Short-term effects of bioenergy cropping on soil carbon and nitrogen dynamics in a Florida Ultisol

Abstract

Although perennial bioenergy crops provide a potential for soil C sequestration, high fertilizer inputs are required to sustain yields. Land application of bioenergy byproducts can be an effective strategy to reduce the use of inorganic fertilizer. This study examined the impacts of elephantgrass [Pennisetum purpureum (L.) Schum.] bioenergy cropping and conventional pasture on soil C and N responses. Treatments included (a) bahiagrass + 50 kg N ha−1 yr−1 (BHG), (b) elephantgrass + 50 kg N ha−1 yr−1 (E50), (c) elephantgrass + 50 kg N ha−1 yr−1 + fermentation residual (E50FR), (d) elephantgrass + 50 kg N ha−1 yr−1 + biochar (E50BC), and (e) elephantgrass + 250 kg N ha−1 yr−1 (E250). Soil C and N responses occurred mainly at the 0- to 10-cm depth. Biochar resulted in the largest increase in soil C stocks at the 0- to 10-cm depth (5.8 Mg C ha−1 in 2012 vs. 40 Mg C ha−1 in 2016), but no differences were observed among the other treatments. Conversely, greater soil N stocks at the 0- to 10-cm depth were associated with the BHG (0.8 and 0.34 Mg N ha−1 for other treatments). Biochar favored N and C accumulation in the mineral-associated (<53 μm) fraction, possibly from the accumulation of fine biochar fragments. In contrast, fermentation residual promoted C accumulation in unprotected light fraction. These results are supported by the δ13C signature and C/N ratios of the fractions. Results underscore the value of recycling biochar in bioenergy cropping systems as a means of improving C in coarse-textured soils.