Biogeochemistry最新文献

Management of drained peatlands may pose a risk or a solution on the way towards climate change mitigation, which creates a need to evaluate the current state of forestry-drained peatlands, the magnitude of degradation processes and indicators for carbon (C) loss. Using a large dataset (778 profiles, 891 peat samples, collected between 1977 and 2017) from peatlands having different fertility classes across Finland, we investigate whether the surface peat profiles of undrained and forestry-drained peatlands differ in C:N, von Post and dry bulk density. The utility of element ratios (C:N:H stoichiometry) as site indicators for degradation were further analyzed from a subsample of 16 undrained and 30 drained peat profiles. This subsample of drained sites had carbon dioxide (CO₂) and methane (CH₄) fluxes measured allowing us to link peat element ratios to annual C gas effluxes. Element ratios H:C, O:C and C:N and degree of unsaturation (combining C, N, H changes) were found widely valid: they captured both differences in the botanical origin of peat as well as its potential decomposition pathway (C lost via a combination of dissolved organic C and C gas loss and/or the gaseous loss predominantly as CO₂). Of the stoichiometric indexes, peat H:C ratio seemed to be the best proxy for degradation following drainage, it indicated not only long-term degradation but also explained 48% of the variation in annual CO₂ emission. The O:C ratio positively correlated with annual CH₄ flux, presumably because high O:C in peat reflected the availability of easily degradable substrate for methanogenesis. The differences in C:N ratio indicated notable decomposition processes for Sphagnum-dominated peatlands but not in Carex-dominated peatlands. Degree of unsaturation showed potential for an integrative proxy for drainage-induced lowering water table and post-drainage changes in peat substrate quality.

In Germany, emissions from drained organic soils contributed approximately 53.7 Mio. t of carbon dioxide equivalents (CO₂-eq) to the total national greenhouse gas (GHG) emissions in 2021. In addition to restoration measures, shifting management practices, rewetting, or using peatlands for paludiculture is expected to significantly reduce GHG emissions. The effects of climate change on these mitigation measures remains to be tested. In a 2017 experimental field study on agriculturally used grassland on organic soil, we assessed the effects of rewetting and of predicted climate warming on intensive grassland and on extensively managed sedge grassland (transplanted Carex acutiformis monoliths). The testing conditions of the two grassland types included drained versus rewetted conditions (annual mean water table of − 0.13 m below soil surface), ambient versus warming conditions (annual mean air temperature increase of + 0.8 to 1.3 °C; use of open top chambers), and the combination of rewetting and warming. We measured net ecosystem exchange of CO₂, methane and nitrous oxide using the closed dynamic and static chamber method. Here, we report the results on the initial year of GHG measurements after transplanting adult Carex soil monoliths, including the controlled increase in water level and temperature. We observed higher N₂O emissions than anticipated in all treatments. This was especially unexpected for the rewetted intensive grasslands and the Carex treatments, but largely attributable to the onset of rewetting coinciding with freeze–thaw cycles. However, this does not affect the overall outcomes on mitigation and adaptation trends. We found that warmer conditions increased total GHG emissions of the drained intensive grassland system from 48.4 to 66.9 t CO₂-eq ha⁻¹ year⁻¹. The shift in grassland management towards Carex paludiculture resulted in the largest GHG reduction, producing a net cooling effect with an uptake of 11.1 t CO₂-eq ha⁻¹ year⁻¹. Surprisingly, we found that this strong sink could be maintained under the simulated warming conditions ensuing an emission reduction potential of − 80 t CO₂-eq ha⁻¹ year⁻¹. We emphasize that the results reflect a single initial measurement year and do not imply the permanence of the observed GHG sink function over time. Our findings affirm that rewetted peatlands with adapted plant species could sustain GHG mitigation and potentially promote ecosystem resilience, even under climate warming. In a warmer world, adaptation measures for organic soils should therefore include a change in management towards paludiculture. Multi-year studies are needed to support the findings of our one-year experiment. In general, the timing of rewetting should be considered carefully in mitigation measures.

Litter decomposition plays an important role in biogeochemical cycling in boreal peatlands, where mosses, especially Sphagnum species, are a determinant. In recent decades, these peatlands have experienced a decline in moss cover due to abrupt climate warming and atmospheric nitrogen (N) deposition. To reveal the effect of the reduction in moss cover on litter decomposition, we adopted a field living moss removal experiment (with the senesced tissues remaining) in a Sphagnum-dominated boreal peatland, and investigated litter mass loss and net N loss of three deciduous woody species decomposing in monocultures and mixtures over 3 years. Based on the observed and predicted mass loss and net N loss of litter mixtures, we divided litter mixing effects into additive (no significant difference), synergistic (observed value greater than predicted value), and antagonistic (observed value lower than predicted value) effects. Across 3 years of decomposition, moss removal increased litter mass loss and net N loss, irrespective of single- or mixed-species compositions. Moss removal generally changed litter mixing effects on mass loss from antagonistic to additive effects in the initial 2 years, but from synergistic to additive effects after 3 years of decomposition. Regarding net N loss of litter mixtures, moss removal often resulted in a shift from additive to synergistic effects or from antagonistic to additive effects after 2 and 3 years of decomposition. Our observations suggest that the declines in living moss cover can accelerate litter decomposition and nutrient release, and highlight that living moss loss makes litter mixture decomposition predictable by reducing non-additive effects in boreal peatlands. Given the widespread occurrence of reduced moss cover in boreal peatlands, the mechanisms explaining living moss controls on litter decomposition and N cycling should receive significant attention in further studies.

Soil nutrient distribution is heterogeneous in space and time, potentially altering nutrient acquisition by trees and microorganisms. Ecologists have distinguished “hot spots” (HSs) as areas with enhanced and sustained rates of nutrient fluxes relative to the surrounding soil matrix. We evaluated the spatial and temporal patterns in nutrient flux HSs in two mixed-conifer forest soils by repeatedly sampling the soil solution at the same spatial locations (horizontally and vertically) over multiple seasons and years using ion exchange resins incubated in situ. The climate of these forests is Mediterranean, with intense fall rains occurring following summers with little precipitation, and highly variable winter snowfall. Hot spots formed most often for NO₃⁻ and Na⁺. Although nutrient HSs often occurred in the same spatial location multiple times, HSs persisted more often for PO₄³⁻ NH₄⁺, and NO₃⁻, and were more transient for Ca²⁺, Mg²⁺, and Na⁺. Sampling year (annual precipitation ranged from 558 to 1223 mm) impacted the occurrence of HSs for most nutrients, but season was only significant for PO₄³⁻, NH₄⁺, NO₃⁻, and Na⁺, with HSs forming more often after fall rains than after spring snowmelt. The frequency of HSs significantly decreased with soil depth for all nutrients, forming most commonly immediately below the surficial organic horizon. Although HSs accounted for less than 17% of the sampling volume, they were responsible for 56–88% of PO₄³⁻, NH₄⁺, and NO₃⁻ resin fluxes. Our results suggest that macronutrient HSs have a disproportional contribution to soil biogeochemical structure, with implications for vegetation nutrient acquisition strategies and biogeochemical models.

Graphical abstract

Tropical peat swamp degradation can modify net peat greenhouse gas (GHG) emissions even without drainage. However, current Intergovernmental Panel on Climate Change (IPCC) guidelines do not provide default emission factors (EF) for anthropogenically-degraded undrained organic soils. We reviewed published field measurements of peat GHG fluxes in undrained undegraded and degraded peat swamp forests in Southeast Asia (SEA) and Latin America and the Caribbean (LAC). Degradation without drainage shifted the peat from a net CO₂ sink to a source in both SEA (− 2.9 ± 1.8 to 4.1 ± 2.0 Mg CO₂–C ha⁻¹ yr⁻¹) and LAC (− 4.3 ± 1.8 to 1.4 ± 2.2 Mg CO₂–C ha⁻¹ yr⁻¹). It raised peat CH₄ emissions (kg C ha⁻¹ yr⁻¹) in SEA (22.1 ± 13.6 to 32.7 ± 7.8) but decreased them in LAC (218.3 ± 54.2 to 165.0 ± 4.5). Degradation increased peat N₂O emissions (kg N ha⁻¹ yr⁻¹) in SEA forests (0.9 ± 0.5 to 4.8 ± 2.3) (limited N₂O data). It shifted peat from a net GHG sink to a source in SEA (− 7.9 ± 6.9 to 20.7 ± 7.4 Mg CO₂-equivalent ha⁻¹ yr⁻¹) and increased peat GHG emissions in LAC (9.8 ± 9.0 to 24.3 ± 8.2 Mg CO₂-equivalent ha⁻¹ yr⁻¹). The large observed increase in net peat GHG emissions in undrained degraded forests compared to undegraded conditions calls for their inclusion as a new class in the IPCC guidelines. As current default IPCC EF for tropical organic soils are based only on data collected in SEA ombrotrophic peatlands, expanded geographic representation and refinement of peat GHG EF by nutrient status are also needed.

Mercury cycles at levels three- to five-fold higher today than the pre-Industrial era, resulting in global contamination of ecosystems. In the western United States (U.S.), mercury mobilization has led to widespread production of methylmercury (MeHg), a potent, bioaccumulating neurotoxin, which has resulted in fish consumption advisories across all states. Mountain regions are particularly sensitive to continued mercury contamination as they receive higher rates of atmospheric deposition, compared to lower elevations, and have aquatic ecosystems on the landscape conducive to MeHg production. In this paper, we focus on the U.S. Rocky Mountain region and synthesize: (1) current knowledge regarding the mercury cycle; (2) impacts of climate change on the mercury cycle connected to hydrology and wildfire; and (3) future research priorities for informing mercury research and regulation. Studies on the interactions between mercury contamination and climate change in mountain ecosystems is still nascent. We use the findings from this synthesis to summarize the following research needs: (1) quantify sources of mercury in wet and dry deposition, as these pathways dictate mercury exposure and toxicity, and are shifting with climate change; (2) investigate MeHg in mountain aquatic ecosystems, which are important pathways of human mercury exposure and provide food resources and habitat to local wildlife; and (3) examine the disproportionate impact of mercury contamination on indigenous communities through community-led research. Although we focus on the Rocky Mountains for this review, the findings are applicable to semi-arid mountain ecosystems globally and must be prioritized to promote the health of ecosystems and people everywhere.

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.