Mapping the forest litterfall mercury deposition in China

Litterfall mercury (Hg) deposition represents one of the biggest Hg inputs to forest ecosystems through assimilation of atmospheric gaseous elemental Hg (Hg-0) to foliage. However, due to the availability of litterfall production and Hg concentration data, a comprehensive quantification of litterfall Hg deposition is still lacking in China. In this study, the forest litterfall production of five major forest types in China was modeled by using the random forest (RF) method and multi-source datasets. A substantial nationwide dataset of litterfall Hg concentration was compiled including the investigation of our research group and previous published data. The litterfall Hg flux of forest was quantified by integrating litterfall production map and litterfall Hg concentration data. The nationwide litterfall Hg concentration ranged from 12.75 to 178.00 ng g(-1) with a mean of 51.99 +/- 34.23 ng g(-1). For litterfall production, the mean value was simulated to be 5.07 Mg ha(-1) yr(-1), with the highest values in tropical areas and the lowest in the northeast and northwest arid regions. The litterfall Hg flux of forest in China was characterized by high in the south and low in the north, ranging from 5.57 to 137.05 mu g m(-2) yr(-1), with an average value of 25.88 +/- 12.53 mu g m(-2) yr(-1). Total Hg deposition from forest litterfall in China was estimated to be 27.0 +/- 13.0 Mg yr(-1), and that of evergreen broadleaf forest, mixed forest, deciduous broadleaf forest, evergreen needleleaf forest and deciduous needleleaf forest were 10.8 +/- 5.3 Mg yr(-1), 8.5 +/- 4.0 Mg yr(-1), 6.1 +/- 2.6 Mg yr(-1), 1.5 +/- 1.0 Mg yr(-1) and 0.2 +/- 0.1 Mg yr(-1), respectively. This is the primary quantitative evaluation of the forest litterfall Hg deposition in China, which is essential for understanding the role and status of Chinese forest in the global Hg cycle.