Mercury (Hg) deposition through litterfall has been regarded as the main input of gaseous elemental mercury (Hg0) into forest ecosystems. We hypothesize that earlier studies largely underestimated this sink because the contribution of Hg0 uptake by moss and the downward transport to wood and throughfall is overlooked. To test the hypothesis, we investigated the Hg fluxes contributed via litterfall and throughfall, Hg pool sizes in moss covers and woody biomass as well as their isotopic signatures in a glacier-to-forest succession ecosystem of the Southeast Tibetan Plateau. Results show that Hg0 depositional uptake and pool sizes stored in moss and woody biomass increase rapidly with the time after glacier retreat. Using the flux data as input to a Hg isotopic mixing model, Hg deposition through litterfall accounts for 27–85% of the total accumulation rate of Hg0 in organic soils of glacial retreat over 20–90 years, revealing the presence of additional sources of Hg0 input. Atmospheric Hg0 accounts for 76 ± 24% in ground moss, 86 ± 15% in tree moss, 62–92% in above ground woody biomass (branch–bark–stem), and 44–83% in roots. The downward decreasing gradient of atmospheric Hg0 fractions from the above ground woody biomass to roots suggests a foliage-to-root Hg transport in vegetation after uptake. Additionally, 34–82% of atmospheric Hg0 in throughfall further amplifies the accumulation of Hg0 from atmospheric sources. We conclude that woody biomass, moss, and throughfall represent important Hg0 sinks in forest ecosystems. These previously unaccounted for sink terms significantly increase the previously estimated atmospheric Hg0 sink via litterfall.

Mercury accumulation in montane forested areas plays an important role in global Hg cycling. In this study, we measured stable Hg isotopes in soil and litter samples to understand Hg accumulation on the forest floor along the eastern fringe of the Tibetan Plateau (TP). The low atmospheric Hg inputs lead to the small Hg pool size (23 ± 9 mg m in 0-60 cm soil horizon), up to 1 order of magnitude lower than those found at sites in Southwest China, North America, and Europe. The slightly negative ΔHg (-0.12 to -0.05‰) in the litter at low elevations (3100 to 3600 m) suggests an influence of local anthropogenic emissions, whereas the more significant negative ΔHg (-0.38 to -0.15‰) at high elevations (3700 to 4300 m) indicates impact from long-range transport. Hg input from litter is more important than wet deposition to Hg accumulation on the forest floor, as evidenced by the negative ΔHg found in the surface soil samples. Correlation analyses of ΔHg versus total carbon and leaf area index suggest that litter biomass production is a predominant factor in atmospheric Hg inputs to the forest floor. Precipitation and temperature show indirect effects on Hg accumulation by influencing litter biomass production in the eastern TP.

Aim Our aims were to quantify climatic and soil controls on net primary productivity (NPP) and leaf area index (LAI) along subtropical to alpine gradients where the vegetation remains relatively undisturbed, and investigate whether NPP and LAI converge towards threshold-like logistic patterns associated with climatic and soil variables that would help us to verify and parameterize process models for predicting future ecosystem behaviour under global environmental change.Location Field data were collected from 22 sites along the Tibetan Alpine Vegetation Transects (TAVT) during 1999 -2000. The TAVT included the altitudinal transect on the eastern slope of the Gongga Mountains in the Eastern Tibetan Plateau, with altitudes from 1900 m to 3700 m, and the longitudinal-latitudinal transect in the Central Tibetan Plateau, of approximately 1000 km length and 40 km width.Methods LAI was measured as the product of foliage biomass multiplied by the ratio of specific leaf area. NPP in forests and shrub communities was estimated as the sum of increases in standing crops of live vegetation using recent stem growth rate and leaf lifespan. NPP in grasslands was estimated from the above-ground maximum live biomass. We measured the soil organic carbon (C) and total and available nitrogen (N) contents and their pool sizes by conventional methods. Mean temperatures for the year, January and July and annual precipitation were estimated from available meteorological stations by interpolation or simulation. The threshold-like logistic function was used to model the relationships of LAI and NPP with climatic and soil variables.Results Geographically, NPP and LAI both significantly decreased with increasing latitude ( P < 0.02), but increased with increasing longitude ( P < 0.01). Altitudinal trends in NPP and LAI showed different patterns. NPP generally decreased with increasing altitude in a linear relationship ( r 2 = 0.73, P < 0.001), whereas LAI showed a negative quadratic relationship with altitude ( r 2 = 0.58, P < 0.001). Temperature and precipitation, singly or in combination, explained 60 -68% of the NPP variation with logistic relationships, while the soil organic C and total N variables explained only 21-46% of the variation with simple linear regressions of log-transformed data. LAI showed significant logistic relationships with both climatic and soil variables, but the data from alpine spruce-fir sites diverged greatly from the modelled patterns associated with temperature and precipitation. Soil organic C storage had the strongest correlation with LAI ( r 2 = 0.68, P < 0.001). Main conclusionsIn response to climatic gradients along the TAVT, LAI and NPP across diverse vegetation types converged towards threshold-like logistic patterns consistent with the general distribution patterns of live biomass both above-ground and below-ground found in our earlier studies. Our analysis further revealed that climatic factors strongly limited the NPP variations along the TAVT because the precipitation gradient characterized ...