Three dimensional soil organic matter distribution, accessibility and microbial respiration in macro-aggregates using osmium staining and synchrotron X-ray CT
The spatial distribution and accessibility of organic matter (OM) to soil microbes in aggregates – determined by the fine-scale, 3-D distribution of organic matter, pores and mineral phases – may be an important control on the magnitude of soil heterotrophic respiration (SHR). Attempts to model SHR at fine scales requires data on the transition probabilities between adjacent pore space and soil OM, a measure of microbial accessibility to the latter. We used a combination of osmium
... ination of osmium staining and synchrotron X-ray CT to determine the 3-D (voxel) distribution of these three phases (scale 6.6 μm) throughout nine aggregates taken from a single soil core (range of organic carbon (OC) concentrations 4.2–7.7 %). Prior to the synchrotron analyses we had measured the magnitude of SHR for each aggregate over 24 hours under controlled conditions (moisture content and temperature). We test the hypothesis that larger magnitudes of SHR will be observed in aggregates with shorter length scales of OM variation (i.e. more frequent, and possibly more finely disseminated, OM and a larger number of aerobic microsites). <br><br> After scaling to their OC concentrations, there was a six-fold variation in the magnitude of SHR for the nine aggregates. The distribution of pore volumes, pore shape and volume normalised surface area were similar for each of the nine aggregates. The overall transition probabilities between OM and pore voxels were between 0.02 and 0.03, significantly smaller than those used in previous simulation studies. We computed the length scales over which OM, pore and mineral phases vary within each aggregate using indicator variograms. The median range of models fitted to variograms of OM varied between 178 and 487 μm. The linear correlation between these median length scales of OM variation and the magnitudes of SHR for each aggregate was −0.42, providing some evidence to support our hypothesis. We require a larger number of observations to make a statistical inference. There was no evidence to suggest a statistical relationship between OM:pore transition probabilities and the magnitudes of aggregate SHR. The solid-phase volume proportions (45–63 %) of OM we report for our aggregates were surprisingly large by comparison to those assumed in previous modelling approaches. We suggest this requires further assessment using accurate measurements of OM bulk density in a range of soil types.