Nitrogen loss, nitrogen functional genes, and humification as affected by hydrochar addition during chicken manure composting.

This study investigated the effect of hydrochar addition on nitrogen (N) transformation, N functional genes, and humification during chicken manure composting. The addition of 10 % hydrochar reduced cumulative ammonia (NH3) and nitrous oxide emissions by 55.24 % and 45.30 %, respectively, and N losses by 32.07 %. Further, it increased the relative abundance of amoA while decreasing that of nirK, nirS, and nosZ in compost. Hydrochar reduces NH3 emissions during composting owing to its acid-carbon properties that lower the pH of the composting pile and promote ammonia oxidation. Moreover, hydrochar addition enhances the humification of the composting pile and significantly increases the content of humic substances. Moreover, after hydrochar addition, the germination index of the compost product reached >80 % 10 days earlier. The results demonstrate that hydrochar is a suitable composting additive for reducing N loss and shortening the composting time.