城市不同地表覆盖类型下土壤有机碳矿化的差异

土壤有机碳(SOC)矿化是陆地生态系统碳循环的重要过程。因受到强烈的人为干扰,城市土壤生态服务功能严重退化,进而对城市土壤地球化学循环尤其是碳循环产生深刻的影响。以北京市奥林匹克森林公园的5种典型地表覆盖类型(草坪、灌木、行道树、植草砖、硬化地表)下土壤为研究对象,研究了城市不同地表覆盖类型下土壤有机碳矿化过程及固碳能力的差异。结果表明,城市5种地表覆盖类型下的土壤有机碳矿化趋势与自然生态系统中的土壤基本一致,都表现为前期矿化较为快速,后期明显减慢并且趋于平稳;不同地表覆盖类型下土壤的有机碳矿化作用有显著差异,灌木、行道树、植草砖覆盖下土壤有机碳矿化能力较强,硬化地表和草坪较弱,与土壤有机碳含量特征类似;一级动力学方程对各土样有机碳矿化过程的模拟结果较好,结果显示草坪覆盖下土壤固碳能力较强,灌木覆盖下次之,行道树、植草砖和硬化地表覆盖下较弱;土壤固碳能力的高低并不对应着土壤有机碳含量的高低,城市人为干扰和外源有机碳的输入对土壤有机碳储量影响较大;硬化地表下不同土层有机碳矿化作用无明显差异,而其他地表覆盖类型下的土壤有机碳矿化作用随土层加深显著减弱,特别是植草砖和行道树特征最为明显;各地表覆盖类型下土壤固碳能力随土层深度变化的规律不显著。城市土壤有机碳矿化的最主要限制因子是土壤有机碳的含量,土壤p H值、养分含量、粘粒含量等性质也通过影响土壤有机碳含量及微生物活动等对土壤有机碳矿化过程产生影响。

Variation in soil organic carbon mineralization under various land cover types in urban areas

Soil organic carbon (SOC) mineralization is a significant process in soil carbon cycling in the terrestrial ecosystem, which is closely related to the soil nutrient release and global climate change. Function of urban soil as an ecosystem has degenerated due to severe human disturbances, which deeply influence the geochemical cycles, especially the carbon cycle, in urban eco-environments. In order to explore the variation in SOC mineralization and SOC sequestration potential under various land cover types, we collected soil samples from five typical land cover types, i.e., grass-covered, shrub-covered, street tree-planted, grass-planting brick, and impervious surface, in the Olympic Forestry Park in the summer of 2016. SOC mineralization and physico-chemical properties, such as pH, particle constitution, and nutrient content, of all soil samples were measured. After the experiment, we analyzed the variation in SOC mineralization under various land cover types and the relationship between SOC mineralization and soil physico-chemical properties. The results showed that the SOC mineralization of urban soil was higher at the early incubation period than that at the late incubation period; the same trend also favored the SOC mineralization in natural ecosystems. However, relatively significant differences in daily and cumulative SOC mineralization were observed among the five land cover types, especially in the early incubation period. Both daily and cumulative SOC mineralization under shrub-covered, street tree-planted and grass-planting brick types were higher than those under the other two land cover types, which was consistent with the variation in the SOC content under the five land cover types. SOC mineralization of all soil samples in this experiment followed the first-order kinetic equation. Simulation results showed that the SOC sequestration potential was relatively weaker under the impervious surface, grass-planting brick and street tree-planted types than under the grass-covered and shrub-covered types. In contrast with the variation in SOC mineralization, the variation in the SOC sequestration potential under the five land cover types was inconsistent with that in the SOC content, because SOC content may be deeply influenced by human disturbances and additional organic carbon input. With regard to the variation of SOC mineralization along layer depth, there was no significant difference among three layers (0-15, 15-30, and 30-45 cm) under the impervious surface type. However, under the other land cover types, especially under the grass-planting brick and street tree-planted types, the SOC mineralization of the top layer was greater than that of the lower layers as a whole. Correlation analysis suggested that the SOC content was the main constraint of SOC mineralization whereas other physico-chemical properties, including pH, nutrient content, and clay content, may indirectly influence SOC mineralization, by affecting the SOC content and microbial activity.