Diversified crop rotations positively impact ecosystem processes in temperate agroecosystems by increasing yield resilience, crop productivity, and soil quality. Furthermore, diversified rotations require less nitrogen (N) fertilizer to achieve similar yields, and we are interested in how changes in soil N cycling could lead to a greater capacity of soil to supply N. To gain a mechanistic understanding of how crop rotational diversity and fertilizer application simultaneously influence N cycling, protein depolymerization (the rate limiting step of N mineralization) of N-containing, high-molecular weight compounds must be considered. Therefore, we ask how does rotational crop diversity influence bioavailable N cycling under different fertilization regimes? We hypothesize that rates of gross protein depolymerization (GPD) under elevated N is influenced by temporal crop diversity. To test this hypothesis, we collected soil from a long-term diversity experiment at the Eastern Nebraska Research and Extension Center in Mead, NE. The experiment is a is split-plot factorial design, with crop as the main plot and fertilization as the subplot. Plots sampled included both 0 kg and 180 kg ha-1 fertilization treatments in the continuous corn, continuous sorghum-soybean, and corn-soybean-sorghum-oat/clover rotations. We conducted a novel, isotopic pool dilution assay that measures GPD rates, amino acid consumption, and net depolymerization rates. In addition, gross rates of N mineralization were measured along with microbial biomass, carbon use efficiency, respiration, enzyme activity, and total C and N.
Results/Conclusions
There was a significant interaction between crop rotation and N application for GPD rates (p = 0.005). Rates of GPD were higher under zero N application than under high N application (p < 0.0001). In the zero N application treatment, rates of GPD in the continuous-corn plots were significantly higher than the corn-soy plots (p = 0.003) and the diverse plots (p = 0.19), respectively. However, there were no significant differences between any of the crop rotation treatments in the high N application plots. Our data suggest that there is strong competition between plants and microbes for N in the zero N plots, especially in the low diversity rotation (continuous corn). The implementation of just one cover crop stimulates a similar N mining response that is seen in the most diverse rotations. Taken together, these results imply that under the most extreme N limitation in agricultural systems, diversifying crop rotations by even one cover crop can help ameliorate the need for additional N fertilizer.