding of the rhizosphere processes leading to models which explicitly account for pore scale processes.

Soil structure formation and management effects on gas emission

The aim of this paper is to clarify the effect of soil management and thus also of soil aggregation on physical and chemical properties of structured soils both on a bulk soil scale, for single aggregates, as well as for homogenized material. Aggregate formation and aggregate strength depend on swelling and shrinkage processes and on biological activity and kinds of organic exudates as well as on the intensity, number and time of swelling and drying events. Thus, soil management like conventional or conservation tillage alter not only the mechanical strength but also the pore continuity and the hydraulic, gas and heat fluxes, and also alter the accessibility of exchange places for nutrients and for carbon storage (global change aspects). The possibility to predict physical properties on these various scales depends on the rigidity of the pore system. In general this rigidity depends on the above-mentioned physical and chemical processes both with respect to intensity and frequency, which again are linked to the soil management systems.

Land use change impacts on floods at the catchment scale : Challenges and opportunities for future research

Abstract Research gaps in understanding flood changes at the catchment scale caused by changes in forest management, agricultural practices, artificial drainage, and terracing are identified. Potential strategies in addressing these gaps are proposed, such as complex systems approaches to link processes across time scales, long‐term experiments on physical‐chemical‐biological process interactions, and a focus on connectivity and patterns across spatial scales. It is suggested that these strategies will stimulate new research that coherently addresses the issues across hydrology, soil and agricultural sciences, forest engineering, forest ecology, and geomorphology.

Effects of Grazing Intensity on Soil Water Regime and Flux in Inner Mongolia Grassland,China

Abstract In the past few decades, the increase in grazing intensity has led to soil degradation and desertification in Inner Mongolia grassland, China, due to population growth and shift in the socio-economic system. Two sites with different grazing intensities, continuous grazing site (CG) with 1.2 sheep ha−1 year−1 and heavy grazing site (HG) with 2.0 sheep ha−1 year−1, were investigated at the Inner Mongolia Grassland Ecosystem Research Station (43° 37′ 50″ N, 116° 42′ 18″ E) situated in the northern China to i) characterize the temporal distribution of soil water content along soil profile; and ii) quantify the water fluxes as affected by grazing intensity. Soil water content was monitored by time domain reflectometry (TDR) probes. Soil water retention curves were determined by pressure membrane extractor, furthermore processed by RETC (RETention Curve) software. Soil matric potential, plant available water and water flux were calculated using these data. Both sites showed an identical seasonal soil water dynamics within four defined hydraulic periods: 1) wetting transition coincided with a dramatic water increase due to snow and frozen soil thawing from March to April; 2) wet summer, rainfall in accordance with plant growth from May to September; 3) drying transition, a decrease of soil water from October to November due to rainfall limit; and 4) dry winter, freezing from December to next February. Heavy grazing largely reduced soil water content by 43%–48% and plant available water by 46%–61% as compared to the CG site. During growing season net water flux was nearly similar between HG (242 mm) and CG (223 mm) sites between 5 and 20 cm depths. However, between 20 and 40 cm depths, the upward flux was more pronounced at HG site than at CG site, indicating that water was depleted by root uptake at HG site but stored at CG site. In semi-arid grassland ecosystem, grazing intensity can affect soil water regime and flux, particularly in the growing season.

The impact of chemical pollution on the resilience of soils under multiple stresses: A conceptual framework for future research.

Soils are faced with man-made chemical stress factors, such as the input of organic or metal-containing pesticides, in combination with non-chemical stressors like soil compaction and natural disturbance like drought. Although multiple stress factors are typically co-occurring in soil ecosystems, research in soil sciences on this aspect is limited and focuses mostly on single structural or functional endpoints. A mechanistic understanding of the reaction of soils to multiple stressors is currently lacking. Based on a review of resilience theory, we introduce a new concept for research on the ability of polluted soil (xenobiotics or other chemical pollutants as one stressor) to resist further natural or anthropogenic stress and to retain its functions and structure. There is strong indication that pollution as a primary stressor will change the system reaction of soil, i.e., its resilience, stability and resistance. It can be expected that pollution affects the physiological adaption of organisms and the functional redundancy of the soil to further stress. We hypothesize that the recovery of organisms and chemical-physical properties after impact of a follow-up stressor is faster in polluted soil than in non-polluted soil, i.e., polluted soil has a higher dynamical stability (dynamical stability=1/recovery time), whereas resilience of the contaminated soil is lower compared to that of not or less contaminated soil. Thus, a polluted soil might be more prone to change into another system regime after occurrence of further stress. We highlight this issue by compiling the literature exemplarily for the effects of Cu contamination and compaction on soil functions and structure. We propose to intensify research on effects of combined stresses involving a multidisciplinary team of experts and provide suggestions for corresponding experiments. Our concept offers thus a framework for system level analysis of soils paving the way to enhance ecological theory.

Development of the polysaccharidic matrix in biocrusts induced by a cyanobacterium inoculated in sand microcosms

Soil inoculation with cyanobacteria (cyanobacterization) is a biotechnological method widely studied to improve soil quality and productivity. During their growth on soil, cyanobacteria excrete exopolysaccharides (EPSs) which glue trichomes to soil particles, in a three-dimensional extracellular polymeric matrix. EPS productivity is an important screening parameter to select proficient inoculants and is affected by growth conditions and abiotic stresses. In this study, we evaluated the capability of the cyanobacterium Schizothrix cf. delicatissima AMPL0116 to form biocrusts when inoculated in sand microcosms under stressing conditions, and the characteristics of the synthesized polymeric matrix. In parallel, we evaluated the characteristics of exopolysaccharidic exudates of the strain when grown in liquid culture, under optimal growth setting. Our results pointed out at significant differences of the exopolymers produced in the two conditions in terms of monosaccharidic composition and molecular weight distribution, and proved the capability of S. cf. delicatissima AMPL0116 to form stable bioaggregates on sandy soils.

The interior of soil aggregates investigated by synchrotron-radiation-based microtomography

Knowledge on the geometry of pore networks of intra-aggregate soil pore spaces are of great value for many soil environmental processes. Advances in non-invasive 3D imaging techniques such as synchrotron-radiation-based microtomography offer an excellent opportunity to study the interrelationship of the pore network geometry with physical processes at a spatial resolution of a few micrometers. This paper presents results of a quantitative 3D pore space geometry analysis of small scale (~5mm across) soil aggregates from contrasting soil management systems. Soil aggregates have been scanned at the SR-μCT facility operated by the GKSS Research Center at HASYLAB / DESY (Hamburger Synchrotron Strahlungslabor / Deutsches Elektronen Synchrotron) in Hamburg/Germany. The achieved isotropic voxel resolution of the scans ranged from 2.4 to 5.4 μm. Three-dimensional reconstructions of the soil aggregates were analysed for various pore space features using a suite of algorithms based on mathematical morphology. Results have shown expected differences in distributions of pore size, throat size, channel length and width as well as tortuosity and connectivity of the intra-aggregate pores with potential implications for soil functions. Underlying image transformations and methods of visualization and quantification of soil pore networks will be discussed in view of their robustness and possible application of such information in soil related research fields.

Influence of non-invasive X-ray computed tomography (XRCT) on the microbial community structure and function in soil.

In this study the influence of X-ray computed tomography (XRCT) on the microbial community structure and function in soils has been investigated. Our results clearly indicate that XRCT of soil samples has a strong impact on microbial communities and changes structure and function significantly due to the death of selected microbial groups as a result of the treatment.