|FOLLETT, RONALD - Retired ARS Employee|
|VOGEL, KENNETH - Retired ARS Employee|
|Mitchell, Robert - Rob|
Submitted to: Global Change Biology Bioenergy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2015
Publication Date: 5/3/2016
Citation: Stewart, C.E., Follett, R.F., Pruessner, E.G., Varvel, G.E., Vogel, K.P., Mitchell, R.B. 2016. N fertilizer and harvest impacts on bioenergy crop contributions to SOC. GCB Bioenergy. doi:10.1111/gcbb.12326.
Interpretive Summary: Belowground C stocks are important to quantify and accurately represent in bioenergy assessments and this study illustrates two contrasting scenarios that have the potential for substantial belowground contributions to soil carbon. Annual no-till-corn produced belowground biomass C proportional to aboveground biomass C with a low C:N ratio and a relatively quick decomposition trajectory into SOC. Under switchgrass, perennial roots built substantial belowground biomass with a higher C:N ratio and slower decomposition and incorporation as POM-C. Despite these differences in C and N cycling, NT-corn had a greater conversion to POM-C per unit root biomass such that after 9 years switchgrass POM-C stocks were similar to NT-corn. These results may explain why no differences in SOC sequestration between NT-corn and switchgrass were observed in the soil profile after the initial 9 years (Follett et al., 2012)
Technical Abstract: Plant contributions to belowground biomass and soil C stocks are important for developing accurate bioenergy lifecycle models. Switchgrass (Panicum virgatum L.) is a native perennial, cellulosic biofuel feedstock with greater root production compared to corn (Zea mays L.,) and potentially contributes to greater soil organic C (SOC) sequestration. Field management (nitrogen (N) fertilizer rate and harvest management) could influence bioenergy crop root biomass allocation and subsequently, SOC and soil properties. We examined plant belowground root biomass, C, N and soil particulate organic matter –C (POM-C) in a 9 year rainfed study of N fertilizer rate (0, 60, 120, and 180 kg N ha-1) and harvest management near Mead, NE USA. Switchgrass was harvested with one pass in either August or post-frost and 50% or no stover was removed for no-till (NT) corn. Switchgrass had greater belowground biomass C & N (6.39, 0.96 Mg ha-1) throughout the soil profile compared to NT-corn (1.30, 0.58 Mg ha-1) and a higher belowground biomass C:N ratio (82.63 vs. 29.41), indicating greater recalcitrant belowground biomass C input beneath switchgrass. There was little difference between the two species in soil POM-C indicating substantially slower decomposition and incorporation into SOC under switchgrass. The highest N rate decreased POM-C under both NT-corn and switchgrass, indicating faster decomposition rates with added fertilizer. Nine years of residue removal reduced corn belowground biomass C by 37% and N by 48% and subsequently reduced POM-C by 22% compared to no residue removal. Agricultural management of bioenergy crops will influence belowground plant allocation, nutrient cycling, and soil C dynamics in this mesic system.