黄土丘陵区不同土地利用类型土壤CO_2,N_2O通量特征

Characteristics of CO_2 and N_2O Emissions Under Different Land-use Types in Loess Hilly Region of China

以黄土丘陵区园则沟小流域农地、撂荒草地、红枣林3种土地利用类型为单元,采用静态箱—气象色谱法对生长季土壤CO_2,N_2O两种温室气体进行定位监测,研究退耕还林(草)工程实施后不同土地利用类型土壤CO_2,N_2O排放通量特征。结果表明:生长季农地、撂荒草地、红枣林土壤CO_2排放通量均值分别为300.39,273.31,173.80mg/(m^2·h),季节变化均呈单峰型;农地、撂荒草地、红枣林N_2O通量均值分别为7.08,9.26,0.52μg/(m^2·h),土地利用类型未明显改变N_2O通量的季节特征,各处理均于6—7月出现较大值,其他时期均较低或出现负排放并呈现较为复杂的源汇特征。土壤10cm温度与土壤CO_2,N_2O相关关系高于土壤水分,而3种土地利用类型下N_2O通量与土壤水分均不相关,二元线性回归结果显示水热双因子解释了54%~78%的土壤CO_2通量变异。综合分析表明黄土丘陵区退耕还草后土壤CO_2未有显著变化,土壤N_2O则随土壤基质条件的改善呈现上升趋势(p<0.01);坡耕地改为经济林后土壤CO_2,N_2O通量均有一定程度减少(42.1%~92.7%),且更容易出现N_2O的负排放。

Investigation of the effects of land-use change on soil CO2 and N2O fluxes under the conversion of cropland to forest and grassland in loess hilly region would be helpful to understand the context of global warming. Using static chambers method, soil COz and N2O fluxes were investigated during the growing season under three land-use types （cropland, jujube orchard and abandoned grassland） in the study area in 2014. Additionally, soil temperature and moisture at 10 cm depth were monitored using mercurial thermometer and potable time domain reflectometry respectively to explore the correlations between environmental factors and soil gas emissions. During the growing season, the emission rates of CO2 in cropland, jujube orchard and abandoned grassland were 110.66-531.79 mg/（m^2 · h）, 74.01-437.27 mg/（m^2 · h） and 52.56-267.20 mg/（m^2 · h）, respectively, which peaked in summer and were correlated with soil temperature in all land-use types. Soil N2O under different land-use types followed similar seasonal patterns which were related to vegetative growth, the higher emission rate occurred in June and July compared with other growing period when negative fluxes frequently occurred, and the values varying -3.82-19.78 μg/（m^2· h）, -4.21-28.10 μg/ （m^2 · h）, -8.77-16.76 μg/（m^2· h） in cropland, jujube orchard and abandoned grassland, respectively. Linear correlation analysis showed that, for both land-use types, relationship between soil CO2 （N2O） fluxes and soil temperature at 10 cm depth was better than the relationship between soil CO2 （N2O） fluxes and soil moisture, while no relationship was found between soil N2O flux and soil moisture due to the complicated mechanism of nitrification and denitrification processes. Moreover, binary linear regression analysis on the relationship of soil temperature and soil moisture could explain 54%-78% of soil CO2 seasonal variations. Our research results showed that soil N2O increased （p〈0.01） with the better substantial condition after converting cropland into grassland, however, no statistics difference was observed in the case of CO2 emissions, which were probably caused by the low temperature in grassland. The conversion of cropland to orchard led to the lower soil CO2 and N2O emission rates （42.1%-92.7%） due to intensive field management which may also contribute to more negative flux of N2O in economic forest.