Changes in microbial community responses to gradients of carbon, nitrogen, and wetting cycles in concentric layers of soil macro-aggregates

This study examined the effect of carbon (C), nitrogen (N), and wetting cycle gradients within soil aggregates on microbial communities and populations. New methods of bacterial extractions and identification were developed for this study. Direct microscopic counts of bacterial populations were 35% higher in moist than air dry aggregates. Bacterial populations from contrasting management systems and soil types were native forest on silt loam soils > conventional-till on silt loam soils ≅ native grassland on loam soils > conventional-till planted to continuous alfalfa on sandy loam soils. A 17% and 32% increase in populations occurred in the interior regions of aggregates exposed to 3 and 6 W/D cycles. Populations in the exterior regions increased 23% with 3 W/D cycles and decreased 29% with 6 W/D cycles. A decrease in polar tensile strength was observed in aggregates exposed to 6 W/D cycles. This decrease in strength may be caused by decreased populations in the exterior regions, leading to fewer microbial by-products produced and lower aggregate strength. C and N concentrations and other factors appear to contribute to bacterial populations. Direct relationships exist between aggregate stability, porosity, and bacterial populations. As greater energy sources are available to soil bacterial populations, those populations increase in biovolume and density. This increase in size and population leads to greater bacterial by-products that in turn lead to increased aggregate formation and stabilization.