干旱对土壤剖面不同深度土壤CO2通量的影响

由于全球气候变化，预计未来我国亚热带地区干旱频率和持续时间将会增加。森林土壤CO₂的释放是陆地生态系统碳循环的重要组成部分，然而，有关不同深度土壤CO₂通量对干旱响应的理解仍相当有限。选择武夷山针叶林（Coniferous Forest，CF）和常绿阔叶林（Evergreen Broadleaved Forest，EBF）为研究对象，于2014年6月至2015年12月，采用梯度法计算10、30 cm和50 cm深度各层土壤CO₂通量，探讨模拟干旱对其影响。结果表明：CF和EBF样地土壤CO₂浓度均随土壤深度的增加而升高。CF和EBF样地对照（CK）处理10 cm深度土壤CO₂生产量分别占总CO₂生产量的53.5%和55.7%，表明土壤CO₂生产量主要来源于浅层土壤，这可能与浅层土壤有高的有机碳含量及细根生物量主要分布区有关。干旱处理使CF和EBF样地不同深度土壤CO₂通量均显著减少。在两个样地土壤CO₂通量的温度敏感性（Q₁₀）值均随着土壤深度的增加而减少。干旱处理显著减少了CF样地浅层土壤的Q₁₀值（P=0.02），对深层土壤影响不显著（30 cm：P=0.30；50 cm：P=0.23）；而在EBF样地干旱处理显著减少了深层土壤的Q₁₀值（30 cm：P=0.02；50 cm：P=0.01），对浅层土壤影响不显著（P=0.32）。

The influence of drought on the soil CO2 flux at different depths in soil vertical profiles

Soil CO₂ flux to the atmosphere is a major factor affecting the global carbon cycle. The CO₂ flux between soil layers is driven by soil temperature, moisture, and substrate supply. Soil CO₂ flux and its major driving factors vary temporally and spatially within soil profiles. However, many studies have focused only on the surface soil, which is insufficient to correctly clarify the actual soil CO₂ release processes, because soil CO₂ flux is the addition of CO₂ production in each soil layer under different biological, chemical, and physical conditions. The vertical distribution of soil CO₂ flux should be considered to better understand the production processes in soil CO₂. Moreover, in the next few decades, an increasing frequency and duration of droughts is expected in subtropical regions in China as a result of global climate change. However, our understanding of the effect of drought on the vertical partitioning of soil CO₂ flux is not well known. In the present study, a throughfall exclusion experiment was established to explore the soil CO₂ flux distribution in the vertical profile in response to simulated drought at two different elevations, including a coniferous forest (CF) at 1450 m.a.s.l and an evergreen broadleaved forest (EBF) at 650 m a.s.l, in a subtropical region in southeastern China from June 2014 to December 2015. We used a CO₂ solid concentration detector to determine the CO₂ concentration at different soil depths, and measured soil CO₂ efflux from surface soils using an Li-8100 soil CO₂ automated measurement system. The soil CO₂ flux at 10, 30, and 50 cm soil depths was estimated using the gradient method. The results showed that soil CO₂ concentrations from the CF and EBF plots gradually increased with soil depth. Soil CO₂ production in the 10 cm soil depths in CF and EBF for the control treatment (CK) was 53.5% and 55.7% of the total soil CO₂ production, respectively, indicating that soil CO₂ production primarily appeared in shallow soil owing to the high soil organic carbon and fine root biomass. Drought treatment significantly reduced soil CO₂ flux in each soil layer from CF and EBF. The temperature sensitivity of soil CO₂ flux decreased with increasing soil depth. Drought treatment in CF significantly decreased Q₁₀ in the shallow soil (P=0.02), but there were no significant differences in the deeper soil layers (30 cm:P=0.30; 50 cm:P=0.23); and in EBF, drought treatment significantly decreased Q₁₀ in the deeper soil (30 cm:P=0.02; 50 cm:P=0.01), but was not significantly different in the shallow soil (P=0.32). The asymmetric response of Q₁₀ in the shallow and deep soil with simulated drought at different elevations implied that the response mechanisms of the shallow and deep soil to drought were different.