Mercury behaviour and C, N, and P biogeochemical cycles during ecological restoration processes of old mining sites in French Guiana

Abstract

Several decades of gold mining extraction activities in the Amazonian rainforest have caused deforestation and pollution. While ecological rehabilitation is essential for restoring biodiversity and decreasing erosion on deforested lands, few studies note the behaviour or toxicity of trace elements during the rehabilitation process. Our original study focused on the potential use of microbial activity and Hg speciation and compared them with As, Cu, Zn and Cr speciation in assessing the chemical and biological quality of ecological restoration efforts. We sampled two sites in French Guyana 17 years after rehabilitation efforts began. The former site was actively regenerated (R) with the leguminous species Clitoria racemosa and Acacia mangium, and the second site was passively regenerated with spontaneous vegetation (Sv). We also sampled soil from a control site without a history of gold mining (F). We performed microcosm soil experiments for 30 days, where trace element speciation and enzyme activities (i.e., FDA, dehydrogenase, β-glucosidase, urease, alkaline and acid phosphatase) were estimated to characterise the behaviour of trace elements and the soil microbial activity. As bioindicators, the use of soil microbial carbon biomass and soil enzyme activities related to the carbon and phosphorus cycles seems to be relevant for assessing soil quality in rehabilitated and regenerated old mining sites. Our results showed that restoration with leguminous species had a positive effect on soil chemical quality and on soil microbial bioindicators, with activities that tended toward natural non-degraded soil (F). Active restoration processes also had a positive effect on Hg speciation by reducing its mobility. While in Sv we found more exchangeable and soluble mercury, in regenerated sites, Hg was mostly bound to organic matter. These results also suggested that enzyme activities and mercury cycles are sensitive to land restoration and must be considered when evaluating the efficiency of restoration processes.

The Impact of Ecological Restoration on Biogeochemical Cycling and Mercury Mobilization in Anoxic Conditions on Former Mining Sites in French Guiana

“…Firstly, there were significant differences in soil microbial biomass, which were related with the density and quality of the vegetation cover on the restored sites. These results, already highlighted by various works on rehabilitated sites, confirm the contribution of successful ecological restoration in the recovery of soil microbial biomass [ 39 , 42 , 43 , 44 ], especially on the rehabilitated sites of Yaoni.…”

“…Several authors have highlighted similar results. Anderson [ 37 ] showed that the affinity for and retention of mercury in organic matter in acidic soils could exceed the affinity for metal oxides, as confirmed by recent studies on mercury speciation at former Guyanese mining sites [ 34 , 42 ]. Roulet and Lucotte’s [ 16 ] work highlighted that mercury retention in Guyanese soils was not always correlated with the presence of metal oxides.…”

“…Mercury concentrations were positively correlated with organic matter concentration ( p = 0.0001) and microbial biomass ( p = 0.0001), which is consistent with the literature, as described by Schuster et al (1991) and Skyllberg et al (2000), showing the high affinity of mercury for organic matter, particularly for sulphide groups [ 40 , 41 ]. In addition, the dense vegetation cover from the Aca and Mix sites may have contributed to fixing mercury in soils and limiting its mobility [ 42 ]. Vegetation allows atmospheric mercury to be fixed in leaves and in litter deposits.…”

“…Indeed, due to its chalcophile character, mercury in natural conditions in well-oxygenated soils is mainly found in the form of cinnabar (HgS), a sulfide that is not easily weathered [ 62 ]. The increased presence of soluble and non-adsorbed forms of mercury at NR ( Figure 4 ) and Lyc sites, including alkyl and inorganic mercury species, may also confirm the solubility and reactivity of mercury at rehabilitated but non-restored sites is greater than that at restored sites [ 42 ]. In this study, soluble mercury is also negatively correlated with total mercury (r = −0.68), which could show that mercury mobility is not related to its total concentration in soil.…”

Responses of plant community to the linkages in plant-soil C:N:P stoichiometry during secondary succession of abandoned farmlands, China

“…This change provides a good template for us to study the process of natural coordinated plant-soil recovery under temporal changes (Zhao et al, 2013). Especially, the ecological stoichiometry that is an important indicator reflecting the carbon (C), nitrogen (N) and phosphorous (P) cycles inside the soil (Zechmeister-Boltenstern et al, 2015) integrates the variability of ecosystem function and is widely used in the study of limitative nutrient element determination (Zhou et al, 2018), biogeochemical cycles and population dynamics (Martiny et al, 2016;Busch et al, 2018;Couic et al, 2018). During the process of secondary succession of abandoned farmlands, the relationship between the changes in ecological stoichiometric ratio and the aboveground plant community succession is unknown.…”