A multi-region study reveals high overwinter loss of fall-applied reactive nitrogen in cold and frozen soils

In cold agricultural regions, animal manure and synthetic fertilizers may be applied in the fall for convenience. However, the fate of applied nitrogen (N) is unclear and may differ depending on N source and interannual and regional variations in winter conditions. A multi-region study using 15N-labelled reactive N (NH4-15N) applied in the fall with pig slurry, dairy cattle slurry, and ammonium sulfate was carried out under a range of climatic conditions. Nitrification and immobilization of applied NH4-N occurred throughout the winter period at all sites. Transformation and losses were slower and less at the sites where significant soil freezing occurred than at the site where soil rarely froze, highlighting the repressive effect of frost. Nevertheless, losses were similar among sites with significant freezing despite marked differences in duration and extent of freezing. This suggests that soil microbes were adapted to prevailing winter conditions at each site and able to use and transform fall-applied N...