人工模拟降水条件下旱作农田土壤“Birch效应”及其响应机制

降水事件引起干土复湿刺激土壤CO_2,脉冲释放的现象被称为"Birch效应",其作用机制可能是降水刺激土壤"底物供给"增加或引起土壤"微生物胁迫"所致。为深入了解土壤"Birch效应"对降水格局改变的响应过程及内在机制,在冬小麦拔节期和夏闲期分别进行了不同降水量(1-32 mm)人工模拟降水实验,系统观测了降水后0-72 h土壤呼吸及土壤碳组分变化特征,结果表明:土壤呼吸随降水量的增大而增强,1-16 mm降水土壤呼吸峰值出现在降水后4h,而32 mm降水土壤呼吸峰值出现时间滞后了4 h。与较小降水量相比,较大的降水量能增加土壤呼吸但会推迟土壤呼吸峰值出现时间。土壤呼吸速率峰值(SRP)与降水量(P)呈幂相关(拔节期:SR-P=0.97P~(0.09),R~2=0.5,P0.05;夏闲期:SR-P=1.07P~(0.09),R~2=0.98,P0.01)。降水后72h累积CO_2释放量(CO_2-P)与降水量呈线性相关(拔节期:CO_2-P=0.03P+5.99,R~2=0.58,P0.05;夏闲期:CO_2-P=0.11P+6.04,R~2=0.86,P0.01)。土壤呼吸温度敏感性系数和降水量之间存在二次曲线关系(拔节期:Q_(10)=-0.007P~2+0.2P+0.7,R~2=0.32,R~20.05;夏闲期:Q_(10)=-0.01P~2+0.3P+0.2,R~2=0.86,P0.01)。逐步回归分析表明,冬小麦拔节期所有降水量处理土壤呼吸与土壤微生物量碳相关性均达到显著水平(P0.05),指示土壤"Birch效应"是由"微生物胁迫"所致。而在夏闲期,当降水量小于8 mm时土壤呼吸与微生物量碳相关性显著,即以微生物胁迫机制占主导;8 mm降水处理下土壤呼吸与氯仿熏蒸-K_2SO_4提取态有机碳相关性达到极显著水平,指示则为两种机制共同起作用,而当降水量大于16 mm时,土壤呼吸主要与可提取态有机碳显著相关,"Birch"效应转为以底物供给机制占主导。与夏闲期相比,冬小麦拔节期作物生长会削弱"Birch效应",并改变其响应机制。

Response of soil "Birch Effect" to simulated rainfalls in dry croplands

In the dry cropping lands, soil rewetting due to precipitation can stimulate soil CO₂ efflux pulses, which is called the "Birch Effect". Two types of responding mechanisms:"Substrate supply" and "Microbial stress", were proposed to explain the soil "Birch effect". To elucidate how precipitation change affects the "Birch effect "and its responding mechanism, we conducted precipitation simulation experiments in a winter wheat field to measure the short-term response of precipitation levels(1-32 mm) on soil respiration from 0 to 72 h at the jointing stage and during the summer fallow period on the Loess Plateau. The results showed that soil respiration peaked at 4 h with 1 to 16 mm of precipitation, but the peak was delayed by another 4 h at 32 mm precipitation. Higher precipitation delayed, but increased soil respiration compared to lower precipitation. The peak value for the soil respiration rate(SR-P) increased exponentially with increased precipitation(P)(jointing:SR-P=0.97P^0.09, R²=0.5, P < 0.05, fallow:SR-P=1.07P^0.09, R²=0.98, P < 0.01). Cumulative soil respiration after 72 h(CO₂-P) increased linearly with the precipitation level(jointing:CO₂-P=0.03P+5.99, R²=0.58, P < 0.05; fallow:CO₂-P=0.11P+6.04, R²=0.86, P < 0.01). The temperature sensitivity of soil respiration(Q₁₀) was correlated binomially with the precipitation level(jointing:Q₁₀=-0.007P²+0.2P+0.7, R²=0.32, P < 0.05; fallow:Q₁₀=-0.01P²+0.3P+0.2, R²=0.86, P < 0.01). Stepwise correlation analysis between soil carbon fractions and soil respiration rate showed that soil respiration rate was significantly correlated with soil microbial biomass C at the jointing stage for all precipitation levels, indicating that a "microbial stress" mechanism dominated the "Birch effect" during growing season. However, during summer fallow period, soil respiration rate was significantly correlated with microbial biomass C, which suggested "microbial stress", when the precipitation amount was lower than 8 mm, but correlated with chloroform-fumigated extracted C, which suggested that both mechanisms were operating when precipitation amount was equal to 8 mm. However, soil respiration was significantly correlated with K₂SO₄-extracted soil C when precipitation amount was equal to or higher than 16 mm, indicating a shift to the "Substrate supply" mechanism. Compared with that during the summer fallow period, the soil "Birch effect" was weaker and its responding mechanism had changed at the wheat jointing stage due to root respiration and shading effects.