Tillage, Cover Crop and Nitrogen Fertilization Effects on Soil Microbial Carbon Dynamics under Long-term Cotton Production

Effective soil management requires a deeper understanding of how internal soil carbon (C) works. This is vital as cycling is tightly linked with nutrient cycling processes. This study evaluated the effect of agronomic practices including tillage (no-till and tilled), cover crops (no cover, hairy vetch - Vicia villosa L, and winter wheat - Triticum aestivum. L ) and different N fertilizer rates ( 0, 34 and 101 kg N per hectare ) on soil microbial C dynamics as measured by soil microbial biomass C (SMBC) along with soil microbial respiration over the 2013 cotton growing season at Jackson, West Tennessee. The study was set as split -split plot with N fertilizer rates as the whole plot, cover crops as the split plot and tillage as the split-split plot. Sampling was carried out four times; in June, July, September and October. Both SMBC and soil respiration responded similarly to the treatment factors. Early in the season, June and July, N fertilizer rate affected SMBC with 101N fertilizer rate having greater SMBC. In July, in addition to N fertilizer rate, tillage affected SMBC with tilled-vetch treatment at 34N and 101N fertilizer rate having a significantly greater levels of SMBC while soil respiration was greater under the no-till no cover treatment across all N fertilizer rates. However in September, tillage and cover crop affected SMBC and soil microbial respiration. The no-till no cover and no-till wheat treatments had greater SMBC and soil microbial respiration compared with no-till vetch at 0N and 34N fertilizer rate. At the end of the growing season prior to harvest, the no-till no cover treatment still had effect on soil microbial respiration while none of the treatment factors affected SMBC. Early in the season N fertilizer influences microbial activities while later in the season tillage and cover crop become the dominating factors. Overall the effects of N fertilizer, tillage and cover crops on these soil properties were season dependent.