Soil Micropore Development and Contributions to Soluble Carbon Transport within Macroaggregates

Abstract

Soil biophysical transport mechanisms promoting biogeochemical sorption of soluble organic carbon (SOC) compounds within macroaggregates control the retention and release of most soil nutrients, C‐ and N‐based polysaccharides, and contaminants. Ecosystems containing continuous supplies of soluble root exudates and particulate organic matter (POM) provide a constant supply of mobile SOC compounds to surfaces and internal pore networks of soil aggregates. Intra‐aggregate pores, especially the ultrafine pores, appear to be developed, interconnected, and blocked or disconnected by repeated drying and wetting (DW) cycling with direct but unknown contributions to movement and retention of SOC compounds. There is evidence that the severity (e.g., range of soil water potential) and frequency of severe DW cycles control intra‐aggregate micro‐ and nanopore formation and function. Heterogeneously distributed microsites within aggregates contain microbial communities that readily mineralize available C and N compounds, producing mobile SOC that can be tightly sorbed to additional mineral surfaces made available within micro‐ and nanosized fissures during repeated DW cycling. Mechanical removal of concentric soil layers of aggregates, synchrotron imaging and computer microtomographic (CMT) image processing software of three‐dimensional pore networks and connectivities, coupled with synchrotron X‐ray small angle scattering to measure pore sizes. Natural isotopes of 13C and 15N to quantify C and N sorption and CO2 respiration provide new and integrated approaches for quantifying spatially heterogeneous changes of pore diameters, connectivities, and organo‐ion‐mineral sorption within intra‐aggregate pore networks. Net C and N alterations at surfaces and within aggregates appear to modify both the microbial activities and bacterial community structures, producing integrated feedback and feed‐forward processes between the soil biological and physical components of soil aggregates.

Modeling the effect of soil meso- and macropores topology on the biodegradation of a soluble carbon substrate

“…The heterogeneity of the arrangement of soil particles (inter-aggregate vs intra-aggregate porosity) and the variation of water saturation conditions contribute to maintain gradients in abiotic conditions (nutrients, pH and redox conditions) and therefore locally promote or not the growth of microorganisms [6]. The patchy distribution patterns of bacteria in soils can result in spatial disconnection between organic residues and decomposers and thereby influence the kinetics of decomposition of organic compounds, as has been shown experimentally by [7,8].…”

Estimating effects of compaction on pore size distribution of soil aggregates by mercury porosimeter

“…Internal aggregate porosities transport and store substantial organic substrates on mineral surfaces within micropores of aggregate interior regions Smucker et al, 2007). Therefore, stable aggregates protect C from microbial decomposition and affect the soil capability in carbon sequestration (Dexter et al, 2000;Kęsik et al, 2010;Lal et al, 2003;Six et al, 2004;Smith, 2004).…”

Exploration of soil micromorphology to identify coarse-sized OM assemblages in X-ray CT images of undisturbed cultivated soil cores

“…Coarse-sized OM can constitute highly reactive sites for pesticides (Benoit et al, 2008). Identification of the spatial location of the different natures of SOM in the soil pore space together with the spatial distribution of micro-organisms in soils will thus help in modelling the fate of pesticides, as suggested by recent studies (Monga et al, 2008;Or et al, 2007;Smucker et al, 2007;Vieublé-Gonod et al, 2009).…”