Light grazing reduces gaseous nitrogen emissions from temperate grassland soils during freeze‒thaw cycles: An intact core incubation study

Abstract

Livestock grazing and soil freeze‒thaw cycles (FTCs) can affect the biogeochemical processes of nitrogen (N) and gaseous N (N₂O, NO, and N₂) emissions from grassland soils. However, the effect of grazing intensity on soil gaseous N emissions during FTCs and the underlying mechanisms are not clearly understood. In this intact core incubation study, soil gaseous N emissions during two FTCs were simultaneously quantified from temperate grasslands that included grazing exclusion (GE), light grazing (LG), and heavy grazing (HG) in Inner Mongolia. Additionally, the abundance of N cycle-related functional genes and the main soil characteristics were determined to better understand the drivers of gaseous N emissions. The results showed that N₂ emissions dominate the gaseous N loss from all investigated soils during FTCs, with cumulative N₂ exceeding NO and N₂O emissions by factors of 47–135 and 71–161, respectively. Increased soil moisture during thawing promoted N₂ and N₂O emissions from all three sites, except for N₂O at the LG site. However, no obvious NO emission peak was observed from all investigated soils during FTCs. Soil C and N availability and aeration changed by grazing regulated soil N₂O fluxes, while the abundances of key functional genes generally did not show significant correlations with gaseous N emissions. Moreover, compared to the GE and HG sites, LG substantially decreased the soil N₂O and total gaseous N emissions during FTCs, suggesting that light grazing rather than long-term grazing exclusion could be a promising measure to reduce gaseous N losses during spring thaw. Our results highlighted the importance of the simultaneous determination of all kinds of gaseous N emissions during FTCs for closing the ecosystem N balance and developing appropriate strategies for grassland management.