Managing Cover Crop and Manure Nitrogen to Increase Nitrogen Use Efficiency and Reduce Nitrous Oxide Emissions.

While organic sources of N have the potential to contribute more to soil health and provide more ecosystem services than mineral fertilizers, there has been limited research on manipulating the 4 Rs (Right Source, Rate, Timing, Placement) to improve their efficiency.  The benefits of using organic N inputs are illustrated by results from a Long-Term Agricultural Research project in Beltsville, Maryland. In an organic corn-soybean-wheat rotation that relies solely on legume cover crops and poultry litter (PL, applied at conservative rates) as N inputs, soil reserves after 13 years were sufficient to increase potentially mineralizable N by 41% compared to a similar conventional rotation receiving only mineral N. Corn grain yield in microplots receiving no N inputs in 2009 was 10.2 Mg ha^-1 in the organic and only 7.12 Mg ha^-1 in the conventional system. The organic system, however, has elevated soil test P and N₂O emissions twice as high as the conventional system. A number of studies conducted by scientists at the USDA-ARS Sustainable Agricultural Systems Lab indicate that organic N management can be improved using the 4R approach when legume cover crops and manure are used in concert. PL application rate studies indicate that applying PL at P removal or lower rates in conjunction with legume cover crops on relatively high N soils produces maximum corn grain yields. Studies comparing timing and placement of PL in systems also using legume cover crops show that corn grain yield is almost always similar or greater when subsurface banded (SSB) PL is applied at sidedress compared to other application methods (broadcast, incorporated) or timing (at planting). Emissions of N₂O were lowered while maintaining comparable N inputs by using SSB PL with grass:legume cover crop mixtures rather than legume monocultures.  SSB PL also decreased N₂O emissions relative to UAN at equivalent rates following a grass:legume cover crop mixture.