Nitrogen And Harvest Effects On Soil Properties Under Rainfed Switchgrass And No-Till Corn Over 9 Years: Implications For Soil Quality

Abstract

Nitrogen fertilizer and harvest management will alter soils under bioenergy crop production and the long￢ﾀﾐterm effects of harvest timing and residue removal remain relatively unknown. Compared to no￢ﾀﾐtilled corn (NT￢ﾀﾐC, Zea mays L.), switchgrass (Panicum virgatum L.) is predicted to improve soil properties [i.e. soil organic C (SOC), soil microbial biomass (SMB￢ﾀﾐC), and soil aggregation] due to its perennial nature and deep￢ﾀﾐrooted growth form, but few explicit field comparisons exist. We assessed soil properties over 9ￂﾠyears for a rainfed study of N fertilizer rate (0, 60, 120, and 180ￂﾠkg Nￂﾠha￢ﾈﾒ1) and harvest management on switchgrass (harvested in August and postfrost) and NT￢ﾀﾐC (with and without 50% stover removal) in eastern NE. We measured SOC, aggregate stability, SMB￢ﾀﾐC, bulk density (BD), pH, P and K in the top 0￢ﾀﾓ30ￂﾠcm. Both NT￢ﾀﾐC and switchgrass increased SMB￢ﾀﾐC, SOC content, and aggregate stability over the 9ￂﾠyears, 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￢ﾀﾓ8% decrease in bulk density in the 0￢ﾀﾓ5 and 5￢ﾀﾓ10ￂﾠcm depths compared to NT￢ﾀﾐC. After 9ￂﾠyears, cumulative decrease in available P was significantly greater beneath NT￢ﾀﾐC (￢ﾈﾒ24.0ￂﾠkg Pￂﾠha￢ﾈﾒ1) compared to switchgrass (￢ﾈﾒ5.4ￂﾠkg Pￂﾠha￢ﾈﾒ1). When all measured soil parameters were included in the Soil Management Assessment Framework (SMAF), switchgrass improved soil quality index over time (ￎﾔSQI) in all depths. NT￢ﾀﾐC without residue removal did not affect ￎﾔSQI, but 50% residue removal decreased ￎﾔSQI (0￢ﾀﾓ30ￂﾠcm) due to reduced aggregate stability and SMB￢ﾀﾐC. Even with best￢ﾀﾐmanagement practices such as NT, corn stover removal will have to be carefully managed to prevent soil degradation. Long￢ﾀﾐterm N and harvest management studies that include biological, chemical, and physical soil measurements are necessary to accurately assess bioenergy impacts on soils.