Data_Ecosystem-level warming reduces microbial necromass in the topsoil but not in deeper soil of a subtropical forest

Tropical and subtropical forests play a crucial role in global carbon (C) pools, and their responses to warming can significantly impact C-climate feedback and predictions of future global warming. Despite earth system models projecting reductions in land C storage with warming, the magnitude of this response varies greatly between models, particularly in tropical and subtropical regions. Here, we conducted a field ecosystem-level warming experiment in a subtropical forest in southern China, by translocating mesocosms (ecosystem composed of soils and plants) across 600 m elevation gradients with temperature gradients of 2.1℃ (moderate warming), to explore the response of ecosystem C dynamics of the subtropical forest to continuous 6-year warming. Compared with the control, the ecosystem C stock decreased by 3.8% under the first year of 2.1°C warming; but increased by 13.4% by the sixth year of 2.1°C warming. The increased ecosystem C stock by the sixth year of warming was mainly attributed to a combination of sustained increased plant C stock due to the maintenance of a high plant growth rate and unchanged soil C stock. The unchanged soil C stock was driven by compensating and offsetting thermal adaptation of soil microorganisms (unresponsive soil respiration and enzyme activity, and more stable microbial community), increased plant C input, and inhibitory C loss (decreased C leaching and inhibited temperature sensitivity of soil respiration ) from soil drying. These results suggest that the humid subtropical forest C pool would not necessarily diminish consistently under future long-term warming. We highlight that differential and asynchronous responses of plant and soil C processes over relatively long-term periods should be considered when predicting the effects of climate warming on ecosystem C dynamics of subtropical forests.

热带与亚热带森林在全球碳库中扮演着至关重要的角色，其对气候变暖的响应可显著影响碳-气候反馈过程与未来全球变暖的预测结果。尽管地球系统模式预测气候变暖将导致陆地碳储量减少，但不同模式间该响应的幅度差异显著，在热带与亚热带地区尤为突出。本研究在中国南方的一处亚热带森林中开展了野外生态系统尺度增温实验：通过将中型生态柱（mesocosm，由土壤与植物构成的生态系统单元）沿600米海拔梯度迁移，构建出2.1℃的温度梯度（即中度增温处理），以此探究亚热带森林生态系统碳动态对持续6年增温的响应。与对照组相比，2.1℃增温处理的第一年，生态系统碳储量下降3.8%；但到第六年时，该储量提升了13.4%。增温第六年生态系统碳储量的提升，主要源于两方面的协同作用：一是植物生长速率维持在较高水平，使得植物碳储量持续增加；二是土壤碳储量未发生显著变化。土壤碳储量未发生变化的原因，是土壤微生物产生了兼具补偿与抵消效应的热适应（土壤呼吸与酶活性无显著响应，微生物群落结构更为稳定）、植物碳输入量增加，以及土壤干燥引发的碳损失受到抑制（碳淋溶量减少，且土壤呼吸的温度敏感性被削弱）。上述结果表明，在未来长期气候变暖背景下，湿润亚热带森林的碳库未必会持续减少。本研究强调，在预测气候变暖对亚热带森林生态系统碳动态的影响时，需考虑植物与土壤碳过程在较长时间尺度上的差异化与异步响应。