Nitrogen fertilizer and harvest management will alter soils under bioenergy crop production and the long-term effects of harvest timing and residue removal remain relatively unknown. Compared to no-tilled corn (NT-C, Zea mays L.), switchgrass (Panicum virgatum L.) is predicted to improve soil properties [i.e. soil organic C (SOC), soil microbial biomass (SMB-C), and soil aggregation] due to its perennial nature and deep-rooted growth form, but few explicit field comparisons exist. We assessed soil properties over 9 years for a rainfed study of N fertilizer rate (0, 60, 120, and 180 kg N ha À1 ) and harvest management on switchgrass (harvested in August and postfrost)and NT-C (with and without 50% stover removal) in eastern NE. We measured SOC, aggregate stability, SMB-C, bulk density (BD), pH, P and K in the top 0-30 cm. Both NT-C and switchgrass increased SMB-C, SOC content, and aggregate stability over the 9 years, reflecting improvement from previous conventional management. However, the soils under switchgrass had double the percent aggregate stability, 1.3 times more microbial biomass, and a 5-8% decrease in bulk density in the 0-5 and 5-10 cm depths compared to NT-C. After 9 years, cumulative decrease in available P was significantly greater beneath NT-C (À24.0 kg P ha À1) compared to switchgrass (À5.4 kg P ha À1). When all measured soil parameters were included in the Soil Management Assessment Framework (SMAF), switchgrass improved soil quality index over time (DSQI) in all depths. NT-C without residue removal did not affect DSQI, but 50% residue removal decreased DSQI (0-30 cm) due to reduced aggregate stability and SMB-C. Even with best-management practices such as NT, corn stover removal will have to be carefully managed to prevent soil degradation. Long-term N and harvest management studies that include biological, chemical, and physical soil measurements are necessary to accurately assess bioenergy impacts on soils.Keywords: harvest timing, no-till corn, P, K, N fertilizer, residue removal, soil C sequestration, soil organic C, switchgrass IntroductionBioenergy production has increased exponentially over the past 30 years (U.S. Department of Energy, 2013) in part leading to a dramatic increase in US acreage planted to corn (Zea mays) USDA-NAAS, 2013 and consuming over 30% of the US corn production in 2009 (Robertson et al., 2011). The primary feedstock for producing cellulosic bioethanol, corn stover, is attractive due to the large quantity of biomass available (Karlen et al., 2011a, b). However, concerns have been raised about the long-term annual removal of 50% or more of the crop residue and its potential to decrease both plant and soil productivity while also increasing the potential for soil erosion-ultimately reducing future yields and decreasing soil organic carbon (SOC) content (Johnson et al., 2011; Karlen et al., 2011a, b).Switchgrass (Panicum virgatum L.) is a perennial, native, C4 grass capable of growing on a broad range of soil types and under non-irrigated conditions (Sander...

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Belowground root biomass is infrequently measured and simply represented in models that predict landscapelevel 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, 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 À1 ) 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 belowground root biomass C and N (6.39, 0.10 Mg ha À1 ) throughout the soil profile compared to NT-corn (1.30, 0.06 Mg ha À1) and a higher belowground root biomass C:N ratio, indicating greater recalcitrant belowground 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 belowground 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 aboveground productivity but potentially reduces SOC sequestration by reducing belowground root biomass and increasing root and soil C decomposition.