Nitrogen and harvest effects on soil properties under rainfed switchgrass and no-till corn over 9 years: implications for soil quality

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-1) 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 (-24.0 kg P ha-1) compared to switchgrass (-5.4 kg P ha-1). When all measured soil parameters were included in the Soil Management Assessment Framework (SMAF), switchgrass improved soil quality index over time (Delta SQI) in all depths. NT-C without residue removal did not affect Delta SQI, but 50% residue removal decreased Delta SQI (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.