N fertilizer and harvest impacts on bioenergy crop contributions to SOC

Authors

Catherine E. Stewart, USDA-ARSFollow
Ronald F. Follett, USDA-ARSFollow
Elizabeth G. Pruessner, USDA-ARSFollow
Gary E. Varvel, USDA-ARSFollow
Kenneth P. Vogel, USDA-ARSFollow
Robert B. Mitchell, USDA-ARSFollow

Date of this Version

2016

Citation

GCB Bioenergy (2016) 8, pp. 1201–1211, doi: 10.1111/gcbb.12326.

Comments

U.S. government work.

Abstract

Below ground root biomass is infrequently measured and simply represented in models that predict landscape level changes to soil carbon stocks and greenhouse gas balances. Yet, crop-specific responses to N fertilizer and harvest treatments are known to impact both plant allocation and tissue chemistry, potentially altering decomposition rates and the direction and magnitude of soil C stock changes and greenhouse gas fluxes. We examined switchgrass (Panicum virgatum L.) and corn (Zea mays L.,) yields, below ground root biomass, C, N and soil particulate organic matter-C (POM-C) in a 9-year rain fed 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 postfrost, and for no-till (NT) corn, either 50% or no stover was removed. Switchgrass had greater below ground root biomass C and N (6.39, 0.10 Mg ha^-1) throughout the soil profile compared to NT-corn (1.30, 0.06 Mg ha^-1) and a higher below ground root biomass C:N ratio, indicating greater recalcitrant below ground root biomass C input beneath switchgrass. There was little difference between the two crops in soil POM-C indicating substantially slower decomposition and incorporation into SOC under switchgrass, despite much greater root C. The highest N rate decreased POM-C under both NT-corn and switchgrass, indicating faster decomposition rates with added fertilizer. Residue removal reduced corn below ground root biomass C by 37% and N by 48% and subsequently reduced POM-C by 22% compared to no-residue removal. Developing productive bioenergy systems that also conserve the soil resource will require balancing fertilization that maximizes above ground productivity but potentially reduces SOC sequestration by reducing below ground root biomass and increasing root and soil C decomposition.