Effect of Quality and Quantity of Residue Inputs On System Efficiency and Productivity in Conventional and Diversified Rotations.

Many studies show that diversified rotations including cover crops can have increased yields, nitrogen (N) use efficiency and soil organic carbon (SOC) storage compared to simple rotations, but the causal mechanisms are still unclear.  Controlled incubation studies have identified several processes by which system input properties interact with the soil environment to determine organic matter dynamics and N release.  Important among these are slow breakdown of lower quality inputs, stimulation of microbial activity by labile material or mineral fertilizer,  initial protection of organic matter by stable macroaggregates and differential placement, shape and quality of root and shoot material.  However, their importance in a system depends greatly on the specific input properties and field conditions, some of which change according to soil depth and time of year.  The objective of this study was to identify the extent to which any of these processes helped explain nutrient cycling and yield differences observed between a conventional and two diversified rotations at the Marsden Crop Rotation Experiment in central Iowa. We characterized organic inputs by measuring the quantity and C to N ratio of compost, harvest residues and mid- and end-of season root mass.   We obtained soil samples from five dates and two depths over a two-year period for analysis.  As metrics of SOC turnover and nutrient cycling, we used external input and soil C and N content to create rough C and N budgets, as well as measuring potentially mineralizable N and particulate organic matter.  To characterize soil environmental factors likely to affect organic matter turnover, we analyzed the soil samples for texture and bulk density.  For evidence of differences in SOC protection in aggregates or microbial activity, we also measured wet aggregate stability and soil enzyme activity.  Our results suggest that in these heavy soils macroaggregate protection of organic matter is not an important mechanism, but that the tillage, manure inputs and large cover crop root systems in the diversified rotations are able to increase yields and nitrogen use efficiency mainly by providing a store of labile material throughout the soil profile.