全球变化研究组知识库

Strong Hg-0 evasion from forestsoil depletesdeposited Hg-0 through foliage uptake in a Chinese tropicalforest. Significant knowledge gaps exist regarding the emissionof elementalmercury (Hg-0) from the tropical forest floor, which limitour understanding of the Hg mass budget in forest ecosystems. In thisstudy, biogeochemical processes of Hg-0 deposition to andevasion from soil in a Chinese tropical rainforest were investigatedusing Hg stable isotopic techniques. Our results showed a mean air-soilflux as deposition of -4.5 & PLUSMN; 2.1 ng m(-2) h(-1) in the dry season and as emission of +7.4 & PLUSMN; 1.2 ng m(-2) h(-1) in the rainyseason. Hg re-emission, i.e., soil legacy Hg evasion, induces negativetransitions of & UDelta;Hg-199 and & delta;(202)Hgin the evaded Hg-0 vapor, while direct atmospheric Hg-0 deposition does not exhibit isotopic fractionation. Usingan isotopic mass balance model, direct atmospheric Hg-0 depositionto soil was estimated to be 48.6 & PLUSMN; 13.0 & mu;g m(-2) year(-1). Soil Hg-0 re-emission was estimatedto be 69.5 & PLUSMN; 10.6 & mu;g m(-2) year(-1), of which 63.0 & PLUSMN; 9.3 & mu;g m(-2) year(-1) is from surface soil evasion and 6.5 & PLUSMN; 5.0 & mu;gm(-2) year(-1) from soil pore gasdiffusion. Combined with litterfall Hg deposition (& SIM;34 & mu;gm(-2) year(-1)), we estimated a & SIM;12.6 & mu;g m(-2) year(-1) net Hg-0 sink in the tropical forest. The fast nutrient cycles inthe tropical rainforests lead to a strong Hg-0 re-emissionand therefore a relatively weaker atmospheric Hg-0 sink.