Julie McKnight, University of Tennessee - Knoxville, Knoxville, TN and Sean Schaeffer, Biosystems Engineering and Soil Science, University of Tennessee, Knoxville, TN
Conservation practices such as no-tillage and cover crops are recommended to maintain or improve the accumulation of soil organic matter, minimize the risks of soil erosion or loss in soil productivity, and decrease system fertilizer nitrogen (N) input requirements and soil N losses. Currently, there is little understanding of the soil microbial and physical processes driving N transformations that affect system N retention. This research assesses the effects of fertilizer application (0 and 67 kg N ha-1) and winter cover crop (no cover, hairy vetch, and winter wheat) treatments on nitrogen transformation and microbial activity rates in a no-till cotton cropland in west Tennessee. Although extractable organic N was similar among all treatments, net N mineralization rates were significantly greater at the vetch sites for both N application treatments. Greater microbial activity at the 0 kg ha-1 N-application vetch site suggests that microbial acquisition at this site is primarily based on soil organic matter decomposition, whereas lower microbial activity at the 67 kg N ha-1 vetch site suggests preferential microbial utilization of fertilizer-based inorganic N which requires less energy for acquisition. However, despite higher immobilization potential, significantly more extractable nitrate was measured at the fertilized (0.00172 µg g-1-dry soil) versus unfertilized (0.00119 µg g-1-dry soil) vetch site, indicating an excess of N that may be at risk of export from the system. Nitrogen content, transformations, and microbial activity were similar between the no cover and winter wheat sites for both fertilization treatments. However, low N mineralization and microbial biomass N at the winter wheat sites suggest a potential difference in soil N availability compared to the no cover sites. Our results suggest that within the scope of no-till management, the combination of fertilization and winter cover crop treatments may have significant implications for soil N transformations and related microbial activity that drive nutrient retention or loss.