Soil CO2 Efflux from Elephantgrass (Pennisetum purpureum L.) Under Different Nutrient Management Regimes.

Elephantgrass (Pennisetum purpureum L. Schum.) is a promising candidate for bioenergy production in the southeastern US, however its high dry matter production is coupled with high nutrient removal. Alternative nutrient management strategies, such as returning biochar or fermentation residual from converting biomass to fuels, could help to offset nutrient demand, but the implications for carbon cycle are not well understood. The objective of this research was to compare seasonal CO₂ flux from the soil for 4-yr old elephantgrass field plots under five different nutrient management scenarios. Treatments included 1) bare soil control; 2) 50 kg N ha^-1 yr^-1 (E50); 3) 50 kg N ha^-1 yr^-1 plus 9 Mg ha^-1 yr^-1 lignocellulosic fermentation residual (E50FR); 4) 50 kg N ha^-1 yr^-1 plus 7 Mg pyrolysis biochar residual ha^-1 yr^-1 (E50BC); and 5) 250 kg N ha^-1 yr^-1 (E250). Flux data were collected in 2016 from May to December (17 sampling events) using a portable FTIR analyzer (DX4040, Gasmet^TM) with a 20-cm survey chamber. Soil CO₂ efflux peaked immediately after treatment application, then gradually declined over the rest of the season. Over the first month following treatment application, soil CO₂ efflux was greater for E50FR (3,115 kg C ha^-1) compared to all other treatments (avg. of 1,136 kg C ha^-1), which did not differ. During the remainder of the season E50FR also showed greater soil CO₂ efflux compared to the other treatments, which did not differ, albeit to a lesser extent (9,310 vs. 4,913 kg C ha^-1, respectively). Despite greater C inputs for E50FR compared to E50BC, much greater soil CO₂ efflux for E50FR suggests greater C retention by the E50BC treatment. This is consistent with soil C measurements that show E50BC in top 10 cm to be 18.1 g kg^-1 compared to 10.6 g kg^-1 for E50FR. These results demonstrate that while biochar and fermentation residual could help to sustainably supply nutrients to bioenergy cropping systems, the application of these residuals will have big differences for carbon cycling and GHG emissions.