冬季土壤呼吸:不可忽视的地气CO_2交换过程

REVIEW OF WINTER CO_2 EFFLUX FROM SOILS: A KEY PROCESS OF CO_2 EXCHANGE BETWEEN SOIL AND ATMOSPHERE

冬季土壤呼吸是生态系统释放CO2的极为重要的组成部分,并显著地影响着碳收支。然而,过去绝大多数工作集中在生长季节土壤呼吸的测定,对年土壤呼吸量的估算大多基于冬季土壤呼吸为零的假设。目前为数不多的研究集中在极地苔原和亚高山,其它植被类型的研究只有零星报道。极地苔原和森林冬季土壤呼吸速率分别为0.002～1.359和0.22～0.67μmolC·m-2·s-1;土壤呼吸的CO2释放量分别为0.55～26.37和22.4～152.0gC·m-2,是地气CO2交换过程中不可忽视的环节。雪是土壤呼吸过程的重要调节者,积雪厚度和覆盖时间的长短均会影响土壤呼吸的强弱;水分的可获取性是重要的限制因素;对于维持活跃的土壤呼吸有一个关键的土壤温度临界值(-7～-5℃),低于这个值会因自由水的缺乏而抑制异养微生物的呼吸。如果存在绝缘的积雪层,可溶性碳底物在自由水存在的情况下可控制异养微生物的活力。该文对冬季土壤呼吸的重要性、研究方法、土壤呼吸强度及其影响机制等进行了综述,并讨论了冬季土壤呼吸研究中存在的问题及未来研究方向。

Winter CO2 efflux from soils is a significant component of annual carbon budgets and can greatly determine carbon balance of ecosystems. However, present estimates of annual soil respiration are mostly based on measurements taken during the growing season and assume that microbial respiration in frozen or snow-covered soils is negligible. We analyze methods used, magnitude of winter soil respiration, and influencing factors. There are very few measurements of winter soil respiration except in tundra and alpine ecosystems. Winter CO2 efflux from soils ranged from 0.002 to 1. 359 μmol C·m^-2·s^-1 and 0.22 to 0.67 μmol C·m^-2·s^-1 in tundra and forest ecosystems, respectively. No direct relationship between soil temperature and winter CO2 efflux from soils was found, but there is a critical threshold for active respiration, typically between - 7 and - 5 ℃, below which lack of free water limits microbial contributions to winter soil respiration. The depth, timing and duration of snow cover greatly influence the magnitude of winter CO2 efflux from soils, with water availability an important limiting factor. If insulating snowpack is present, carbon availability also controls heterotrophic activity. We discuss current problems and future research needs.