Carbon budget in permafrost and non-permafrost regions and its controlling factors in the grassland ecosystems of Mongolia

Although many studies have found that global warming has caused permafrost to thaw, we still lack understanding of the mechanism relating permafrost thawing and ecosystem carbon budgets. To compare the effects of freeze–thaw cycles on the grassland ecosystem carbon budget between a permafrost area (PA) and a non-permafrost area (NPA), we established two carbon dioxide flux towers since 2015 to monitor the net ecosystem exchange by eddy covariance (EC) systems at the site of Nalaikh in PA and Hustai in NPA. The gross primary production (GPP), respiration by ecosystems (Reco), and net ecosystem production (NEP) from 2016 to 2019 were estimated using EddyPro 7 and Tovi™. The result showed that, at the PA and NPA sites, the annual GPP was 686.3 and 654.9 g C m−2 y−1, Reco was 611.5 and 699.6 g C m−2 y−1, and NEP was 73.8 and − 45.5 g C m−2 y−1, respectively, which implies that the grassland ecosystem was a carbon sink in the PA but a carbon source in the NPA. Then, the effect of the freeze–thaw cycles on the carbon budget was also analyzed. The NEP in the PA (35.3 g C m−2) was significantly larger than in the NPA (0.3 g C m−2) during the thawing period and, similarly, the NEP in the PA (121.7 g C m−2) was also larger than in the NPA (72.1 g C m−2) during the thawed period, implying significantly larger carbon absorption in the PA than in the NPA during both the thawing and thawed periods. Finally, correlation analysis results revealed that the soil water content (SWC) plays an important role in maintaining the ecosystem carbon budget. The degradation of permafrost might accelerate soil thawing and promote the transfer of soil water, and thus greatly affect the carbon budget of grassland ecosystems in Mongolia.