Influence of cover crop species and fertilization on nitrogen dynamics and loss in organic and conventional conservation agriculture systems

Background/Question/Methods

Unintentional nitrogen (N) loss from agroecosystems produces greenhouse gases, induces eutrophication, and is costly for farmers; therefore, adoption of conservation agricultural management practices, such as no-till, cover cropping, and inter-cropping has increased in the United States.  However, the ecosystem consequences of adopting these practices have not been well-studied, and in practice, many different species of cover crops and combinations of these conservation techniques are employed, complicating the quantification of ecosystem benefits that they may provide. To aid in the development of best management practices that increase N use efficiency while decreasing N loss and environmental impact, we assessed N loss via leaching, NH₃ volatilization, N₂O emissions, and N retention in plant and soil pools of Kentucky corn conservation agroecosystems across one year.  Three systems were evaluated in this field study: 1) an unfertilized, organic system with cover crops hairy vetch (Vicia villosa), winter wheat (Triticum aestivum), or a mix of the two (bi-culture); 2) an organic system with a hairy vetch cover crop employing three fertilization schemes (0 N, organic N, or a cover crop N-credit approach); and 3) a conventional system with a winter wheat cover crop and three fertilization schemes (0 N, urea N, or organic N). 

Results/Conclusions

During cover crop growth, in the unfertilized, organic system, species affected NO₃-N leaching (vetch > bi-culture > wheat; p=0.0015), but gaseous emissions were low across all treatments. Similarly, during corn growth, vetch and bi-culture cover crops increased NO₃-N leaching (p=0.0010) and gaseous N₂O-N loss (p=0.0197) compared to wheat, but also produced higher yields.  Fertilization increased soil inorganic N, gaseous N loss, N leaching, and yield in the organic vetch and conventional wheat systems, while fertilizer type affected soil inorganic N, N leaching, and gaseous fluxes temporally and in magnitude, but not yield.  In the organic vetch system, the organic N fertilizer treatment had the greatest gaseous N₂O-N loss across the growing season (p=0.0096). In the conventional system, NO₃-N leaching with urea was approximately 5x greater than with organic N, and urea fertilization immediately stimulated N₂O and NH₃ fluxes, while in the organic N treatment, fertilizer associated increases in fluxes were delayed.  Overall, fertilizer and legume residue increased soil inorganic N and yield, but differences in temporal trends in N release and availability illustrate that N conserving management techniques can be developed and employed in conservation agriculture systems to reduce N leaching and greenhouse gas emissions without sacrificing yield.