All soils harbor microaggregates, i.e., compound soil structures smaller than 250 μm. These microaggregates are composed of diverse mineral, organic and biotic materials that are bound together during pedogenesis by various physical, chemical and biological processes. Consequently, microaggregates can withstand strong mechanical and physicochemical stresses and survive slaking in water, allowing them to persist in soils for several decades. Together with the physiochemical heterogeneity of their surfaces, the three-dimensional structure of microaggregates provides a large variety of ecological niches that contribute to the vast biological diversity found in soils. As reported for larger aggregate units, microaggregates are composed of smaller building units that become more complex with increasing size. In this context, organo-mineral associations can be considered structural units of soil aggregates and as nanoparticulate fractions of the microaggregates themselves. The mineral phases considered to be the most important as microaggregate forming materials are the clay minerals and Fe-and Al-(hydr)oxides. Within microaggregates, minerals are bound together primarily by physicochemical and chemical interactions involving cementing and gluing agents. The former comprise, among others, carbonates and the short-range ordered phases of Fe, Mn, and Al. The latter comprise organic materials of diverse origin and probably involve macromolecules and macromolecular mixtures. Work on microaggregate structure and development has largely focused on organic matter stability and turnover. However, little is known concerning the role microaggregates play in the fate of elements like Si, Fe, Al, P, and S. More recently, the role of microaggregates in the formation of microhabitats and the biogeography and diversity of microbial communities has been investigated. Little is known regarding how microaggregates and their properties change in time, which strongly limits our understanding of micro-scale soil structure dynamics. Similarly, only limited information is available on the mechanical stability of microaggregates, while essentially nothing is known about the flow and transport of fluids and solutes within the micro-and nanoporous microaggregate systems. Any quantitative approaches being developed for the modeling of formation, structure and properties of microaggregates are, therefore, in their infancy. We respond to the growing awareness of the importance of microaggregates for the structure, properties and functions of soils by reviewing what is currently known about the formation, composition and turnover of microaggregates. We aim to provide a better understanding of their role in soil function, and to present the major unknowns in current microaggregate research. We propose a harmonized concept for aggregates in soils that explicitly considers the structure and build-up of microaggregates and the role of organo-mineral associations. We call for experiments, studies and modeling endeavors that will link informatio...

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.

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Pore network geometries of intra‐aggregate pore spaces are of great importance for water and ion flux rates controlling C sequestration and bioremediation. Advances in non‐invasive three‐dimensional imaging techniques such as synchrotron‐radiation‐based x‐ray microtomography (SR‐μCT), offer excellent opportunities to study the interrelationships between pore network geometry and physical processes at spatial resolutions of a few micrometers. In this paper we present quantitative three‐dimensional pore‐space geometry analyses of small scale (∼5 mm across) soil aggregates from different soil management systems (conventionally tilled vs. grassland). Reconstructed three‐dimensional microtomography images at approximate isotropic voxel resolutions between 3.2 and 5.4 μm were analyzed for pore‐space morphologies using a suite of image processing algorithms associated with the software published by Lindquist et al. Among the features quantified were pore‐size distributions (PSDs), throat‐area distributions, effective throat/pore radii ratios as well as frequency distributions of pore channel lengths, widths, and flow path tortuosities. We observed differences in storage and transport relevant pore‐space morphological features between the two aggregates. Nodal pore volumes and throat surface areas were significantly smaller for the conventionally tilled (Conv.T.) aggregate (mode ≈ 7.9 × 10−7 mm3/≈ 63 μm2) than for the grassland aggregate (mode ≈ 5.0 × 10−6 mm3/≈ 400 μm2), respectively. Path lengths were shorter for the Conv.T. aggregate (maximum lengths < 200 μm) compared with the grassland aggregate (maximum lengths > 600 μm). In summary, the soil aggregate from the Conv.T site showed more gas and water transport limiting micromorphological features compared with the aggregate from the grassland management system.

Abstract:Temporal stability of soil moisture spatial patterns has important implications for optimal soil and water management and effective field monitoring. The aim of this study was to investigate the temporal stability of soil moisture spatial patterns over four plots of 105 m ð 135 m in grid size with different grazing intensities in a semi-arid steppe in China. We also examined whether a time-stable location can be identified from causative factors (i.e. soil, vegetation, and topography). At each plot, surface soil moisture (0-6 cm) was measured about biweekly from 2004 to 2006 using 100 points in each grid. Possible controls of soil moisture, including soil texture, organic carbon, bulk density, vegetation coverage, and topographic indices, were determined at the same grid points. The results showed that the spatial patterns of soil moisture were considerably stable over the 3-y monitoring period. Soil moisture under wet conditions (averaged volumetric moisture contents >20%) was more stable than that under dry (Â j,t < 10%) or moist (Â j,t D 10-20%) conditions. The best representative point for the whole field identified in each plot was accurate in representing the field mean moisture over time (R 2 ½ 0Ð97; p < 0Ð0001). The degree of temporal persistence varied with grazing intensity, which was partly related to grazing-induced differences in soil and vegetation properties. The correlation analysis showed that soil properties, and to a lesser extent vegetation and topographic properties, were important in controlling the temporal stability of soil moisture spatial patterns in this relatively flat grassland. Response surface regression analysis was used to quantitatively identify representative monitoring locations a priori from available soil-plant parameters. This allows appropriate selection of monitoring locations and enhances efficiency in managing soil and water resources in semi-arid environments.

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