Losses of native mineral-associated organic nitrogen through microbial mineralization and gaseous emissions induced by ammonium and nitrate addition

Abstract

Atmospheric nitrogen (N) deposition has a significant impact on terrestrial N cycling. However, it has not been well understood how external inorganic N addition affects the dynamics of soil organic N (SON) that supports long-term plant productivity and carbon (C) sequestration. Here, we added six levels of ammonium (NH4+-N) or nitrate (NO3−-N) to a temperate grassland soil to reveal the mechanisms of SON responses to inorganic N addition. Results showed that the SON content decreased by 7% ± 2%–15% ± 1%, which corresponded with the increase in the abundance of functional gene related to SON mineralization (i.e., chiA) by both NH4+-N and NO3−-N. The inorganic N addition stimulated gaseous losses from the native soil N, leading to at least 6% ± 2%–12% ± 1% of the initial SON lost at different N addition levels (i.e., N-priming). However, the responses of nitrification- and denitrification-related functional genes to NH4+-N and NO3−-N treatments varied remarkably. The positive responses of nitrification- and denitrification-related functional genes (i.e., hao, nirS, and narG) to NO3−-N addition contributed to gaseous N losses. The negative responses of some denitrification-related functional genes (i.e., nirK, narG, and napA) to NH4+-N addition resulted in less gaseous N losses by NH4+-N compared with that by NO3−-N addition. Additionally, the release of H+ by NH4+-N addition may liberate organic matter from associated minerals, showing a decrease in mineral-associated organic matter (MAOM) but an increase in extractable Fe. These findings provide direct evidence that the addition of inorganic N results in a net native SON loss through microbe-mediated mineralization and gaseous emissions. The study also emphasizes the different mechanisms of reduced and oxidized N additions in affecting SON mineralization and gaseous N losses.