Trade-Offs and Synergies: A Meta-Analysis of Fertilizer Management Affecting Nitrous Oxide and Nitrate Losses.

Faced with economic, environmental, and social pressures, agriculture must produce more food with fewer inputs, while improving air and water quality. How does fertilizer management aimed at mitigating nitrous oxide (N₂O) emissions affect nitrate (NO₃) leaching, and vice versa? Combining almost 800 site-year-treatment observations from 50 field studies of corn in North America, we found very little overlap in climate, soil type, and cropping systems for studies that measured N₂O and NO₃ losses. This makes it difficult to determine how managing fertilizer to reduce one loss pathway affects the other. We calculated classic meta-analysis effect sizes for select fertilizer treatments that had sufficient side-by-side comparison data, primarily fertilizer source effects on N₂O emissions. In the observed systems, urea reduced N₂O losses when compared with anhydrous ammonia, and SuperU^TM reduced N₂O losses when compared to urea, UAN, and coated urea. Using hierarchical (multi-level) regression models we used all observations – grouping by location and weighting the data accordingly – to estimate effects of management, climate, and soil factors on N losses. Yield-scaled emissions of N₂O were reduced with nitrification inhibitors and side-dress fertilizer application (timing); but increased with N rate, injection of fertilizer N, and with higher average July temperatures. Yield-scaled NO₃ losses were positively related to precipitation, but decreased with soil carbon content and seemed to increase with aqueous ammonia fertilizer as compared with UAN. Thus, while we find that N₂O and NO₃ losses respond to different factors, more data is crucial if we are to determine when and where fertilizer management practices are trade-offs or synergies for air and water quality.