CH₄ Flux Of Returning Farmland To Grassland In An Agro-pastoral Ecotone In Inner Mongolia

The area of semi-arid temperate grassland in northern China accounts for about 78%of the country’s grassland,and due to long-term unreasonable use,the region is facing serious ecological problems.In order to improve and protect the ecological environment,the project of returning farmland to grassland was carried out in the agro-pastoral ecotone in China,which gradually restored ecosystem functions and changed greenhouse gas exchange fluxes.Methane（CH₄）is the second largest greenhouse gas after carbon dioxide（CO₂）.Many studies have shown that grassland is a sink of atmospheric CH₄,but when farmland is converted into grassland,how this sink changes is unclear.This study studies the ecological function of returning farmland to grassland from the perspective of soil-atmospheric CH₄ exchange flux.We take the artificial grassland,which was converted from degraded farmland,and cultivated farmland in the agro-pastoral ecotone of Inner Mongolia as the research object.We use the static chamber-gas chromatography method to study the effect of returning farmland to grassland on soil-atmosphere CH₄ exchange flux.The results will provide a more scientific and comprehensive theoretical basis to evaluate the policy of returning farmland to grassland.Artificial grassland includes continuous grazing（CG）,rotation grazing（RG）and ungrazed grassland（UG）.Farmland includes corn field（CF）,potato patch（PP）and abandoned land（AL）.The main results are as follows:（1）The annual average CH₄ uptake of artificial grassland was significantly lower than that of farmland（p<0.05）.The annual average total CH₄ uptake of artificial grassland was 11.4%～45.3%lower than that of farmland,indicating that returning farmland to grassland would reduce the uptake of atmospheric CH₄ by soil.（2）The CH₄ uptake of artificial grassland and farmland during the crop growing period accounted for the largest proportion of the whole year,but during the non-growing period which also accounted for a higher proportion of annual CH₄ uptake（18%-48%）.So the contribution during the non-growing period is important for the estimation of annual CH₄fluxes.（3）Soil temperature was an important environmental factor influencing CH₄ fluxes,and the soil-atmospheric CH₄ fluxes of each treatment showed a significant negative correlation（p<0.05）with soil temperature in artificial grasslands and farmland.Soil moisture was also an important environmental factor affecting CH₄ fluxes.In addition to the abandoned land（AL）,the CH₄ fluxes of other treatments（CG,RG,UG,CF,and PP）were significantly correlated with soil moisture（p<0.05）.The combination of mean annual soil temperature and mean annual soil moisture explained 47%of the annual accumulation of CH₄ fluxes.（4）Soil ammonium nitrogen was also an important factor influencing CH₄ fluxes in artificial grassland and farmland.During the study period,the CH₄ fluxes of three treatments（CG,RG and UG）of artificial grassland and two treatments（CF and PP）of farmland were significantly correlated with soil ammonium nitrogen content（p<0.05）.The annual accumulation of CH₄ fluxes in all treatments and the annual average soil ammonium nitrogen content reached a very significant positive correlation（p<0.01）.During the study period,soil dissolved organic carbon and soil nitrate nitrogen had no significant effect on CH₄fluxes in artificial grasslands and farmland.However,in all treatments,the annual accumulation of CH₄ fluxes was significantly correlated with the average annual soil dissolved organic carbon content（p<0.05）and which was significantly negatively correlated with the average annual soil nitrate nitrogen content（p<0.05）.