Nutrient Cycling in Switchgrass Grown for Biomass.

Switchgrass (Panicum virgatum L.) has been extensively studied as an energy crop, but questions remain about its nutrient management and optimizing its nutrient-use efficiency. We argue here that managers can take advantage of the species’ inherent physiology to significantly reduce nutrient additions and simultaneously improve feedstock quality. Studies conducted in Virginia and elsewhere show that N cycles seasonally between switchgrass roots and shoots. Indirect evidence suggests that >50 kg N ha^-1 can be translocated from standing biomass into belowground reserves at the end of the growing season – but only if the biomass harvest is delayed until late fall. Harvests taken midseason (or late season but before N is translocated below ground) interrupt the annual N cycle, remove additional N from the system, and obligate the grower to replace larger N losses. Adding as little as 50 kg N ha^-1 may maximize switchgrass yields when harvested only once at the end of the seasonal cycle. Our studies and those of others also suggest that allowing aboveground biomass to die and be leached (by rainfall) can “recycle” P and K in situ and reduce requirements for adding those nutrients. Coincidentally and importantly, reductions in N, P, and K are often considered desirable when switchgrass might be used as feedstock for a biorefinery. Management practices that optimize nutrient use – such as delaying harvests till late fall – will be vital in developing sustainable switchgrass-based energy cropping systems.