增温促进亚热带森林底层土壤有机碳矿化损失机制

Forest subsoil layer sequestrates more than half organic carbon of the whole soil profile. The latest data shows that soil warming accelerates soil organic carbon mineralization of subsoil. But the mechanism behind it is still far from being understood. Both in situ and laboratory soil warming experiment are applied in this project to gain a insight of soil organic carbon mineralization of subsoil from subtropical evergreen broad-leaved forest. 18O abundance of CO2 and H2O were analyzed with isotopic mass spectrometry. Microbial community compositions are determined by high-throughout sequencings and CO2 fluxes from subsoil are measured with non-scattering infrared CO2 sensor. Contribution of pumped 18O2 to Increased aerobic respiration and N18O3-addition to enhanced anaerobic respiration are calculated. Carbon in subsoil may be leached outside of soil system and finally mineralized elsewhere. This phenomenon could be simulated with incubation experiment. The phenomenon found from in situ soil warming experiment are tested by laboratory soil column incubation experiment. Results from incubation experiment could help gain a deep insight of mechanisms of subsoil carbon mineralization and better understand the feedback mechanisms of climate-carbon cycle of terrestrial ecosystem.

森林底层土壤（>20 cm）有机碳储量占土壤总有机碳的50%以上。新近研究表明增温加速底层土壤有机碳矿化损失，但其机制仍知之甚少。本项目依托土壤增温实验平台（增温4℃），通过野外观测和室内土柱培养相结合，研究亚热带常绿阔叶天然林底层土壤有机碳矿化。采用18O2、N18O3-同位素标记技术测定底层土壤CO2、H2O的18O丰度；高通量测序技术测定底层土壤微生物群落组成；非散射红外固态CO2探头测定底层土壤CO2排放量；准确测定增温后底层土壤通透性增强（O2增加）和NO3-输入增加分别对底层土壤有机质的有氧呼吸和无氧呼吸的贡献；通过对深层土壤渗滤液的培养实验，揭示底层土壤“向下淋溶”异地转移的C损失。室内以土柱培养实验（设置不同温度、湿度梯度）验证原位观测的结果。研究结果进一步揭示增温促进底层土壤有机碳矿化损失的作用机制，对于深入理解全球气候变暖背景下陆地生态系统气候－碳循环反馈具有重要意义。

底层土壤（20-100 cm）有机碳贮量占1 m土层土壤总有机碳贮量的比例超过50%，但全球变暖对底层土壤有机碳的影响，很可能表现出完全不同于表层土壤的模式。为弄清增温如何促进底层土壤CO2排放，本项目主要研究了对照和增温两种处理下不同土层土壤CO2生产量的差异，并通过测定两种处理对应土层土壤溶液DOM的数量和质量的变化，以及不同土层土壤微生物的变化，探讨增温如何促进亚热带森林底层土壤有机碳损失。结果表明，增温后不同土层土壤温度显著提高，土壤含水量显著下降，且不同土壤土壤CO2浓度也都有提高趋势，这是导致增温后不同土层土壤CO2生产量显著增加的根本原因。而不同土层温度和含水量作用于周围的环境，导致土壤溶液DOM和土壤微生物酶活性的改变，增温导致增温后底层土壤CO2生产量变化的直接原因。本研究的成果对于准确评估全球气候变暖后森林土壤碳排放具有重要的意义，研究数据还能为预测气候变化对生态系统影响的相关模型提供矫正数据；此外本项目研究中自主研发的技术可运用于农田生态系统和草原生态系统等多个研究领域。