不同利用方式对红壤CO_2排放的影响

CO_2 emissions from upland and paddy red soils in mid-subtropical China

采用静态箱法研究了我国亚热带红壤区农田利用方式 (旱地或水田 )对土壤 CO2 排放及其相关因子的影响 ,并估算了旱地和水田 CO2 的年排放通量。结果表明 ,水田在淹水植稻期 (夏季 ) ,其排放通量明显低于旱地 ,而在非淹水期 (排水落干或休闲期 ) ,其排放通量则显著高于旱地。 CO2 排放通量呈现明显的季节性变异 ,旱地以夏季最高、春秋季次之、冬季最低 ;而水田则以秋季最高、其次是春冬季、夏季最低。土壤温度和湿度分别是影响旱地和水田 CO2 排放的主导因子 ,可将二者与通量的指数关系作为模型 ,分别进行旱地和水田 CO2 排放的估算。经模型估算 ,我国中亚热带旱地和水田红壤 CO2 的年排放通量分别为 1.37和 2 .73kg CO2 / (m2 · a) )。

Although CO_2 efflux plays a key role in carbon exchange between the biosphere and the atmosphere, our understanding the factors affecting its temporal and spatial variations is rather limited. Field experiments located in mid-subtropical China were conducted with the closed chamber method to investigate the effects of land use pattern (upland and paddy) on red soil CO_2 evolution and its related factors including soil temperature, soil moisture, dissolved organic carbon and microbial biomass carbon. Meanwhile, the annual soil CO_2 emission fluxes in upland and paddy soils was also estimated in this paper. The results obtained from this study indicated that, CO_2 emission fluxes for paddy soil were significantly lower than those for upland soil in summer (flooded), but significantly higher in other seasons (non-flooded or fallow). Soil CO_2 fluxes had marked seasonal fluctuations, for upland soil, with the tendency showing maximum value in summer, minimum in winter and intermediate in spring and autumn; However for paddy soil, with maximum value in autumn, minimum in summer and intermediate in spring and winter. Soil temperature was the dominant factor influencing CO_2 emission from upland soil, and the exponential relationship between soil CO_2 fluxes and soil temperature could be used as a model for estimating the annual soil CO_2 emission flux for upland soil. As to paddy soil, the soil CO_2 fluxes were mainly affected by soil moisture, and could be used to build an exponential equation for computing the annual CO_2 fluxes for paddy soil. The two exponential equations mentioned above are Fd_1=0.379×e^(0.0887×t), r=0.805~* (p≤0.05) for upland soil; Fd_2=317.45×e^(-0.049×m), r=-0.760~* (p≤0.05) for paddy soil, where Fd1 (Fd2), t and m are the diurnal soil CO_2 flux (as g CO_2/(m^(2)·d)), soil 5 cm temperature (℃) and soil moisture (% WHC, soil water holding capacity), respectively. The annual fluxes were estimated with the following method: Firstly, the polynomial equations were built to describe the soil temperature (for upland soil) or soil moisture (for paddy soil) in relation to the cumulating time (d) during the entire experimental period. The equations are as follows: y_1=27.56+0.3527x_1-0.0102x^2_1+7×10^(-5)x^3_1-2×10^(-7)x^4_1+1×10^(-10)x^5_1, R^2_1=0.9881 y_2=86.15-0.8083x_2+0.014x^2_2-1×10^(-4)x^3_2+3×10^(-7)x^4_2-3×10^(-10)x^5_2, R^2_2=0.9179where y_1 and y_2 were soil temperature and soil moisture, respectively. Both x_1 and x_2 were the cumulating time after the beginning of the experiment. Secondly, as the relationship between the diurnal and annual soil CO_2 flux could be described by a differential function: Fd=d Fa/dx, where Fd is the diurnal soil CO_2 flux (g CO_2/(m^(2)·d)), Fa is the annual soil CO_2 flux (kg CO_2/(m^(2)·a)), and x is the cumulating time (d). Thus, the annual soil CO_2 fluxes for upland and paddy soil could be calculated by the following integral functions: Fa1=∫^(365)_0{0.379×EXP[0.0887×(27.56+0.3527x_1-0.0102x^2_1+7×10^(-5)x^3_1-2×10^(-7)x^4_1+1×10^(-10)x^5_1)]}dx_1 Fa2=∫^(365)_0{317.45×EXP[-0.049×(86.15-0.8083x_2+0.014x^2_2-1×10^(-4)x^3_2+3×10^(-7)x^4_2-3×10^(-10)x^5_2)]}dx_2which were from the combination of the above exponential and polynomial equations using soil temperature and soil moisture, where Fa1 and Fa2 is the annual soil CO_2 flux (kg CO_2/(m^(2)·a)), and x is the cumulative days after the experimental beginning (d). With the integral functions, the annual soil fluxes were predicted as 1.37 and 2.73 kg CO_2/(m^(2)·a) for upland and paddy soils, respectively.