Nitrogen and Harvest Treatment Effects On Soil Properties Under Non-Irrigated Switchgrass and Maize After 9 Years.

Nitrogen fertilizer and harvest management will alter soils under bioenergy production.  The long-term effects of the timing, harvest, and residue removal rates remain relatively unknown.  Compared to no-tilled corn (Zea mays L.), switchgrass (Panicum virgatum L.) is predicted to have a greater positive effect on 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 rain-fed study of N fertilizer rate (0, 60, 120, and 180 kg N ha^-1) and harvest management on switchgrass (harvested at Aug. and post-frost) and no-till corn (with and without stover removal) in eastern, NE. We measured SOC, aggregate stability, microbial biomass, bulk density (BD), pH, P and K in the top 0-30 cm. Both no-till corn (NT-C) and switchgrass increased SMB-C, SOC, and aggregate stability over the nine 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. Cumulative soil P loss was significantly greater beneath NT-C (-14.79 kg P ha^-1) compared to switchgrass (-9.30 kg P ha^-1) after 9 years. When all measured soil parameters were included in the Soil Management Assessment Framework (SMAF), switchgrass improved soil quality indicators (SQI) over time and NT-C decreased soil quality (0-30cm), or had no effect. NT-C residue removal of 50% decreased SQI. Long-term N and harvest management studies that include a diverse set of field measurements - biological, chemical, and physical – are necessary to accurately assess bioenergy impacts on soils.