Converting bahiagrass pasture land to elephantgrass bioenergy production enhances biomass yield and water quality

Changing pasture land to the production of bioenergy crops will affect regional water dynamics. Returning by-products of industrial conversion of bioenergy crops like fermentation residual or biochar back to the field could be used to improve sustainable nutrient management, but could also impact water quantity and quality in ways that are poorly understood. The objective of this study was to assess the effects of land-use conversion from low-input bahiagrass (Paspalum notatum Flüggé) pastures to elephantgrass [Pennisetum purpureum (L.) Schum.] for bioenergy production under different nutrient management practices on biomass yield, crop water dynamics and nitrate-nitrogen (NO3-N) leaching during growing and dormant seasons. Treatments evaluated were 1) bahiagrass + 50 kg N ha−1; 2) elephantgrass + 50 kg N ha−1; 3) elephantgrass + 50 kg N ha−1 + fermentation residual; 4) elephantgrass + 50 kg N ha−1 + biochar; and 5) elephantgrass + 250 kg N ha−1. Data were collected on crop evapotranspiration (ET), water use efficiency, drainage, NO3-N leaching, and aboveground dry matter accumulation. Dry matter yield of elephantgrass was 4- to 7-fold greater than bahiagrass after the first growing season, but was similar among elephantgrass treatments. Elephantgrass produced with no residual amendments reduced drainage (approx. 43% across all growing seasons) compared to bahiagrass, and this reduction was exacerbated in the residual treatments. Reduced drainage was associated with increased ET. Elephantgrass, regardless of treatment, reduced the amount of NO3-N lost through drainage compared to bahiagrass. Therefore, replacing bahiagrass with elephantgrass will increase cropping system water use and diminish the rate of groundwater replenishment during the growing season, which could have detrimental effects for other ecosystem processes that rely on this water resource. However, elephantgrass increased the efficiency with which water was used to produce biomass and reduced NO3-N leaching to groundwater.