Global Biogeochemical Cycles最新文献

Increasing freshwater discharge from the cryosphere has the potential to affect mechanisms that regulate the supply of macronutrients to primary producers in the ocean. Although numerous studies have characterized the biogeochemical effects of freshwater discharge from the Greenland Ice Sheet within fjords, it is unclear to what extent these effects propagate offshore and whether or not seasonal macronutrient availability in Greenland's shelf seas has changed since annual freshwater discharge increased in the mid-1990s. Here, we collate and scrutinize extensive work undertaken over the past century to constrain macronutrient distributions in Greenland's coastal seas. From 1929 to 2022, over 38,000 publicly available measurements of macronutrient concentrations were made on the Greenland shelf, with over 7,000 within inshore waters. Low salinity waters around Greenland are associated with a modest dSi enrichment (23.01 ± 6.16 μM at zero salinity) matching prior estimates from runoff, whereas extrapolated phosphate and nitrate freshwater endmembers are small or negligible. A north-to-south dSi enrichment is evident along both east and west coastlines, which, as well-described in prior oceanographic work, reflects Arctic Ocean outflow. The direct impact of processes related to the Greenland Ice Sheet on macronutrient distributions is largely confined to fjords, although there are a few cases of nutrient anomalies propagating to the shelf associated with large marine-terminating glaciers in Sermilik Fjord, Disko Bay, and Nioghalvfjerdsbrae (“the 79° North Glacier”). Furthermore, we identified several oceanic areas of the Greenland shelf with elevated (>2 μM) surface nitrate or dSi concentrations in summer, which likely reflect regionally distinct shelf processes.

The polar ice sheets, their surrounding land fringes and oceans (68 × 10⁶ km²; 13% of Earth's surface) are hot spots for carbon cycle perturbation under future climate change due to glacier retreat, rising meltwater fluxes, reduced sea ice, thawing permafrost, warming land-surfaces and increased precipitation. Here we assess carbon stored and exchanged with the atmosphere (as carbon dioxide and methane) across an expansive bipolar ice-to-ocean domain. We show that the polar regions harbor large reserves of carbon stored in sediments, rocks and the ocean, which differ in their reactivity and turnover times: 5,300–22,200 PgC of organic carbon and 5,600–8,600 PgC of inorganic carbon. These carbon reservoirs include potential reserves of marine and subglacial methane hydrate (80–570 PgC), which could become destabilized under future warming scenarios. Oceans (270–360 PgC) and ice sheets (14–96 PgC Greenland, 5,000–21,000 PgC Antarctica) dominate organic carbon stores, with smaller (but regionally important) stocks found in ice sheet land fringes (13–58 PgC). Estimates of natural CO₂ and CH₄ fluxes from these polar regions to the atmosphere present high uncertainty but highlight oceanic CO₂ sinks in Greenland (−110 to −49 TgC-CO₂ a⁻¹) and in the ICE and SPSS biomes of the Southern Ocean (−480 to 55 TgC-CO₂ a⁻¹), with potential CH₄ sources associated with the Greenland Ice Sheet. Such high uncertainty in polar carbon reservoirs and fluxes is important to resolve if future feedbacks between the polar regions, Earth's carbon cycle and climate are to be conclusively determined.

Forests currently mitigate anthropogenic climate change by sequestering substantial amounts of carbon, but future carbon dynamics are expected to vary across the temperate forest biome. Previously cold-limited ecosystems with low disturbance activity could increase their carbon uptake, while water-limited ecosystems with high disturbance activity could become a carbon source to the atmosphere. However, forests dynamically adapt to changing climate and disturbance regimes by reorganizing their composition and structure, with unclear consequences for future carbon dynamics. We asked how the carbon dynamics of reorganizing forests differ from those of resilient forests, that is, forests that conserve their composition and structure under climate change, and how shifts in composition and structure drive future forest carbon. We simulated long-term forest and carbon dynamics under current and future climate conditions for three contrasting temperate forest national parks, spanning a gradient from low disturbance activity in Shiretoko, Japan, to intermediate disturbance activity in Berchtesgaden, Germany, and high disturbance activity in Grand Teton, Japan. Under climate change, carbon stores increased in Shiretoko, remained close to current levels in Berchtesgaden, and decreased in Grand Teton. Forests that reorganized, that is, exhibited compositional and/or structural changes, generally took up more carbon than resilient forests. Changes in forest carbon cycling were consistently associated with changes in forest structure across systems, whereas the effects of tree species composition change were less consistent. We conclude that resilience in composition and structure does not guarantee continuity in ecosystem functioning, suggesting that reorganization is necessary to maintain forest carbon stocks in a changing climate.

Fertilizer nitrogen (N) can accumulate in the soil-vadose zone-groundwater continuum as legacy N, exerting a consistent influence on key biogeochemical processes such as crop N uptake, N leaching, and gaseous N emissions. However, the regional stock of legacy N in soils and its contribution to soil N pools remain unknown due to the lack of effective estimation methods, hindering the understanding of its environmental and agricultural effects. Here, we proposed a method for quantifying legacy N stock at the regional scale by identifying an exponential decay pattern in multi-year N retention rates and establishing a validated power function fitting between a parameter related to the initial retention rate and the N application rate. We further proposed a novel N isotope model to constrain the contribution of legacy N to soil N pools. Overall, China's upland croplands have accumulated legacy N of 54.7 ± 24.9 Tg N in top 30 cm soils over the past six decades (1961–2020). Croplands in southern China are hotspots for legacy N accumulation, likely due to their rich soil organic carbon. Legacy N may account for 43%–89% of soil organic N sequestration. The N isotope model estimates that legacy N constitutes 21.4% ± 6.9% of total soil N, comparable with the compiled proportion of total N increases due to fertilization (16.0%). These findings suggest that the legacy N from historical fertilizers substantially enhances soil fertility, and may inform biogeochemical parameterization given that the components of soil N pools may differ in stability.

Fertilizer nitrogen (N) can accumulate in the soil-vadose zone-groundwater continuum as legacy N, exerting a consistent influence on key biogeochemical processes such as crop N uptake, N leaching, and gaseous N emissions. However, the regional stock of legacy N in soils and its contribution to soil N pools remain unknown due to the lack of effective estimation methods, hindering the understanding of its environmental and agricultural effects. Here, we proposed a method for quantifying legacy N stock at the regional scale by identifying an exponential decay pattern in multi-year N retention rates and establishing a validated power function fitting between a parameter related to the initial retention rate and the N application rate. We further proposed a novel N isotope model to constrain the contribution of legacy N to soil N pools. Overall, China's upland croplands have accumulated legacy N of 54.7 ± 24.9 Tg N in top 30 cm soils over the past six decades (1961–2020). Croplands in southern China are hotspots for legacy N accumulation, likely due to their rich soil organic carbon. Legacy N may account for 43%–89% of soil organic N sequestration. The N isotope model estimates that legacy N constitutes 21.4% ± 6.9% of total soil N, comparable with the compiled proportion of total N increases due to fertilization (16.0%). These findings suggest that the legacy N from historical fertilizers substantially enhances soil fertility, and may inform biogeochemical parameterization given that the components of soil N pools may differ in stability.

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.