Synchrotron Observations of Soil Pore Continuities Modified by Related Biophysical Processes.

Advances in X-ray microtomography enabled the examination of intact pore networks, as small as 3 to 5 microns, within soil volumes.  Field management practices controlled clay-organic-ion sorptivity which modified the formation and function of pores undergoing alterations during frequent dry/wet (DW) cycling. Soil aggregates subjected to multiple DW cycling developed greater spatial correlation ranges of ¹³C sequestration away from respiring microorganisms. T-RFLP electropherograms of PCR-amplified 16S rDNA microbial nucleotides demonstrated significant shifts in the abundance of unique microbial ribotypes among porosities within exterior and interior regions of macroaggregates. Differences in micro-scale pore structures substantially influenced the flux and fate of E. coli within soil volumes. Bacterial flux rates through soil aggregates inoculated with E. coli and incubated under controlled temperature and changing soil water potential environments, which doubled soil water diffusivity, were more uniformly distributed among regions of soil volume fractions containing elevated C and N compounds and cations. E. coli became more uniformly distributed throughout aggregates from conventionally tilled loam soils, having greater porosity throughout, while more abundant E. coli  resided in aggregate surfaces, exhibiting larger porosity than their centers, from non-tilled and forest soils. A Sphingobacterium spp. significantly increased erosive strength of stable aggregates while Flavobacterium spp. reduced erosive strength of native soil aggregates by contributing to greater losses of organic carbon from soils during near freezing conditions.