Soil Biogeochemical Feedback to Fire in the Tropics: Increased Nitrification and Denitrification Rates and N2O Emissions Linked to Labile Carbon and Nitrogen Fractions

Although tropical ecosystems have become increasingly vulnerable to fire over the past century, the mechanisms by which fire disturbance influences N2O emissions in these regions remain poorly understood. This study investigated the effects of fire on nitrous oxide (N2O) emissions, the gross nitrification rate (GN), denitrification genes, and carbon (C) and nitrogen (N) fractions in a tropical forest. The results showed that fire increased the GN by 41.5%. The abundance of the nirK and nirS genes encoding nitrite reductase increased by 16.3% and 27.5%, respectively, while the abundance of the nosZI gene encoding N2O reductase increased by 28%, suggesting a potentially enhanced denitrification capacity. This enhancement in nitrification and denitrification was mainly due to increased easily oxidizable organic C (EOC, +35%), light fraction organic C (LFOC, +32%), hydrolyzable ammonium N (HAN, +13%), and amino sugar N (ASN, +11%), which provided additional substrates for nitrification and denitrification. As a result, soil N2O emissions increased by 18% in response to fire. Soil N2O emissions showed a significant and positive linear correlation with GN, EOC, LFOC, HAN, nirK, nirS, and nosZI. Thus, the post-fire increase in N2O emissions is likely driven by enhanced nitrification and denitrification processes, facilitated by the elevated availability of labile C and N fractions. Our findings provide new evidence for the role of soil C and N fractions in controlling N2O emission and nitrification–denitrification under fire disturbances in tropical soils.