Soil greenhouse gas flux and nitrogen mineralization following manure application from tannin-fed dairy cows

Growing concerns about environmental impacts of dairy farms have driven producers to address greenhouse gas (GHG) emissions and nitrogen (N) losses from soil following land application of dairy manure. Tannin dietary additives have proved to be a successful intervention for mitigating GHG and ammonia (NH₃) emissions at the barn scale. However, it is unknown how land application of dairy manure from cows fed tannin diets affects crop–soil nitrogen dynamics and soil GHG flux. To test this, cows were fed diets at three levels of tannins (0.0%, 0.4%, and 1.8% of dry matter intake) and their manure was field applied at two N rates (240 and 360 kg N ha⁻¹). Soil NH₄⁺-N, NO₃⁻-N, corn silage yield, and soil GHG flux were then measured over a full growing season. Soils amended with tannin manure had lower initial NH₄⁺-N concentrations and lower total mineral N (NH₄⁺-N + NO₃⁻-N) concentrations 19 days after application, compared to soils amended with no tannin manures. Despite lower early season N availability in tannin-fertilized plots, there were no differences in corn silage yield. No differences in soil GHG and NH₃ emissions were observed between manure-amended treatments. These results demonstrate that while tannin addition to dairy cow feed does not offer short-term GHG or NH₃ emissions reductions after field manure application, it can promote slower soil N mineralization that may reduce reactive N loss after initial application.