全球变暖是否会削弱湿润亚热带森林土壤的保氮能力？

With a number of long-term soil warming experiments have been done in the past two decades in temperate forests where poor soil nitrogen are common , The research results indicate that warming could stimulate soil nitrogen mineralization and nitrification but it would not significantly increase soil NO3--N leaching and soil gaseous nitrogen losses. However, the red earth of humid subtropical area in China is characterized with nitrogen saturation and formed the unique soil nitrogen retention mechanism, which minimize the loss of soil NO3--N leaching and soil nitrogen gaseous. According to the theory of soil nitrogen cycle, however, we speculate that warming is very likely to enhance not only soil heterotrophic nitrification and denitrification in this region, but also promote N2O emission from soil, reduce the absorption of N2O and inhibit the DNRA process as well, which weaken the nitrogen retention capacity of this area. So whether the red earth nitrogen retention mechanism changes has become major concern in the background of global warming. To confirm the hypothesis of warming effect on forest soil nitrogen cycling in this region, natural evergreen broad-leaved forest and Chinese fir plantation are selected and nitrogen isotope labeling, combined with PLFA and DGGE techniques are adopted to determine whether warming could cause the increase of soil nitrogen leaching and gaseous nitrogen losses in the subtropical forests. The project try to reveal the mechanism that warming minimize the capacity of nitrogen retention of red earth, and provide data support for forest management measures.

中高纬度地区的森林（氮相对缺乏）进行了20多年的增温实验，发现增温能显著提高土壤氮的矿化和硝化进程，但不会增加土壤氮淋溶和气态氮损失。我国湿润亚热带森林红壤氮相对饱和，因其具有非常独特的氮保持机制，可以避免湿润环境下的硝态氮淋溶和反硝化损失。在全球变暖背景下，其氮保持机制是否会发生改变备受人们关注。增温可能促进红壤占主导地位的异养硝化过程，提高共同反硝化进程并抑制DNRA进程，此外还可能减少对N2O的吸收，极大削弱其氮保持能力。本项目拟以常绿阔叶树天然林和在其采伐迹地上营造的杉木人工林红壤为研究对象，依托先进的自主研发野外原位增温实验平台结合室内15N同位素示踪实验，辅以生物化学和分子生物学技术来验证这些假设，本项目试图揭示全球变暖削弱湿润亚热带地区森林土壤氮保持能力的可能机制，为将来探索更有效应对这种变化的森林管理措施提供数据支撑。

项目以湿润亚热带天然林和杉木人工林为研究对象，通过自主研发的野外原位土壤PID增温系统和室内培养实验相结合的方法，探讨增温对湿润亚热带森林土壤氮保持机制的影响。结果发现：1）增温显著增加了天然林和杉木幼林的净氮矿化速率和净硝化速率（P < 0.05），且对非生长季的影响更显著。增温导致非生长季杉木幼林土壤的硝态氮、铵态氮和可溶性氮含量显著增加（P < 0.05），但显著降低了生长季自然更新幼林土壤的铵态氮含量。增温后，不同土层杉木幼林土壤的硝态氮含量显著增加，但自然更新幼林不同深度土壤硝态氮含量变化不显著。同时增温导致自然更新幼林土壤N2O排放量显著提高，但在杉木幼林未达到显著水平（P > 0.05）。以上结果表明增温增加土壤N矿化，提高硝态氮含量，增加N2O的排放而降低土壤N保持能力。2）培养试验也发现增温显著提高土壤氮矿化速率（P﹤0.05），且在培养初期增温处理的氮净矿化速率高于对照，在遮光条件下，增温后土壤N2O的排放通量高于对照，但在光照条件下呈现相反的变化趋势。室内培养观测结果也支持增温使得土壤N素更易损失。3）增温后土壤微生物多样性指数显著降低，并显著改变了真菌和细菌的群落结构。增温后土壤AOA含量增加，nirK和narG降低。说明增温削弱了土壤反硝化过程，增加了土壤氨氧化作用，使得土壤中硝态氮含量增加，进而增加了亚热带森林土壤N的淋溶损失风险。本研究表明，增温会增加土壤氮矿化速率，提高土壤硝态氮的含量和N2O的含量，增加了N的淋溶和气态损失，而土壤氮功能基因AOA、nirK和narG的变化是增温后亚热带森林土壤N保持能力降低的重要生物调控机制。