中亚热带常绿阔叶林凋落物分解过程中汞的动态变化及迁移机理

森林凋落物对于汞在林地土壤的生物地球化学循环中起到重要作用,为研究森林凋落物分解过程中汞的迁移转化特征,以重庆四面山风景名胜区典型林分(常绿阔叶林)作为研究对象。于2014年3月—2015年3月连续监测典型林分凋落物中各形态汞浓度和有机质变化量,同时监测周围土壤中汞浓度变化。结果表明:四面山典型林分凋落物分解过程中汞浓度整体上升,总汞浓度(初始浓度:78 ng/g)的增幅最高达53%,甲基汞浓度(初始浓度:0.32 ng/g)最高增幅达138%;在春季和夏季,水溶态和酸溶态两种活性态汞含量分别增加了851%和96%,在分解前期和末期,凋落物汞的中惰性汞比例最高,占比达75%。土壤腐殖质层中总汞和甲基汞浓度比较稳定。凋落物中活性态汞通过雨水淋洗进入土壤与有机质络合并发生甲基化/去甲基化过程,通过地表径流、地下径流进入水体。凋落物中C含量减少了22%,N含量增加了15%,总汞浓度与C/N比呈负相关,与N含量呈正相关。凋落物中微生物C、N含量整体增加,与汞浓度峰值同步,且夏季含量显著高于冬季含量(P0.05),说明微生物与凋落物固定汞和汞的甲基化过程密切相关。

The migration and fate of mercury during litter decomposition in a subtropical evergreen broad-leaf forest

Forest ecosystem is the largest terrestrial ecosystem and plays a critical role in the biogeochemical cycling of mercury (Hg). Litterfall is confirmed to be a dominant pathway for Hg to reach the ground surface under a forest canopy. The objective of this research was to understand the characteristics of Hg migration and transformation in the litterfall and underlying soil during litter decomposition in a subtropical evergreen broad-leaf forest. Therefore, the dynamics of Hg concentrations and speciation distribution in the decomposing litterfall and underlying soils of Simian Mountain Nature Reserve were investigated for 1 year, from March 2014 to March 2015. The results indicated that initial total Hg (THg) concentration in the litterfall was 78.40 ±1.4 (standard error, SE) ng/g, which increased with the decomposition of litterfall. THg content increased up to 101.80±7.6 ng/g after decomposition for 1 year, while THg concentration increased by 30%. THg concentration reached its maximum (120.45 ng/g) after decomposing for 90 days. Compared with THg, the increase of methylmercury (MeHg) in the decomposing litter was more remarkable (P < 0.01). MeHg concentration was 2.38 times that of its initial value (0.32 ng/g) at the end of the decomposition, and its content peaked (0.86 ng/g) after decomposing for 210 days. Generally speaking, the concentrations of both THg and MeHg increased with the decomposition of litterfall. The proportion of inert Hg in the decomposing litterfall was highest at the initial and advanced stages of decomposition, accounting for 75%. Moreover, Hg in the litter was relatively stable at these stages. Therefore, the mobility of Hg was slow, and thus its ecological risk were low. Both the soluble and acid soluble Hg contents increased significantly in the sfpring and summer. At this time, THg and MeHg in the decaying litterfall were inclined to be transported to the downstream water with a large amount of rainwater, thereby causing high ecological risk. On the contrary, the concentrations of THg and MeHg in the organic layer of the underlying soil were quite stable. A comparison of the THg concentrations in the litter and underlying soil revealed that Hg in the soil could enter the litter, so the soil could act as the source of Hg during litterfall decomposition. Results of MeHg content showed that MeHg in the litter could be exported to the underlying soil, and was finally stored in the soil and gradually demethylated. The volatilization of Hg could be one of the reasons why its concentration in the soil was stable. The decomposing litter was also an important source of THg and MeHg to the surface water because it could be transported by the runoff and percolate. The content of organic C in the litterfall decreased by 22%, while the organic N content increased by 15% during decomposition. THg concentration was negatively correlated with the C/N ratio but positively correlated with the N content. The increase of THg concentration was presumed to be closely related with the nitrogen fixation microorganisms. Microbial carbon and nitrogen in the litter increased and were significantly higher in summer than that in winter (P < 0.05). The microorganisms activated in the decomposing litterfalls could immobilize more nutrients as decomposition rates and microbial activities peaked in the summer, and thus contributed to microbial biomass of the soil. Consequently, the concentration of THg increased during this process, especially MeHg, which increased exponentially.