Migration characteristics and potential determinants of mercury in long-term decomposing litterfall of two subtropical forests

It is of great importance to elucidate the mechanism of mercury (Hg) migration in the forest litterfall so as to clearly understand global Hg deposition. However, it is still unclear for the migration and transformation of Hg in different forest litters during long-term decomposition. Therefore, the dynamics of total Hg (THg), methylmercury (MeHg), carbon, nitrogen, microbial biomass carbon and nitrogen in the litterfall of the evergreen broadleaf (EB) and mixed broadleaf-conifer (MBC) forests, southwest China were investigated, aiming to understand the migration characteristics of Hg in the two-year decomposing litterfall. Results showed that carbon decreased, while nitrogen accumulated slightly in the process of litterfall decomposition. THg levels in the second year of the EB and MBC forests decreased by 16.9% and 11.3%, while MeHg levels reduced by 141.4% and 210.7% respectively comparing with those in the first year. The total percentage of hydrochloric acid-soluble mercury (Hg-h) and water-soluble mercury (Hg-w) had a significant impact on the migration of THg and MeHg in the two forest stands. The C/N ratio in the EB forest bore a positive correlation with THg and MeHg levels, whereas that in the MBC forest was adverse. Besides, microbial biomass C and N were positively related with THg and MeHg levels in both the EB and MBC forests. It is proposed that THg and MeHg accumulation in the second year drastically decreased probably due to finite nutritional conditions, which implies that Hg accumulation risks alleviate with degradation time.