Journal of Geophysical Research: Biogeosciences最新文献

Diapycnal mixing supplies nutrients to the euphotic zone, which in oligotrophic regions may substantially support rates of new production. However, the consensus view that diapycnal nutrient fluxes support new production within the entire euphotic zone is challenged by deep living autotrophs that likely consume some, if not all, of the diapycnal flux at depth. Quantifying how much of the diapycnal nitrate flux is trapped by biological consumption immediately above the nitracline remains challenging and the implications of nutrient trapping for comparisons of cross-nitracline diapycnal fluxes with euphotic zone integrals of new production remains unclear. It is increasingly important therefore to determine where in the euphotic zone the diapycnal flux has impact. In this study, a simple assessment is presented of the strength of the “nutrient trap,” which is attributed to picoeukaryotes, a widely distributed group of autotrophic picoplankton found in the subtropical and tropical ocean. This study finds significant potential for the total consumption of diapycnal nutrient fluxes within a few meters of the nitracline, thus largely negating the significance of vertical diffusive fluxes for processes occurring at shallower depths. These results suggest that the significance of diapycnal nutrient fluxes for integrated productivity estimates is lower than generally assumed. Yet, although diapycnal fluxes cannot be entirely discounted from nutrient budgets due to seasonality in the consumption of such fluxes at depth, this likely makes harder current modeling efforts to constrain future ocean productivity where predictions of increased stratification generally favor greater reliance upon the diapycnal pathway to support production.

A significant quantity of soil organic carbon (SOC) is buried in mangrove ecosystems. However, recent research has revealed that substantial amounts of carbon are exported to the atmosphere as CO₂ or to the ocean as dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). This carbon outflow highlights the need for a comprehensive understanding of carbon burial, outgassing, and outwelling to clarify the role of mangroves in net atmospheric CO₂ removal. To elucidate the carbon cycle within a mangrove ecosystem, we quantified CO₂ outgassing through high-resolution measurements CO₂ and porewater-derived fluxes of total alkalinity, DIC, DOC, and POC using radium isotopes (²²⁴Ra and ²²³Ra) in an island mangrove ecosystem in Japan. Our findings showed that the dominant carbon flux was DIC outwelling, at 189 ± 18 mmol m⁻² day⁻¹, approximately 2.3 times higher than the global median. This pronounced DIC outwelling likely reflects the presence of a vast reservoir of poorly stabilized SOC. A comparison with ecosystem-scale CO₂ emissions revealed that approximately 89% of the DIC was transported into the estuary without being emitted as CO₂. This high DIC transport appears to result from the efficient water exchange characteristic of island mangroves, with a creek residence time of ∼1 day. Surprisingly, the adjacent estuary acted as a net CO₂ sink, surpassing CO₂ outgassing from the mangrove creeks. These results suggest that efficient water exchange in island mangroves, coupled with high biological productivity at the adjacent estuary, promotes long-term sequestration of mangrove-derived DIC in the ocean.

Understanding the balance between denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in riparian systems is essential for managing watershed nitrogen (N) budgets and evaluating restoration practices. This balance is influenced by several factors including concentrations and ratio of various organic and inorganic electron donors (dissolved organic carbon [DOC], Fe²⁺) to acceptor (NO₃⁻). In riparian sediments, these factors can change rapidly over space and time, complicating measurement and quantification. We used a PFLOTRAN batch reactor model calibrated to laboratory microcosm experiments where the denitrification and DNRA rates in riparian sediments were measured using ¹⁵N stable isotopes. Although DOC/NO₃⁻ ratios influenced the relative proportions of denitrification and DNRA, the processes were also affected by elemental concentrations. For a starting DOC concentration of 0.12 mgL⁻¹, DNRA exceeded denitrification at DOC/NO₃⁻ = 6; however, this shift was not observed within a range of DOC/NO₃⁻ = 30 at higher DOC concentration of 12 mgL⁻¹. Heterotrophic pathways dominated NO₃-N reduction with smaller contribution from autotrophic pathways. These findings suggest that although heterotrophic pathways are important in carbon-rich sediments, autotrophic pathways can be significant in carbon-depleted conditions in the presence of inorganic electron donors such as Fe²⁺. Our simulations also highlighted key challenges with constraining model rate constants and parameters and the need for site specific calibrations. This work highlights the value of process-based modeling in quantifying denitrification-DNRA partitioning and the variable controls of electron donors and acceptors. Such simulations could be extended to riparian buffers to determine if they are effective management sinks for N mitigation.

The consequences of climate change on boreal ecosystems are evident in declining permafrost extent, amplifying positive climate feedback loops, and altering the timing and intensity of hydrologic events. Thawing permafrost in the discontinuous permafrost zone could affect carbon and nutrient cycling in stream ecosystems. We examined stream chemistry and climate trends over a 20+-year period across catchments in the Caribou Poker Creeks Research Watershed underlain with varying extents of permafrost (4%–53%). The study aimed to evaluate patterns in dissolved inorganic carbon (DIC, pCO₂), dissolved organic carbon (DOC), nitrogen (Dissolved organic nitrogen, and NO₃⁻), geochemical solutes (Ca²⁺, Mg²⁺, SO₄²⁻), and discharge to determine how altered terrestrial flowpaths and climate change-related trends in temperature and precipitation have transformed solute transport in high-latitude watersheds during the ice-free season. We analyzed long-term trends in stream chemistry using Thiel-Sen analysis and a mixed effects model to quantify the influence of abiotic factors on solute concentrations. Results indicate significant declines in DOC (−109.0 to −169.9 μg L⁻¹ yr⁻¹) and pCO₂ (−24.1 ppmv yr⁻¹) in higher permafrost extent sub-catchments. The highest permafrost catchment is experiencing the greatest amount of change, contrasting sharply with opposite to fewer trends in the catchments with lower permafrost extent. Model results indicate the importance of moisture conditions and discharge (p < 0.05), especially for changes in organic solutes. As climate change progresses, the role of these abiotic factors and permafrost thaw will remain important for solute transport dynamics in boreal headwater streams, with consequences for in-stream communities and downstream solute yields.

Arctic rivers are intricate water networks that chemically and biologically process carbon before releasing it as carbon dioxide (CO₂) into the atmosphere or carrying it to the ocean. Primary producers use inorganic carbon to build biomass at the base of the trophic chain. Little is known about how biogeochemical properties in Arctic rivers adapt to climate warming and changes in hydrology. To quantify net and gross biological productivity, we measured the dissolved oxygen-to-argon (O₂/Ar) ratios and O₂ triple isotopologues composition in the river Kolyma and in its tributary Ambolikha during late freshet (June) and low-flow conditions (August) in 2019. We found that hydrological factors restricted river productivity. The river system released CO₂ into the atmosphere in June and August, however August emissions were only 6% of late freshet emissions. In June, higher river flow and turbidity restricted river production, but in August, lower flows allowed more light penetration and a phytoplankton bloom at the tributary-main Kolyma channel confluence. CO₂ emissions per area during June and August accounted for 5 ± 11% of the gross carbon uptake estimated during a bloom event. Thus, in-stream metabolism can exceed riverine CO₂ emissions under certain flow and light conditions. Arctic climate change may promote biological productivity in particular locations along with changes in dissolved organic matter signature and microbiome, and contribute to Arctic river carbon budgets as flow slows during prolongued open water periods.

Saltmarshes provide vital ecosystem services, including coastal protection and habitat for fisheries. While feedbacks influencing vertical sediment accretion in marshes are well-documented, including those between relative sea level and primary production, relationships among nutrient cycles, flooding, and primary production remain less explored. This study presents a unique 30-year data set from the North Inlet estuary, South Carolina, tracking monthly growth of the dominant macrophyte, Spartina alterniflora, and porewater concentrations of sulfide, ammonium, and phosphate. Our findings reveal correlated seasonal and decadal patterns of sulfides, nutrients, and plant growth, with periodicities linked to seasonal climate and decadal tidal cycles. S. alterniflora thrives in anaerobic sediments where sulfides are toxic byproducts of sulfate-reducing bacteria (SRB) produced through the oxidation of organic acids. Dissolved organic compounds fluctuated seasonally in tandem with plant growth. Organic acids utilized by SRB are released as root exudates and produced via cellulosic biomass fermentation. SRB fix nitrogen and produce sulfides that solubilize phosphate, compensating for nutrients lost in drainage and rafting of aboveground biomass. By directly and indirectly producing the substrates, Spartina effectively regulates SRB activity, thereby orchestrating biogeochemical cycles and accumulating sulfides that inhibit competing species. Our results challenge the traditional view of sulfides as mere phytotoxins and saltmarshes as passive nutrient transformers. Instead, we propose a model wherein S. alterniflora sustains its growth and supports estuarine productivity by regulating its nitrogen and phosphorus supplies through a mutualistic relationship with SRB.

The operation of reservoirs significantly impacts the cycling of dissolved inorganic carbon (DIC) in rivers, and yet such effects in highly turbid river reservoirs remain poorly understood. This study collected water samples from the Xiaolangdi Reservoir (XLDR) located in the lower reaches of the Yellow River, China, during four distinct periods: June and December 2017, and April and August 2018. The DIC concentration, isotopic composition, and total alkalinity were analyzed to investigate the seasonal variations and controlling mechanisms of DIC cycling and fluxes under different regulation regimes within this turbid reservoir. Artificial regulation affects the hydraulic residence time of the reservoir, which varies from 22 days in summer to 99 days in winter. This variation leads to a transition from thermal stratification to homogeneous mixing, resulting in changes in physicochemical factors. Consequently, there is a pronounced seasonal variation in DIC flux and storage. During the spring release period, DIC storage was second only to winter, despite the net output flow peaking at 8.0 × 10⁴ t/month. In contrast, during the summer flood control period, DIC output flow reached its maximum while DIC storage in the reservoir was at its lowest, measuring 4.3 × 10⁴t. Major factors influencing DIC cycling in the XLDR include photosynthesis, organic matter decomposition, and carbonate precipitation and dissolution, exhibiting regional and depth-dependent variations. These findings highlight the relationship between artificial regulation and biogeochemical processes, positioning the XLDR as a valuable natural laboratory for studying carbon cycling in inland waters in the Anthropocene.

The priming effect (PE) of soil organic matter (SOM) mineralization plays a crucial role in regulating soil carbon (C) dynamics. However, in flooded ecosystems like rice paddies, where the water-soil interface is a critical hotspot for carbon turnover, the influence of periphytic biofilms (PB) on SOM mineralization and PE remains poorly understood. Here, we employed ¹³C-labeled PB and glucose to investigate PB-mediated effects on SOM mineralization and PE in paddy soils. PB amendments significantly increased dissolved organic carbon (DOC), redox potential (Eh), and the microbial biomass carbon-to-nitrogen ratio (MBC:MBN), while reducing MBN and pH. These shifts stimulated CO₂ emissions but suppressed CH₄ emissions. In the early stage (day 7), CO₂ PE was negative and inversely correlated with PB-derived CO₂ emissions, suggesting preferential utilization of labile PB-C. As labile C was depleted, CO₂ PE shifted to positive values under higher biomass inputs, with negative correlations to total nitrogen, implicating nitrogen mining. In contrast, CH₄ PE remained consistently negative, indicating sustained suppression of SOM-derived methanogenesis when PB served as an alternative substrate. Over 60 days, cumulative PE increased linearly with PB biomass (R² = 0.98, p < 0.05). In glucose-amended soils, PB presence also lowered MBN and pH, reduced glucose-induced PE, and enhanced net glucose-C retention. These findings reveal PB's regulatory role by directly influencing SOM mineralization through stoichiometric constraints and indirectly modulating PE by restructuring soil biogeochemistry, offering mechanistic insights into C sequestration and greenhouse gas mitigation in paddy ecosystems.

Infrequent soil wetting in deserts can induce large nitrogen (N) trace gas pulses; however, how other abiotic mechanisms interactively control the timing and magnitude of these pulses are not clear. In particular, production of nitric(NO) and nitrous (N₂O) oxide may be differentially sensitive to temperature, carbon (C), and N availability. At a desert field site in Southern California, USA, we used an automated sensor system in 4 years of field campaigns to track NO and N₂O pulse responses to experimental manipulations of C and N across a range of ambient temperatures and shrub fertile islands. We observed rapid onset and shorter duration of N₂O pulses immediately after wetting compared to lagged and extended pulses of NO, suggesting preferential incorporation of N initially into N₂O in anoxic microsites and then to NO as soils dry. We identified strong nitrogen limitation and exponential temperature dependence of NO pulses, particularly for soils located under shrubs. N₂O pulses were less responsive to experimental manipulations but showed evidence of C and N colimitation as well as seasonal temperature differences. As atmospheric N deposition and temperatures continue to increase in desert systems, we can expect larger losses of N from soils as pulse-based emissions.

Restored mangroves are increasingly recognized as vital nature-based solutions for atmospheric CO₂ sequestration. We hypothesize that the seasonal dynamics of carbon fluxes and coupled regulatory mechanisms may be the key in understanding their sequestration strength, especially in the northernmost mangroves experiencing pronounced seasonality. In this study, we measured net ecosystem CO₂ exchange from 2017 through 2023 using the eddy covariance technique in the northernmost restored mangrove ecosystem in southern China. These mangroves acted as carbon sinks, with an annual net ecosystem production (NEP) of 530 g C m⁻². Throughout the study period, NEP was greater during the wet seasons than the dry seasons, primarily driven by elevated photosynthetically active radiation (PAR). Machine learning identified PAR as the most influential environmental driver of seasonal NEP differences, with its positive effect being significantly stronger in wet seasons compared to dry seasons (p < 0.01). Air temperature (TA), soil temperature (TS), and soil water content (SWC) were also key drivers of NEP. When TA and TS exceeded thresholds of 27.81°C and 27.06°C, respectively, NEP was negatively affected, although such conditions occurred in 21% of whole observation period. High SWC had a more pronounced inhibitory effect on NEP during dry seasons, potentially because of reduced soil salinity impairing photosynthetic efficiency. As mangroves evolved, NEP's sensitivity to PAR and TA increased, while its sensitivity to TS and SWC was reduced. This study enhances our understanding of seasonal carbon fluxes and their interactions with environmental drivers in the northernmost restored mangrove ecosystem.