Grazing Reduces the Soil-Atmosphere Exchange of Greenhouse Gases During Freeze-Thaw Cycles in Meadow Steppes in Inner Mongolia

Fangfang Wang,Bojie Fu,Dongbo Wang,Cong Wang,Yongzhe Chen,Yihe Lv,Ting Li,Xing Wu

doi:10.3389/fevo.2021.795203

Abstract

Both livestock grazing and soil freeze-thaw cycles (FTCs) can affect the soil-atmosphere exchange of greenhouse gases (GHGs) in grasslands. However, the combined effects of grazing and FTCs on GHG fluxes in meadow steppe soils remain unclear. In this study, we collected soils from paired grazing and enclosed sites and conducted an incubation experiment to investigate the effect of grazing on soil GHG fluxes in the meadow steppes of Inner Mongolia during three FTCs. Our results showed that FTCs substantially stimulated the emissions of soil N2O and CO2 and the uptake of CH4 in the meadow steppes. However, compared with enclosure treatments, grazing significantly reduced the cumulative N2O, CO2 and CH4 fluxes by 13.3, 14.6, and 26.8%, respectively, during the entire FTCs experiment. The soil dissolved organic carbon (DOC) and nitrogen (DON), NH4+-N and NO3–-N, significantly increased after three FTCs and showed close correlations with N2O and CO2 emissions. Structural equation modeling (SEM) revealed that the increase in NO3–-N induced by FTCs dominated the variance in N2O emissions and that DOC strongly affected CO2 emissions during thawing periods. However, long-term grazing reduced soil substrate availability and microbial activity and increased soil bulk density, which in turn decreased the cumulative GHG fluxes during FTCs. In addition, the interaction between grazing and FTCs significantly affected CO2 and CH4 fluxes but not N2O fluxes. Our results indicated that livestock grazing had an important effect on soil GHG fluxes during FTCs. The combined effect of grazing and FTCs should be taken into account in future estimations of GHG budgets in both modeling and experimental studies.

Highlights

Greenhouse gases (GHGs), including carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), can significantly promote climate warming on Earth
We proposed three hypotheses: (1) soil Freeze-thaw cycles (FTCs) will stimulate the emissions of N2O and CO2 and the uptake of CH4 by meadow steppe soils; (2) livestock grazing might reduce the soil-atmosphere exchange of GHGs during FTCs; and (3) grazing and FTCs might have an interactive effect on soil GHG fluxes
The aboveground, litter and belowground biomass were significantly lower in the grazing treatments than in the enclosed treatments, but the soil bulk density in the grazing treatments was significantly higher (Supplementary Table 1)

Summary

Introduction

Greenhouse gases (GHGs), including carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), can significantly promote climate warming on Earth. Freeze-thaw cycles (FTCs), which occur predominantly in the soils of some temperate and most high-latitude and highaltitude regions, can greatly affect soil-atmosphere exchange of GHGs in the soil (Smith et al, 2018; Li et al, 2021). Several mechanisms can potentially explain the release of soil GHG fluxes during FTCs: the release of previously generated gases from the unfrozen parts of the soil during soil thawing (Congreves et al, 2018), the increased available carbon and nitrogen substrates (de Bruijn et al, 2009) and the changes in the microbial community during FTCs (Hu et al, 2015). The contribution of the mechanisms to the soil GHG exchanges is not well quantified (Congreves et al, 2018)

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