Soil Aggregate Heterogeneity Improves Soil C Sequestration and Reduces Greenhouse Gas Emissions

Soil clay and organic matter contents combined with natural dry/wet cycling alters intra-aggregate pore continuity, connectivity, dead-end storage volumes, and tortuosity. These changes alter gaseous compositions, C and N storage capacities and microbial communities among different regions within soil aggregates. Recent advances in X-ray microtomography enable the examination of intact pore networks within soil aggregates at resolutions as small as 3 microns. We hypothesize repeated and nondestructive quantitative visualizations of aggregates and soil volumes treated with multiple D/W cycles can be used to compare and identify the most ideal intra-aggregate pore structures for maximizing soil C sequestration in natural ecosystems. Geostatistical and multifractal methods provide concise characteristics of pore spatial distributions within the aggregates and are useful for comparing these alterations among soils. We compared similar air-dried aggregates from the same soil type which were subjected to multiple wetting and drying (WD) cycles. Soil aggregate samples were scanned on the bending beam line, station 13-BM-D of the GeoSoilEnvironCARS (GSECARS) at the Advanced Photon Source (APS) of Argonne National Laboratory (ANL). Incident energy of 73 keV was used to establish image resolutions from 3.3 to 13.1 microns. Geostastical analyses confirmed our hypothesis that repeated WD cycles dramatically alter the spatial arrangements of soil particles and associate pore geometries. Aggregates with WD pretreatments developed greater spatial correlation ranges that parallel with increased 13C sorption away from respiring microorganisms. T-RFLP electropherograms of PCR-amplified 16S rDNA microbial nucleotides demonstrated significant shifts in the abundance of unique microbial ribotypes within exterior and interior regions of macroaggregates subjected to 0 and 5 D/W cycles. Modifications of intra-aggregate pores to reduce aerobic and anaerobic greenhouse gases will be described in greater detail.