Dinitrogen (N$_{2}$) pulse emissions during freeze-thaw cycles from montane grassland soil

Abstract

Short-lived pulses of soil nitrous oxide (N$_{2}$O) emissions during freeze-thaw periods can dominate annual cumulative N$_{2}$O fluxes from temperate managed and natural soils. However, the effects of freeze thaw cycles (FTCs) on dinitrogen (N$_{2}$) emissions, i.e., the dominant terminal product of the denitrification process, and ratios of N$_{2}$/N$_{2}$O emissions have remained largely unknown because methodological difficulties were so far hampering detailed studies. Here, we quantified both N$_{2}$ and N$_{2}$O emissions of montane grassland soils exposed to three subsequent FTCs under two different soil moisture levels (40 and 80% WFPS) and under manure addition at 80% WFPS. In addition, we also quantified abundance and expression of functional genes involved in nitrification and denitrification to better understand microbial drivers of gaseous N losses. Our study shows that each freeze thaw cycle was associated with pulse emissions of both N$_{2}$O andN$_{2}$, with soil N$_{2}$ emissions exceeding N$_{2}$O emissions by a factor of 5–30. Increasing soil moisture from 40 to 80% WFPS and addition of cow slurry increased the cumulative FTC N$_{2}$ emissions by 102% and 77%, respectively. For N$_{2}$O, increasing soil moisture from 40 to 80% WFPS and addition of slurry increased the cumulative emissions by 147%and 42%, respectively. Denitrification gene cnorB and nosZ clade I transcript levels showed high explanatory power for N$_{2}$O and N$_{2}$ emissions, thereby reflecting both N gas flux dynamics due to FTC and effects of different water availability and fertilizer addition. In agreement with several other studies for various ecosystems, we show here for mountainous grassland soils that pulse emissions of N$_{2}$O were observed during freeze-thaw.More importantly, this study shows that the freeze-thaw N$_{2}$ pulse emissions strongly exceeded those of N$_{2}$O in magnitude, which indicates that N$_{2}$ emissions during FTCs could represent an important N loss pathway within the grassland N mass balances. However, their actual significance needs to be assessed under field conditions using intact plant-soil systems