Effects of Precipitation on Mercury Accumulation on Subtropical Montane Forest Floor: Implications on Climate Forcing

Abstract

Processes facilitated by precipitation play an important role on mercury (Hg) accumulation on forest floor and therefore key to Hg cycling in forest ecosystems. Sites along the windward slope of 1,250 to 2,400 m at Mt. Ailao, Southwestern China, have higher precipitation than the leeward slope sites. In this study, measurements of Hg concentration and associated stable isotope composition for soil, fresh, and degraded litterfall samples were made at sites along two slopes of Mt. Ailao to quantify the direct and indirect effects of precipitation on Hg accumulation on forest floor. Higher soil Hg concentrations, larger litterfall Hg depositions, and faster litter decomposition rates were observed on the windward slope (1,250-2,400 m). Data of Hg isotopic signatures suggest that Hg in surface soils is mainly derived from litterfall Hg input. Precipitation enhances litterfall Hg deposition by increasing litter biomass production, reduces litter decomposition rate, facilitates short-term Hg uptake to decomposing litter, and potentially increases microbial activity that increases Hg loss via microbial reduction or runoff. Structural equation modeling results support that the indirect effect of precipitation on increased biomass production merge as the most important factor controlling soil Hg variation. Given the climate forcing on global precipitation pattern and vegetation growth cycle, Hg biogeochemical cycling is likely to continue to evolve under the changing climate.Plain Language Summary Precipitation has direct and indirect effects on Hg accumulation and transformation on forest floor. The direct effect is that atmospheric Hg enters into forest floor via precipitation, throughfall, and cloud water deposition. In addition, precipitation likely has an indirect effect on Hg accumulation by influencing litter biomass production and postdepositional processes of Hg, such as litter decomposition, Hg reemission, and surface runoff. The objective of this study is to better quantify the direct and indirect effects of precipitation on Hg accumulation on forest floor. The study sites are located along the leeward and windward slopes of Mt. Ailao in Southwest China. Precipitation intensity is the main variable at the same altitude between two slopes. Our results showed that influences from precipitation on soil Hg accumulation are largely through the indirect effects caused by influencing litterfall Hg deposition. We also suggest that the changed precipitation pattern can force variations of litterfall Hg deposition and soil Hg accumulation globally.

Global warming accelerates uptake of atmospheric mercury in regions experiencing glacier retreat

“…The soil Hg concentrations in Oe (underlain by a partially decomposed layer) and Oa (a very dark layer of well-decomposed humus) are higher than those in Oi (litter and twigs) at Hailuogou, which can be attributed to the greater litter mass loss during the initial litter decomposition, leading to Hg accumulation in Oe and Oa. This is consistent with earlier studies showing Hg concentration changes during litter decomposition (16,17) and also supported by the increase in the Hg/C ratio from Oi to Oa (SI Appendix, Fig. S5).…”

“…Overall, we found that soil Hg originated primarily from atmospheric sources at Hailuogou and from geological sources at Midui. This is also supported by a significant positive correlation (r = 0.8235, P < 0.001, n = 93) between the content of soil organic carbon (SOC), which can serve as a surrogate for plant productivity in glacier-retreated regions (4,17), and the fraction of atmospheric Hg 0 in soil. Higher plant productivity at Hailuogou thus induces greater atmospheric Hg 0 accumulation than at Mingyong and Midui.…”

Impacts of Extreme Weather on Mercury Uptake and Storage in Subtropical Forest Ecosystems

“…In the SMD forest, it ranges from 0.5 ± 0.2 μg m −2 month −1 to 4.8 ± 3.5 μg m −2 month −1 , with an overall mean monthly deposition of 1.6 ± 0.9 μg m −2 month −1 (Figures 2a and 2b). These values are 30%-40% lower than those reported in our earlier studies (Wang, Yuan, Lu, et al, 2019;, because the litterfall refers mainly to the leaf litterfall in this study, but includes all the litterfall in our earlier studies. The r value between litterfall Hg deposition and litterfall biomass production is up to 0.99, much higher than that (0.26) between litterfall Hg deposition and litterfall Hg concentration (Figure 3a).…”

“…For forest soil sampling, six replicates of 10 m × 10 m subplots were established at each site (Z. Y. Lu et al., 2016; Wang, Lin, Lu, et al., 2016; Wang, Yuan, Lu, et al., 2019). We mainly collected forest humus soils (3–5 cm depth from the forest floor; reflecting short‐term (decadal) variations and top mineral soils (0–20 cm; reflecting long‐term [hundreds of years]) variations in this study.…”

“…Our earlier studies have reported the atmospheric Hg deposition, Hg storage in soils, and Hg isotopic signatures at ASSFERS (Z. Y. Lu et al, 2016;Wang, Lin, Lu, et al, 2016;Wang, Yuan, Lu, et al, 2019;, and in this study we examine the response of forest Hg to extreme weather. We selected two forest types.…”