Global Biogeochemical Cycles最新文献

Nitrogen-fixing (N-fixing) trees are widely planted in forests and agroforestry ecosystems due to their benefits in soil fertility and carbon sequestration. However, their effects on soil fluxes of nitrous oxide (N₂O), methane (CH₄), and carbon dioxide (CO₂) compared to non-fixing trees remain uncertain, potentially challenging their assumed role in greenhouse gas (GHG) mitigation. Through a meta-analysis of 276 observations from 55 publications, we found that N-fixing trees increased soil N₂O emissions (Hedge's d = 0.42) and enhanced CH₄ uptake (Hedge's d = −0.59) without significantly affecting CO₂ emissions and non-CO₂ global warming potential. The type of symbiotic bacteria was critical: actinorhizal N-fixing trees increased N₂O emissions (Hedge's d = 0.70) but had no effect on CH₄ or CO₂ fluxes, whereas rhizobial N-fixing trees increased N₂O emissions (Hedge's d = 0.37), CH₄ uptake (Hedge's d = −0.66) and CO₂ emissions (+13%). The effects of N-fixing trees on N₂O are mainly influenced by elevation and clay content, on CH₄ by clay content and bulk density, and on soil CO₂ fluxes by clay content, mean annual temperature, and soil organic carbon. This study highlights the importance of symbiotic relationships in N-fixing trees when designing climate change mitigation strategies, as different types have distinct effects on ecosystem GHG balances.

Warming bottom waters on the Arctic shelf are thawing subsea permafrost, unlocking large amounts of old organic carbon. This thaw is expected to accelerate with continued sea-ice loss and ocean warming. However, the rate at which thawed subsea permafrost organic matter (OM) is degraded into CO₂ and CH₄ remains uncertain. Here, we use data from 156 subsea and terrestrial permafrost incubation experiments, combined with a reactive continuum model, to estimate permafrost OM reactivity (i.e. parameters a, ν) and quantify OM degradation rates after thaw. Our results show that the reactivity of subsea permafrost OM is similar to terrestrial permafrost OM degraded under anoxic conditions, underscoring that the terrestrial data set provides a strong empirical basis for constraining subsea permafrost OM reactivity. (Subsea) permafrost OM is, on average, less reactive (a_mean = 7.39 × 10⁻⁴ yr; ν_mean = 1.85 × 10⁻³) than terrestrial or marine OM but retains a small, highly reactive fraction driving high initial degradation rates. These initially high rates decline rapidly over years to decades and most OM degrades slowly under anoxic conditions. Using a 1,000-member ensemble of thaw and degradation scenarios, we estimate cumulative subsea permafrost OC loss of up to 96 Pg C (18–126 Pg C) over 300 years, with mean annual degradation rates of ∼350 Tg C yr⁻¹ (60–450 Tg C yr⁻¹) under moderate thawing. If fully converted by methanogens, CH₄ production could exceed current global ocean CH₄ emissions by tenfold. This study provides the first quantitative framework for informing subsea permafrost degradation models over long timescales and can help improve estimates of greenhouse gas emissions and their uncertainties under future warming scenarios.

The ocean's biological carbon pump transports organic carbon from the surface to depth via three main pathways: the gravitational sinking of particles, active transport by vertically migrating zooplankton, and mixing and advection of suspended and dissolved organic carbon. Here, we use a global data-assimilated ocean biogeochemical model to diagnose the seasonal variability of carbon export and sequestration by these gravitational, migrant, and mixing pumps. The total carbon export and sequestration are 10.2 ± 0.8 PgC yr⁻¹ and 1,339 ± 17 PgC, respectively, similar to previous estimates that did not consider seasonality. However, the seasonality of the export and sequestration pathways is highly variable, especially in the high latitudes. In subpolar regions, the seasonal amplitude of the pumps is ∼40%–60% of the annual mean: export and sequestration by the gravitational and migrant pumps peak in the summer, while the mixing pump strongly opposes this seasonality, reaching a maximum during the winter. The sequestration time of exported carbon is generally higher during winter than summer in the subpolar regions, helping to augment carbon sequestration during the less productive winter months. The gravitational “e-ratio,” or ratio of gravitational carbon export to net primary production, has a seasonal variability of ∼0.1 at high latitudes, with higher values in the summer compared to winter. Resolving seasonality reduces the inferred geographic variability of the e-ratio compared with annual-mean models, demonstrating the importance of seasonal observations and models to understand and quantify the processes regulating carbon export and sequestration.

Litter carbon (C) release refers to the amount of litter C lost via microbial respiration or leaching, while the net litter C sink represents the C retained in soil following decomposition. Both are critical but understudied processes in forest C sequestration, particularly under variable water availability driven by climate and land-cover changes. Here, we modeled litter C decomposition rates ($��K_c�$) using 668 field observations with the Random Forest (RF) algorithm at 1 km resolution across Chinese forests. The RF model demonstrated good performance (R² = 0.86), with predicted $��K_c�$ exhibiting a clear latitudinal zonation. Using modeled $��K_c�$, we then estimated the spatial and temporal patterns of litter C release and net litter C sink. Litter C release increased toward lower latitudes, whereas net litter C sink peaked at mid-latitudes and declined toward both lower and higher latitudes. Over 2000–2018, litter C release increased (0.49 ± 0.07 g C m⁻² yr⁻²), while the net litter C sink decreased (−0.10 ± 0.04 g C m⁻² yr⁻²), driven primarily by changing water availability mediated by climate and canopy dynamics. Reduced water limitation in persistently arid zones enhanced decomposition, whereas increased water limitation in seasonally dry regions suppressed productivity. In humid regions, a reduction in water surplus led to asynchronous increases in litter input and decomposition. Our findings highlight the central role of water availability in regulating litter C fluxes and underscore the sensitivity of forest C cycling to climate variability, with important implications for projecting terrestrial carbon–climate feedback under future environmental change.

Global atmospheric carbon dioxide (CO₂) concentrations have increased rapidly in recent years, raising concerns about air-sea CO₂ exchange. The Arabian Sea, being a major CO₂ source with uncertain coastal fluxes, necessitates assessing its governing mechanisms and key drivers. To address this issue, nine basin-scale observations were conducted in the eastern Arabian Sea (EAS) between January 2018 and January 2019. Surface pCO₂ (289–1,310 μatm) exhibited the lowest mean in the early spring (428 ± 30 μatm) and the highest in the late summer monsoon (606 ± 97 μatm). Strong upwelling coupled with cyclonic eddies elevates surface pCO₂ in the south and central EAS during the summer monsoon (SM), while moderate upwelling and strong winds drive CO₂ enrichment in the north. Coastal stratification and benthic production lower pCO₂ during non-monsoons. Annually, mixing accounted for 36%–52% of pCO₂ variability, followed by air-sea exchange (5%–34%), biological processes (10%–29%) and temperature (3%–26%); freshwater influence was significant only in the south (12%) during the peak SM. Consistently higher surface pCO₂ than the atmosphere makes the EAS a perennial source (4.7 ± 8.4 mmolC m⁻² d⁻¹), with maximum efflux during the peak SM (15.9 ± 19.5 mmolC m⁻² d⁻¹). The SM contributes 66% of annual emissions (9.9 TgC y⁻¹) as upwelled CO₂ exceeds biological uptake. The north EAS, though productive in winter and summer, emits the most (6.5 TgC y⁻¹), with substantial input from turbid macro-tidal nearshore regions (2.1 TgC y⁻¹), making it a hotspot for CO₂ emissions. Although no clear pCO₂ trend emerged over two decades, climate cycles and monsoon intensity strongly modulate interannual variability.

The biological carbon pump sequesters carbon through passive fluxes of biologically derived carbon, and by active vertical movement of marine organisms. Trophic coupling between pelagic and benthic communities increases the efficiency of the biological carbon pump as less carbon is lost to remineralization. Such fish-mediated benthic-pelagic coupling, which can be described as the sum of carbon fluxes that is “passed on” when predators eat prey that occupy different vertical habitats in the water column, remains highly uncertain. Here, we applied a size- and trait-based food web model to estimate the amount of carbon that fish actively transport through benthic-pelagic coupling to the seafloor across the shelf-slope-abyssal continuum in different systems of the North Atlantic. The model estimates that benthic-pelagic coupling transports on average 813 kg C km⁻² yr⁻¹ to the demersal fish communities in North Atlantic shelf-slope-abyssal systems, which is equivalent to 5% of the modeled detritus flux reaching the sea floor. In some slopes, midwater fishes mediate up to 50% of the carbon transported downwards via benthic-pelagic fish coupling. We validated model-estimated biomasses of demersal fishes with biomass estimates of bottom trawl-surveys in the same area. Both modeling and survey approaches show that demersal fish biomass estimates are at the same order of magnitude and decrease with bottom depth following a similar trend. Our study shows that benthic-pelagic coupling is an important mechanism transporting carbon to demersal communities, supplying energy to sustain abundant seafloor fish fauna and fueling commercially valuable fisheries.

Tropical peatlands contain around one-sixth of the global peat carbon stock. Decomposition is a key determinant of tropical peat persistence, but there is a scarcity of data on decomposition in tropical peatlands. To further understand decomposition in tropical peatlands, we conducted an 8-year field experiment in a primary peat swamp forest in Brunei. We tracked mass loss and the organic matter composition of Shorea albida wood buried at multiple depths over 8 years, including blocks buried with and without termite exclusion mesh. The proportion of time wood blocks spent above the water table explained the majority of the variation in wood decomposition over time. Carbon loss from wood that spent <1% of the time under the water table was 32.1%–86.5% higher on average than from wood that spent 30%–100% of the time under the water table. We estimate that termites enhanced wood decomposition by ∼2% per year. Despite significant decomposition, we did not observe a strong shift in wood organic matter composition. To contextualize our results, we synthesized past work on wood decomposition across tropical peatlands. We found that burial in waterlogged peat soils slows decomposition across tropical peatlands and that decomposition is also strongly influenced by peatland trophic status. Overall, our results affirm that waterlogging is the key to tropical peat persistence. Our study highlights the vulnerability of tropical peat carbon stocks to lowered water tables by either drainage or prolonged dry spells, as well as the promise of peatland rewetting to mitigate carbon losses from disturbed peatlands.

Marine primary production in the subtropical oceans is strongly regulated by (micro)nutrient availability, yet the types of nutrient stress in the South Pacific remain poorly resolved. Here we assessed this along a >10,000-km transect of the subtropical South Pacific Ocean (GEOTRACES Section GP21). The transect was separated into three regimes in terms of phytoplankton photophysiology: a heterogeneous coastal margin, an eastern gyre boundary transition zone, and the oligotrophic subtropical gyre. The transition zone exhibited the lowest surface apparent photochemical efficiency (F_v/F_m; 0.09–0.26) and significant responses to experimental supply of both iron (Fe) and combined nitrogen (N) and Fe (with differences relative to controls, ΔF_v/F_m, of 0.07–0.11) but depressions to N supply alone (ΔF_v/F_m of −0.03 to −0.07), diagnosing this zone as Fe stressed with low N availability. The offshore coastal margin showed intermediate surface F_v/F_m (0.21–0.41) that increased after N addition but not Fe alone, suggesting prevalent N limitation and no Fe stress. In the nutrient-depleted gyre, surface F_v/F_m was elevated (mean ± SD; 0.42 ± 0.07, n = 41), remained unchanged following any nutrient addition, and showed dawn/dusk peaks with relatively small nocturnal declines (∼33%), consistent with the absence of Fe stress and steady-state N limitation. Basin-wide, nitrate to dissolved Fe ratios best predicted surface F_v/F_m and thereby Fe stress status (R² = 0.54). Additional observations and experiments suggested a basin-wide absence of Fe stress at the deep chlorophyll maximum. Such findings are important for predicting ecosystem responses to climate-driven shifts in nutrient supply.

Methylated mercury (MeHg), including dimethylmercury and monomethylmercury (MMHg), is a pollutant of concern because it biomagnifies in marine biota. The formation of MeHg in the oceans, specifically at highly productive regions and at high oxygen levels, remains elusive. We investigated dissolved and particulate total (THg) and MeHg in the water column and sediments at six stations in a highly productive area of the Southern Atlantic Ocean. Total MeHg concentrations and proportions of THg in seawater were higher (50%–73%) at eutrophic stations. We found that the distribution of MeHg in the mixed layer is strongly controlled by the biological pump. Concentrations of THg and MMHg were highest in particles within the chlorophyll maximum, suggesting THg and MMHg scavenging or assimilation by phytoplankton and an oxic pathway of MMHg formation. Particle breakdown and respiration appeared to increase dissolved MeHg concentrations and MMHg concentrations in small particles (2–51 μm) at greater depths. MMHg concentrations in the sediments were consistently lower than in particles from the mixed layer, indicating MMHg release during particle sinking and that deep-sea sediments are unlikely to be an important source of MeHg in the water phase. Our study identifies productive marine areas as hotspots of MeHg formation and suggests increasing Hg methylation with increasing ocean eutrophication and may amplify biomagnification in marine food webs.

Highly productive summer phytoplankton blooms in the central North Pacific Subtropical Gyre (NPSG) are an annual occurrence that leads to the export of considerable amounts of surface particulate carbon to depth. The mechanisms that control the formation of these blooms remain unresolved, but iron (Fe) availability may be an important factor. From July to October 2022, a large, persistent phytoplankton bloom was detected near 23.3°N, 154.6°W in satellite imagery and in situ measurements. Elevated Fe concentrations and nitrogen (N₂) fixation activity measured within the bloom suggest that high Fe may have supported enhanced diazotrophic activity. To evaluate whether aerosol deposition created favorable conditions for bloom formation, we reconstructed the Fe deposition history of the bloom's source waters by integrating surface water back trajectory analyses with aerosol Fe flux simulations. Our results show that waters that hosted the diatom-diazotroph assemblage bloom received up to 20% more soluble Fe through aerosol deposition than its surrounding waters, primarily from a strong wet deposition event that occurred approximately 1 month before the bloom. The observed lag between deposition and bloom suggests a delayed biological response to atmospheric Fe inputs. Although this moderate increase does not represent incontrovertible evidence that the bloom was stimulated by aerosol Fe deposition, our findings establish the potential for episodic delivery of atmospheric Fe to stimulate diazotrophic activity and phytoplankton growth over month-long timescales in the NPSG.