Global Mercury Assimilation by Vegetation

Assimilation of mercury (Hg) by vegetation represents one of the largest global environmental Hg mass fluxes. We estimate Hg assimilation by vegetation globally via a bottom-up scaling approach using tissue Hg concentrations synthesized from a comprehensive database multiplied by respective annual biomass production (NPP). As global annual NPP is close to annual vegetation die-off, Hg mass associated with global NPP approximates the transfer of Hg from plants to soils, which represents an estimate of vegetation-mediated atmospheric deposition. Annual vegetation assimilation of Hg from combined atmospheric and soil uptake is estimated at 3062 ± 607 Mg yr–1, which is composed of 2491 ± 551 Mg yr–1 from aboveground tissue uptake and 571 ± 253 Mg yr–1 from root uptake. Assimilation of atmospheric Hg amounts to 2422 ± 483 Mg yr–1 when considering aboveground tissues only. Atmospheric assimilation increases to 2705 ± 504 Mg yr–1 when considering that root Hg may be partially derived from prior foliar uptake and transported internally to roots. Estimated atmospheric Hg assimilation by vegetation is 54–137% larger than the current model and litterfall estimates, largely because of the inclusion of lichens, mosses, and woody tissues in deposition and all global biomes. Belowground, about 50% of root Hg was taken up from soils with currently unknown ecological and biogeochemical consequences.

植被对汞（Hg）的同化作用是全球范围内最大的环境汞质量通量之一。我们采用自下而上的尺度化方法，通过将综合数据库整合得到的植物组织汞浓度，乘以相应的年生物量生产量（净初级生产力，NPP），对全球植被汞同化量进行估算。由于全球年净初级生产力与植被年枯落量相近，与全球NPP相关的汞质量近似等于汞从植物向土壤的转移量，这一指标可表征植被介导的大气汞沉降估算值。
全球植被对大气与土壤来源汞的年同化总量估算为3062 ± 607 兆克/年，其中2491 ± 551 兆克/年来自地上组织吸收，571 ± 253 兆克/年来自根系吸收。仅考虑地上组织时，植被对大气汞的同化量为2422 ± 483 兆克/年。若考虑根系汞可能部分来自先前的叶片吸收并通过内部运输抵达根系，则大气汞同化量提升至2705 ± 504 兆克/年。
本次估算得到的植被大气汞同化量较当前模型与枯落物沉降估算值高54%至137%，这在很大程度上是因为本研究将地衣、苔藓与木质组织纳入了沉降考量，且覆盖了全球所有生物群系。在地下部分，约50%的根系汞来自土壤吸收，但其对应的生态与生物地球化学后果目前尚不明确。