Do Long-Term Manure Applications ′Prime′ the Soil for Increased N2Ο Emissions?.

Greenhouse gas emissions from agricultural practices generate some of the largest nitrous oxide (N₂O) emissions in North America. The use of fertilizer amendments to increase crop productivity is a major source of N₂O emission losses especially when nitrogen applications are above crop requirements. A large component of N₂O emissions can be directly attributed to soil microbial activity through nitrification and denitrification. While nitrification emissions are highest during dryer periods with aerated soils, denitrification emissions occur when the soils are more anaerobic. Microbial activity and composition may also change with fertilizer amendment applications. However, changes to the microbial communities and their activities contributing to N₂O fluxes as a result of a change over from long-term manure applications to a chemical fertilizer (urea) is not well understood. The potential changes in soils microbial communities or ‘priming effects’ as a result of this fertilizer amendment changeover needs to be better understood for sustainable agricultural practices and greenhouse gas mitigation.

This study is investigating the potential ‘priming effects’ for N₂O emissions from microbial communities in an agricultural soil with a 12-yr history of manure amendments. The final manure application occurred in the fall of 2009 and since 2011 the plots have received an annual spring application of urea. Microbial populations have therefore experienced a changeover in N source and availability which may lead to a change in N₂O emissions. Two of three field seasons of gas emissions monitoring using vented, non-steady state chambers to assess greenhouse gas emissions have been completed. The results from two seasons of microbial activity analyses by use of denitrification enzyme assays will also be used to assess the potential ‘priming effects’ as a result of excess nutrient availability from long-term manure applications. The preliminary 2012 results show higher microbial potential for denitrification in historically applied manure plots.