Crop/livestock integration effects on soil quality, crop production, and soil nitrogen dynamics

Regional integration of potato and dairy farms has developed in Maine through arrangements where manure, feed, and sometimes land, are exchanged between neighboring farms. The effects of integration on soil quality, crop production, nitrogen (N) cycling, and N loss were investigated in field and laboratory studies of contrasting amended (manure, compost, green manure, and supplemental fertilizer) and nonamended (fertilizer only) soil management systems within 2-year potato (Solanum tuberosum L.) rotations in the Maine Potato Cropping Systems Project (MPEP). Additionally, soil quality of 48 integrated and nonintegrated Maine potato and dairy farm fields was assessed. The MPEP's amended soil system enhanced soil quality and demonstrated aspects of increased resilience for crop production and N cycling. The amended system produced higher and more stable potato yields than the nonamended system by reducing the impact of adverse growing conditions. It also demonstrated the potential to buffer excess N by retaining a greater proportion of net N inputs than the nonamended system. Possible mechanisms to explain increased N retention include better early-season synchrony between N release and crop uptake, as observed in in situ soil monitoring; carbon-enhanced immobilization of excess N, as observed in a laboratory study; increased recalcitrance of N sources; and physical protection. Nitrogen loss, in absolute terms, however, was higher in the amended system due to higher N inputs and a build-up of soil organic N. Soil amendment history had the largest impact on soil N mineralization capacity---fall nitrate levels were higher in the amended system in two of three years, and residual manure N contributed more N than predicted using the standard decay-series model---but it also reduced the availability of recently added N. As currently practiced in Maine, integrated potato systems appear to need greater increases in carbon inputs (preferably as sod crops and trap crops) and reductions in tillage to produce changes in soil carbon that can be detected at a landscape level Future work should focus on finding balance points for soil organic matter content that enhance soil's crop production and N cycling functions while avoiding N excesses and loss.