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Background and aims: Forest soil has numerous ecological functions, some of which are regulated by biogeochemical processes involving trace elements (TEs). However, the accumulation and transport of TEs in remote pure forest ecosystems have been poorly understood to date. Methods: We investigated the concentrations and pools of Cu, Zn, Cd, Pb, Ni, Cr, Hg, and As in the soil horizons of Qinghai spruce and Qilian juniper forests at various elevations in the central Qilian Mountains, northwest China. The Enrichment Factor and Biogeochemical Index were used to evaluate the ecological security of TEs in forest soils. Results: The results indicate that the distribution of TEs concentrations in soil horizons of the two forest stands had spatial heterogeneity and element specificity. The TEs pools in the soil horizons of these stands increased with increasing soil depth. Among the elements studied, the total pools of Zn and Cr in the soil of the two stands were the largest, while the Hg and Cd pools were the smallest. Results further demonstrated that the forest ecosystem in the Tianlaochi catchment had large TEs storage, which was higher in Qinghai spruce forest than in Qilian juniper forest. Conclusions: Forest soils with lower TEs concentrations have higher enrichment degrees and potential ecological risk. Our findings highlight the importance of TEs accumulation in forest soils and warrant more research on their cycling in the region. [ABSTRACT FROM AUTHOR]

Litter plays a key role in maintaining the nutrient cycle of the forest ecosystem. The dynamics of litter trace elements (TEs) can influence litter decomposition and biogeochemical cycling across plant and soil systems. However, our understanding of the biogeochemical cycle of TEs during litter decomposition remains limited. We investigated litter production, the concentrations and fluxes of needle litter TEs over 1 year, and the accumulation and release patterns of TEs at different elevations and canopy coverage during litter decomposition over 3.9 years for the Qinghai spruce in the Qilian Mountains. The concentrations and fluxes of TEs in the needle litter decreased in the following order: Zn > Ni > Cr > Cu > Pb > Co > Cd > Ag. TEs concentrations increased with decomposition time at different elevations, and Co, Cr, Cu, Ni, Pb, and Cd accumulated the fastest at 2850 m. Zn and Ag accumulated the fastest at 3450 and 3050 m, respectively. The fastest accumulation trends were for Co, Cu, Ni, Pb, Zn, Ag, and Cd concentrations at low canopy coverage. Cr accumulated the fastest at middle canopy coverage. The concentrations of Co, Cu, Pb, Zn, Ag, and Cd followed the trend of enrichment–release–enrichment, while Ni and Cr concentrations followed the trends of release–enrichment and sustained enrichment, respectively. Our study is important for an improved understanding of the TE cycle during litter decomposition. It provides theoretical support for healthy soil management and element cycling in the forest of the Qilian Mountains. [ABSTRACT FROM AUTHOR]

Forest litter is an important guarantee for maintaining forest soil fertility and circulating material in forest ecosystems. The input of litter plays an important role in soil organic matter formation and biogeochemical cycles in forest ecosystems. However, the production and elements concentrations of Juniperus przewalskii (JP) litter in the Qilian Mountains are still unknown. In this study, we investigated the production of needle, branch and bark, cone, and impurity litters. We determined the concentrations and fluxes of major (K, Mg, Al, and Fe) and trace (Na, Mn, Zn, Cr, Ni, Cu, Pb, Co, Cd, and Ag) elements in needle litter of JP from September 2020 to August 2021. The results showed that the annual litter production was 4040.74 ± 495.96 kg ha−1 a−1. Needle and cone litters were the main components of the total litter production. The major elements (MEs) and trace elements (TEs) fluxes of litter were consistent with the litter production trend. The concentrations and fluxes of MEs and TEs in needle litter decreased in the order: K > Mg > Al > Fe > Na > Mn > Zn > Cr > Ni > Cu > Pb > Co > Cd > Ag. These results have important implications for understanding the migration processes of MEs and TEs in forest ecosystems of the Qilian Mountains. [ABSTRACT FROM AUTHOR]

[Display omitted] • Concentrations of Cu in rainwater and Zn in particulate were the highest among TEs. • Forest soils had higher Pb, Ni concentrations and lower Cd, Hg concentrations. • The Zn, Ni, Pb, and Cu have accumulated in forest vegetation and tissues. The biogeochemical cycling of trace elements (TEs) in forest ecosystems is important because it plays a role in providing essential nutrients to plants and soils and because it can potentially have toxic effects. In this study, we investigated the concentration of TEs in atmospheric wet deposition, vegetation and soil in Qinghai spruce (QS) and Qilian juniper (QJ) forests of the Qilian Mountains. Our results show that the average concentrations of Cu in rainwater in QS and QJ forests were 10.30 and 5.35 μg L−1, respectively, the highest concentrations of all TEs in these environments. We suggest that the particulate matter present in the air was the main contributor of TEs in atmospheric wet deposition, which is affected by element specificity, regional factors, and the scavenging process during rainfall events. Most vegetation and tissues had high concentrations of Zn, Ni, Pb, and Cu, suggesting that these elements have accumulated in plants. The Zn, Pb, and Ni levels in forest plants may be correlated with those in forest soils. Our study highlights the role that atmospheric wet deposition can play in affecting TEs cycling across forest ecosystems. Managers need to further reduce TEs levels in emissions from surrounding sources and improve long-term observation of TEs in forest ecosystems. [ABSTRACT FROM AUTHOR]

• Atmospheric wet deposition fluxes of TEs to forest were analyzed in Qilian Mountains. • Similarities and differences of wet deposition were discussed in two forest stands. • Potential influencing factors of TEs in wet deposition were investigated. • Three major sources were identified for particulate matter. • Industrial activities and waste incineration were the largest contributors of PM. Atmospheric deposition is the main pathway by which trace elements (TEs) enter the terrestrial ecosystems, but few studies have investigated atmospheric TE deposition in remote forests. To understand the concentration, fluxes, influencing factors, and sources of TEs in atmospheric wet deposition, 104 samples from 13 rainfall events in two forest stands of the Qilian Mountains between July and September 2018 were collected and analyzed for eight TEs. The order of volume-weighted mean (VWM) concentration of TEs was Cu > Pb > Ni > Cr > Cd > As > Hg. Cu was the most abundant TE, with VWM concentrations of 6.33–10.90 μg L−1 in Qinghai spruce (Picea crassifolia Kom.) forest and 3.21–7.48 μg L−1 in Qilian juniper (Juniperus przewalskii Kom.) forest, which were 2 or 3 orders of magnitude higher than those of Hg, respectively. The wet deposition fluxes of TEs in Qinghai spruce forest were larger than those in Qilian juniper forest. The rainfall amount, rainfall intensity, rainfall duration, relative humidity, and wind speed were found to be important factors influencing atmospheric TE deposition. Based on correlation analysis, Principal Component Analysis (PCA) and Positive Matrix Factorization (PMF) modeling, three major sources of particulate matter (PM) were identified, namely industrial activities and waste incineration, traffic emissions and coal combustion, and biomass burning. Industrial activities and waste incineration were the dominant contributors, accounting for 79% of PM mass. Our results confirm that long-range transport of TEs has a significant impact on remote forest ecosystems in the Qilian Mountains. [ABSTRACT FROM AUTHOR]