Cover crops can provide numerous benefits to organic agroecosystems, including the provision of plant-available nitrate and ammonium to the soil. When planted in mixtures, cover crop residues decompose at varying rates, releasing plant-available nitrogen over a broad range of time. However, it is not yet understood how cover crop composition affects the rate of plant-available nitrogen release. The present study aims to determine how cover crop treatments (monocultures or mixtures) alter the soil nitrogen environment (nitrate, ammonium and potentially mineralizable nitrogen) on two organic farms in Illinois in 2015-2016. Soils were collected at multiple days after cover crop termination to measure the change in nitrogen pools over time. Potentially mineralizable nitrogen was quantified after a 7-day anaerobic incubation, and nitrate and ammonium concentrations were determined by colorimetric assay. Analysis of variance (ANOVA) and classification and regression tree (CART) analyses were performed to quantify the relative effects of treatment and time post-termination on nitrogen concentrations.
Results/Conclusions
Results show that nitrate and ammonium concentrations peak at 2 weeks post-cover crop termination. All cover crop treatments reduced nitrate except for the weed-free control and field pea. In contrast, time post-termination had a stronger effect on ammonium levels, which generally decreased one month post-termination. Only five treatments, consisting of both monocultures and mixtures, showed an increase in ammonium after one month. Potentially mineralizable nitrogen (PMN), which measures the maximum organic nitrogen in the soil that could be transformed into plant-available forms throughout the growing season, peaked one month after cover crop termination on one of the farms; the other farm saw no change in PMN over time. Mustard grown in monoculture had the highest PMN concentration after one month. While site did not appear to have an overall effect on nitrogen concentrations, differences in PMN between farms may be due to site-specific soil microbes. Overall, there was an observed pulse in nitrate and ammonium after two weeks, with some cover crop treatments exerting a greater effect on nitrogen concentration than others. This work suggests that cover crop mixtures may be “fine tuned” to synchronize crop nitrogen demands with nitrogen release from cover crop residues.