干旱影响森林土壤有机碳周转及积累的研究进展

随着全球气候变暖趋势加剧,伴之而来的干旱问题成为全球关注的热点。干旱对森林生态系统碳积累和周转可能产生显著影响,其主要过程包括植被地上部分和地下部分凋落物对土壤有机碳的输入、凋落物的分解及土壤有机碳的矿化等。笔者综合分析了近年来国内外相关研究成果,对干旱影响森林土壤有机碳的主要过程与机制进行了归纳和总结,结果表明:①干旱通过促进叶片提前脱落,短期增加森林凋落物量,长期干旱则影响森林植物生长,降低森林初级生产力从而降低植物地上凋落物量。轻度和中度干旱下植物为补偿水分缺失增加细根生物量维持植物生命力,重度干旱下植物丧失自我修复能力导致细根生物量降低,干旱也会造成细根死亡率增加。平均而言,全球范围内干旱会造成森林凋落物量降低(1.9%)和细根生物量降低(8.7%),最终减少植物有机碳向土壤的输入量。②干旱可通过改变凋落物化学性质,对分解者——土壤动物、微生物产生胁迫,从而引起凋落物分解速率下降(10%~70%)。干旱使凋落物碳氮含量变化,造成凋落物次生代谢物,如纤维素、木质素、单宁等积累,改变根系分泌物化学组分,从而影响凋落物分解。干旱导致真菌生物量和分解者等土壤动物丰度降低,增加分解者捕食压力,使相关微生物和酶活性下降,造成凋落物分解速率下降。③干旱驱动微生物群落组成变化(真菌细菌比、革兰阳阴细菌比增加),造成微生物生物量下降,活性减弱,此外还会降低腐食动物的摄食活性、酶活性,最终导致土壤有机碳矿化速率下降(10%~50%)。④干旱对土壤有机碳不同组分影响不同,干旱会减小土壤微生物生物量碳(MBC)库(2%~30%),造成表层土壤溶解性有机碳(DOC)积累(30%~60%)。而在全球范围内的不同区域,干旱对土壤有机碳积累的影响也不同,亚热带森林中干旱对土壤有机碳积累的影响多是负面的,热带森林中则相反。总体而言,干旱对森林土壤有机碳库储量影响可能不大,但降低了土壤碳周转效率。而森林土壤有机碳周转过程不仅受干旱这一单一因素影响,温度、物种等因素会共同作用于土壤有机碳的周转与积累,且单因子的简单叠加模拟可能与现实环境中多因子综合对土壤碳通量的影响有一定差别。未来需要通过长期观测、延长控制实验时间、模拟原生环境条件等,开展多因素综合实验,加强干旱对土壤动物和微生物影响的研究,以深入了解干旱对森林土壤有机碳影响的生物学与生态学的过程与机制。

Progresses in drought stress on the accumulation and turnover of soil organic carbon in forests

With global warming, drought has become a serious issue in the world. Drought can significantly affect the soil carbon accumulation and transformation in forest ecosystems. Forest soil organic carbon (SOC) is the largest carbon pool in terrestrial ecosystems, and its dynamic changes significantly affect the global carbon cycle. Drought stress affects all processes of soil organic carbon dynamics, including the input of organic carbon from the above-ground and belowground litter and the transformation and decomposition of litter, and then causes changes in the soil carbon pool. Here, we reviewed the research progress of the effects of drought on soil organic carbon in forests. The results show that (1) short-term drought increases forest litterfall by promoting leaf shedding in advance, while long-term drought affects forest plant growth, reduces forest primary productivity, and thus decreases plant litterfall. To compensate for water loss, plants in mild and moderate drought increase fine root biomass to maintain plant vitality. In severe drought, plants lose self-repair ability, resulting in reduced fine root biomass and increased fine root mortality. On average, drought decreases forest litter (1.9%)and fine-root biomass(8.7%) worldwide, ultimately reducing the input of plant organic carbon to soil. (2) Drought can reduce the decomposition rate of litter(10% to 70%) by changing its chemical properties and stressing the soil animals and microorganisms responsible for its decomposition. Drought changes the carbon and nitrogen concentration of litters; causes accumulation of secondary metabolites such as cellulose, lignin and tannin; and changes the chemical components of root exudates, thus affecting the decomposition of litter. Moreover, drought results in the decrease of fungal biomass and abundance of soil fauna in decomposers, increases the predation pressure of decomposer animals, and decreases the activities of related microorganisms and enzymes, resulting in the decrease of litter decomposition rate.(3) Drought-driven changes in microbial community composition (the ratio of fungi to bacteria and of Gram-positive to Gram-negative-bacteria increased) result in the decrease of microbial biomass and activity, and drought reduces the feeding and enzyme activities of scavengers, which eventually lead to the decrease of soil organic carbon mineralization rate(10% to 50%).(4) The effects of drought on different components of soil organic carbon are different. Drought produces a smaller and more sensitive soil microbial biomass carbon (MBC) pool(2% to 30%), resulting in the accumulation of dissolved organic carbon (DOC) in surface soil(30%-60%). However, the effects of drought on SOC accumulation vary in different regions of the world. In subtropical forests, the effects of drought on SOC accumulation are mostly negative, while in tropical forests, the effects are positive. In general, drought may have little effect on the forest SOC pool, but reduces soil carbon turnover. Additionally, forest SOC turnover is not affected only by drought; temperature, species, and other factors work together in the turnover of SOC and accumulation. The simple superposition simulation may be single-or multi-factor in the real-world impact on soil carbon flux, which has certain differences. For example, the effect of increasing temperature and drought produce antagonism; the effect of their interaction on soil carbon input is lower than the superposition effect on soil carbon input, and the interaction effect of the two on soil carbon loss is not significant. We propose that it is important to conduct long-term observations, prolonging control experiment through simulating native environments and multi-factor comprehensive influence, to understand the effects of drought on the dynamic process and mechanism of soil organic carbon.