Linking Soil Microbial Ecology to Ecosystem Functioning in Integrated Crop-Livestock Systems.

Enhanced soil stability, nutrient cycling and C sequestration potential are important ecosystem functions driven by soil microbial processes and are directly influenced by agricultural management. Integrated crop-livestock agroecosystems (ICL) can enhance these functions via high-residue returning crops as part of a crop rotation, converting to conservation tillage practices, or inclusion of perennial forages. We compared archived baseline soil samples collected in 1997 of an unmanaged perennial grassland to samples collected in 2010 after conversion to a continuous cotton system (CTN1) and an ICL composed of no-till annual crop rotation and perennial forage paddocks. In a separate study, three additional ICLs and two cotton systems (CTN2 and CTN3) were sampled in 2010. Samples were fractionated into three water-stable aggregate sizes (macroaggregates, microaggregates, and silt+clay) with macroaggregates further separated into three intra-aggregate size fractions (particulate organic matter, microaggregates, and silt+clay). Mean weight diameter (MWD), an indicator of aggregate stability was also calculated. Macroaggregates decreased 2.9 times in CTN1 whereas they were slightly increased (1.1 times) in the ICL. Among the five agroecosystems, macroaggregate formation was between 2.7and 6.0 times higher in the ICLs compared to CTN2 and CTN3. Among ICLs, the ICL comprised of dryland perennials and tilled annuals had the lowest macroaggregate concentration with  the highest concentration in the system comprised of perennial forages and fully irrigated crop rotation. Similar trends were found for soil organic carbon concentrations in the intra-aggregate microaggregate fraction, an indicator of C sequestration potential. The number of operational taxonomic units for fungi (Fungal diversity index) were 3-4 times greater in the ICLs compared to CTN. Strong positive correlations existed between the abundance of arbuscular mychorrhizal fungi (measured by FAME profiling) and MWD (r= 0.70) and phosphodiesterase activity (r=0.75). Our research suggests that soil fungi play critical roles in supporting ecosystem functions including C sequestration and aggregate stability.