黄土高原小麦田土壤呼吸对强降雨的响应

全球气候变化的可能后果之一是干旱频繁，强降雨增多。土壤呼吸是全球碳循环的关键组成部分，探讨强降雨对土壤呼吸的影响，有助于预知在全球变化背景下，土壤CO2排放的可能反馈机制。然而，由于测定技术限制，目前在降雨前后，对土壤呼吸进行原位、全天候、高频率测定的研究尚不够深入。研究选取黄土高原半干旱区小麦田土壤为研究对象，采用全自动多通量箱系统，对降雨前后的土壤呼吸及环境因子在原位置进行了全天候连续监测，分析了3次强降雨前后的土壤呼吸变化。结果表明，（1）强降雨对土壤呼吸促进还是抑制取决于雨前、雨中、以及雨后的土壤水分状态。土壤水分相对亏缺条件下的强降雨促进土壤呼吸，降雨结束后土壤呼吸的平均水平是降雨发生前的1．5—2倍；湿季的强降雨整体上抑制土壤呼吸，降雨过程中观测到呼吸波谷，雨中及雨后土壤呼吸分别下降了约33％和15％。（2）土壤呼吸与土壤水分之间存在二次曲线关系，此关系同时受土壤水分状况和温度的影响。当土壤由干旱和水分相对亏缺状态过渡到湿润时，上述二次曲线关系可靠；当土壤水分充裕时，该二次曲线关系减弱。在于湿交替情况下，二次曲线拐点是土壤呼吸因土壤水分增加而受到抑制的临界点，并且当温度升高时，该临界点相应升高。（3）温度和水分共同影响土壤呼吸。在土壤水分相对亏缺时，水分的增加是影响土壤呼吸的关键因子，温度对土壤呼吸的影响处于相对次要的位置；在水分充裕时，温度是影响土壤呼吸的关键因子，水分的增加会抑制土壤呼吸，但其对土壤呼吸变化的影响相对弱化。

The great rainfall effect on soil respiration of wheat field in semi-arid region of the Loess Plateau

To discern the response of soil respiration to heavy rainfall in semi-arid region of the Loess Plateau, soil respiration of wheat field was monitored during three heavy rainfall periods. The first heavy rainfall event happened on May 15, 2005, after a one-month arid period. The soil was in semi-arid state before the second heavy rainfall on May 30. And the soil was wet before the third heavy rainfall which started on July 1 and lasted nearly 3 days. Soil respiration of wheat field was continuously monitored for about 20 days around each rainfall separately (i.e. 10 days before and 10 days after each of the three rainfalls), using three chambers of a multi-channel automated chamber system. Environmental factors were also simultaneously measured. The main factors affecting the soil respiration were selected by stepwise regression method and the relationship between the soil volume water content (SVWC) and soil respiration (SR) was analyzed. The results showed that, (1) Heavy rainfall could both promote and inhibit soil respiration and its effect on SR depended on the soil water state before, during and after the rainfall event. The rainfall promoted the SR when the soil was in arid or water deficit state, but restrained it when the soil was wet. Comparing the values after and before the rain, the SR increased almost 2 and 1.5 times respectively for the first and second rain, but reduced nearly 15% for the third one. The SR reduced 33% during the third rain course. (2) The relationship between the SR and SVWC could be described in a quadratic equation. According to this equation, the inflexion could be considered as a critical SVWC. The SR increased with the increment of SVWC when the SVWC was below the critical value, but decreased with the building up of SVWC when the SVWC was above the critical value. (3)The quadratic relationship was influenced by soil water state and temperature. It was more reliable when the soil changed from arid or water deficit state to soil wet state due to rainfall than that from wet to wetter. On the other hand, the inflexion of the quadratic equation increased with temperature when the soil changed from water deficit state to superfluous state. (4) Temperature and SVWC were interdependent in their effects on SR, and the effects depended on the soil water state before the rainfall. The SVWC was the most influencing factor when the soil was in arid or semiarid state before rain, but its effect on soil respiration weakened and the temperature became the most influencing factor when the soil was enough wet before rain.