Increasing agricultural N use efficiency is one of society’s ‘Grand Challenges’, becoming more urgent as the farming industry attempts to adopt more sustainable farming methods under climate change. Sustainable management practices, like increasing crop diversity, can help tighten the N cycle in agroecosystems by promoting the internal recycling of N derived from organic sources. 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 (e.g. amino acids) must be considered. Therefore, we ask how does bioavailable N cycling respond to rotational crop diversity under different fertilization rates? We hypothesize that increased crop diversity combined with different levels of fertilization will influence gross protein depolymerization (GPD) rates, which will subsequently dictate N mineralization rates. To test this hypothesis, we collected soil from a long-term diversity experiment at the Eastern Nebraska Research and Extension Center in Mead, NE. 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. A novel isotope pool dilution assay was used to measure GPD rates, AA consumption, and net depolymerization rates. Gross rates of N mineralization were also measured using isotope pool dilution methods.
Results/Conclusions
The interaction of crop rotational diversity and fertilization treatments impacted GPD (p=0.018) and AA consumption rates (p<0.001). GPD and AA consumption rates were significantly higher in the zero-fertilizer, diverse crop rotation (p=0.003), however there were no interaction effects of crop rotational diversity and fertilization on net depolymerization rates. Finally, crop rotational diversity significantly influenced gross N mineralization rates (p=0.021) and fertilization treatments significantly impacted gross N consumption rates (p=0.007). Taken together, there was a significant interaction between crop rotational diversity and fertilizer treatment on net N mineralization (p=0.021). Net mineralization rates were highest in the zero N treatment of the high diversity rotation and lowest in the continuous corn rotation receiving high nitrogen fertilization. Increased AA flux in the zero-fertilizer, diverse treatments is also reflected in higher net rates of N mineralization. These results suggest that bioavailable N cycling is very sensitive to alternative management strategies, especially the recycling of AA-N. These findings show that high rotational crop diversity can be strategically implemented to tighten the N cycle and reduce fertilizer inputs.