Effect of Organic Input Quality On Soil Macroaggregate Stability in Conventional and Diversified Cropping Systems.

Macroaggregate dynamics are an important part of the carbon (C) and nitrogen (N) cycles, particularly in disturbed systems such as agricultural fields.  A better understanding of how system management affects macroaggregate stability will improve our ability to tighten nutrient cycles and so promote a more sustainable agriculture.  While many factors affect soil macroaggregate behavior, one of the most important and least understood is the biochemical quality of organic matter additions.  The objective of this experiment was to evaluate how the macroaggregate dynamics of soils in the Marsden Plots, IA, which compare three cropping systems (a conventional 2-year corn-soybean rotation and two diversified rotations which included cover crops and manure additions), were affected by the different quality inputs that each received.  For each rotation, soil samples were obtained in fall prior to residue incorporation, in spring directly after planting and in summer at maximum root biomass. Input quality was analysed as the C and N contents of the compost, inorganic N and tilled-in cover crops, as well as the quality of the large visible residues removed from the sample.   From the soil, water-stable macro- and micro-aggregates were determined by wet-sieving.  Particulate organic matter that had been occluded within the macroaggregates (OPOM) was extracted and quantified, and its C, N and lignin content was measured. Soil macroaggregation, microaggregation and OPOM quality and quantity were correlated to the input quality measures.  Results will be used to test the hypothesis that more recalcitrant POM would act as a stabilizing agent for aggregates, meaning that more recalcitrant residue inputs (both on short-term (ie, crop) and long-term (ie, system) scales) would be associated with increased proportions of stable aggregates and more recalcitrant and plentiful OPOM.