Biomass Cropping Effects on Soil C and N Dynamics in a Subtropical Ecosystem.

Although perennial crops managed for bioenergy production may enhance soil C sequestration, it is unclear how soil and crop management affect the accumulation, stabilization, and dynamics of soil C in coarse-textured Coastal Plain soils. The objective of this study was to investigate the impacts of converting low-input bahiagrass (Paspalum notatum Flüggé) pastures to elephantgrass [Pennisetum purpureum (L.) Schum.] bioenergy production on soil C and N stocks and distribution among the various size-density fractions. Treatments were 1) bahiagrass + low N (50 kg N ha^-1 yr^-1); 2) elephantgrass + low N (50 kg N ha^-1 yr^-1); 3) elephantgrass + high N (250 kg N ha^-1 yr^-1); 4) elephantgrass + low N + fermentation residual; and 5) elephantgrass + low N + biochar. After 4 yr following conversion to bioenergy cropping, soil C concentrations (0-20 cm) increased by as much as 311% compared with the initial values. The largest increases in soil C were associated with the treatments receiving biochar, particularly at the 0 to 10 cm depth (9 g C kg^-1 soil for elephantgrass + low N vs. 18 g C kg^-1 soil for elephantgrass + low N + biochar). Bahiagrass also promoted soil C accumulation (4.4 g C kg^-1 soil in 2013 vs. 12.3 g C kg^-1 soil in 2016). Elephantgrass amended with biochar increased the mineral-associated C fraction while bahiagrass promoted C accumulation in the particulate organic matter fraction. Results showed that addition of biochar and fermentation residuals altered the δ¹³C signature of size-density fractions. More negative δ¹³C values were generally associated with biochar (-28.4‰) compared to elephantgrass alone (-23.4‰) or bahiagrass (-21.9‰). Soil N concentrations also increased (by an average of 182%) in response to bioenergy cropping. Results from this study underscore the value of bioenergy perennial grass for their potential to sequester soil C.