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The spatial distribution and accessibility of organic matter (OM) to soil microbes in aggregates –\uddetermined by the fine-scale, 3-D distribution of OM, pores and mineral phases – may be an important control\udon the magnitude of soil heterotrophic respiration (SHR). Attempts to model SHR on fine scales requires data\udon the transition probabilities between adjacent pore space and soil OM, a measure of microbial accessibility\udto the latter. We used a combination of osmium staining and synchrotron X-ray computed tomography (CT) to\uddetermine the 3-D (voxel) distribution of these three phases (scale 6.6 μm) throughout nine aggregates taken from\uda single soil core (range of organic carbon (OC) concentrations: 4.2–7.7 %). Prior to the synchrotron analyses\udwe had measured the magnitude of SHR for each aggregate over 24 h under controlled conditions (moisture\udcontent and temperature). We test the hypothesis that larger magnitudes of SHR will be observed in aggregates\udwith (i) shorter length scales of OM variation (more aerobic microsites) and (ii) larger transition probabilities\udbetween OM and pore voxels.\udAfter scaling to their OC concentrations, there was a 6-fold variation in the magnitude of SHR for the nine aggregates.\udThe distribution of pore diameters and tortuosity index values for pore branches was similar for each of\udthe nine aggregates. The Pearson correlation between aggregate surface area (normalized by aggregate volume)\udand normalized headspace C gas concentration was both positive and reasonably large (r D0.44), suggesting that\udthe former may be a factor that influences SHR. The overall transition probabilities between OM and pore voxels\udwere between 0.07 and 0.17, smaller than those used in previous simulation studies. We computed the length\udscales over which OM, pore and mineral phases vary within each aggregate using 3-D indicator variograms. The\udmedian range of models fitted to variograms of OM varied between 38 and 175 μm and was generally larger\udthan the other two phases within each aggregate, but in general variogram models had ranges <250 μm. There\udwas no evidence to support the hypotheses concerning scales of variation in OM and magnitude of SHR; the\udlinear correlation was 0.01. There was weak evidence to suggest a statistical relationship between voxel-based\udOM–pore transition probabilities and the magnitudes of aggregate SHR (r D0.12).We discuss how our analyses\udcould be extended and suggest improvements to the approach we used

soil

Three-dimensional soil organic matter distribution, accessibility and microbial respiration in macroaggregates using osmium staining and synchrotron X-ray computed tomography