Journal of Geophysical Research: Biogeosciences最新文献

Methylmercury (MeHg) in coastal systems has drawn extensive concerns because of its biomagnification through food chain and great threats to human health. Although terrestrial regions bordering the East China Sea (ECS) rank among the “hot spots” for Hg emissions in the world, there is still a lack of knowledge on the dominant sinks/sources, and controlling factors for MeHg in this system, restricting the understanding of its long-term risks. In this study, MeHg and relevant parameters in various media (i.e., seawater, sediment, overlying water above ∼10 cm of surface sediment, and porewater) of the ECS were measured, while MeHg mass budget in the ECS was established by calculating the in situ MeHg degradation/production and external MeHg output/input fluxes. The results showed that MeHg contents in ECS seawater and sediment were relatively higher compared with other coastal systems. The mass budget estimation indicated that the degradation in the seawater and net production in the sediment are the dominant MeHg sink and source in the ECS, a coastal region significantly influenced by a large river (the Yangtze River). Additionally, approximately 10 ± 7 tons of MeHg were exported from the ECS to the Northwestern Pacific Ocean (NWPO) per year, accounting for ∼10% of the MeHg inventory in upper 500 m seawater in the NWPO. This study highlights that in situ processes dominate MeHg cycling in coastal seas even for those significantly affected by large rivers, emphasizing the need for long-term monitoring of Hg methylation/demethylation to assess Minamata Convention's effectiveness.

Mercury (Hg), in the form of methylmercury (MeHg), is a neurotoxin that bioaccumulates in marine organisms and biomagnifies up the food chain. Understanding its behavior in coastal ecosystems is important, especially in fishing grounds such as the Gulf of Maine (GoM). Elevated inputs of terrestrial organic matter (OM) have been observed in the GoM, driven by increased watershed runoff from climate and land use changes. OM is an important vector for Hg transport in rivers, and OM character and concentration can influence the methylation and bioavailability of MeHg. To evaluate how increased watershed inputs would impact the GoM, the relationship between MeHg, Hg, OM quality and quantity, and other water parameters was assessed between April 2023–2024 during four cruises in the GoM and a major tributary, the historically Hg-contaminated Penobscot River, as well as during two trips further upstream. OM quality was evaluated via fluorescence spectrophotometry and a six-component parallel factor analysis (PARAFAC) model identified two marine and four terrestrial OM components. Terrestrial OM proxies were strong predictors of, and positively correlated with, MeHg in the Penobscot River/Estuary, suggesting that co-transport of MeHg and OM outweighs terrestrial OM's attenuation of in situ methylation in the water column. We concluded however that the Penobscot River was not an important source of Hg to the GoM. Rather, a seasonally consistent hotspot of MeHg at depth in Jordan Basin was identified, correlating with elevated apparent oxygen utilization, nitrate, and other proxies for OM degradation suggesting its formation in situ in the deeper waters.

Volatile sulfur compounds (VSCs) play a crucial role in regulating global climate change and atmospheric environment. However, the impact of nitrogen input on sulfur cycling in coastal wetland sediments remains unclear. Here, we conducted in situ experiments to explore the impact of inorganic nitrogen on VSC emissions from a salt marsh wetland in Jiaozhou Bay, China along with comparison between Suaeda glauca and mudflat sediment across different seasons. In autumn, we found that the emissions of dimethyl sulfide (DMS) and carbon disulfide (CS₂) from sediments covered by S. glauca (DMS and CS₂ fluxes: 29.86 ± 2.23 and 9.94 ± 2.00 nmol m⁻² s⁻¹) were significantly higher compared to the mudflat sediment (DMS and CS₂ fluxes: −2.94 ± 0.90 and 7.56 ± 3.07 nmol m⁻² s⁻¹). Conversely, carbonyl sulfide (COS) fluxes were generally lower in S. glauca area (10.5 ± 8.32 nmol m⁻² s⁻¹) than in mudflat area (23.2 ± 3.58 nmol m⁻² s⁻¹). In spring, DMS, CS₂, and COS fluxes in S. glauca area were all lower than those in mudflat area. Moreover, adding nitrogen at soil levels typically led to reduced VSC emissions from S. glauca area in autumn, while, in spring, it increased the emissions of DMS, CS₂, and COS from mudflat sediments by 221.5%, 243.8%, and 375.0%, respectively. These variations in flux were primarily influenced by microbial activity, sediment temperature, and sulfate levels. In summary, these findings enhance our understanding of soil sulfur cycling in the context of future nitrogen deposition scenarios in coastal wetlands.

Warming and elevated atmospheric CO₂ profoundly impact peatland ecosystems, particularly through changes in plant species composition. Plants regulate the initial input of organic compounds to peatland belowground systems, controlling the availability of electron donors and electron acceptors that fuel microbially mediated organic matter decomposition to CO₂ and CH₄. However, explicit links between porewater CO₂ and CH₄ dynamics and plant-derived chemical compounds remain relatively undefined. In a whole ecosystem warming experiment, we investigated how warming affects plant leaf chemical composition and species assemblages, and how the alteration of leaf-derived organic compounds supplied to the subsurface impacts belowground CO₂ and CH₄ production. While earlier studies at our site found no temperature-dependent changes in CH₄ production pathways, our extended timeseries has revealed increased acetoclastic methanogenesis at higher temperatures in certain peat depths, correlated with elevated porewater phenolics. These changes appear driven by the observed increased plant productivity and altered vegetation inputs, which accelerate decomposition and fuel CH₄ production through enhanced substrate availability. We observed warming-induced changes in molecular composition both between and within plant species, suggesting that plant-mediated controls on belowground carbon processing are more complex than previously recognized.

The Greater Cape Floristic Region (GCFR) of South Africa is globally recognized for its exceptional plant diversity and endemism, yet faces mounting threats from habitat loss, altered fire regimes, and invasive species. Fire is a key ecological driver in the Fynbos (shrubland) biome of the GCFR, influencing vegetation structure, composition, and nutrient cycling. Understanding the dynamics of Fynbos sites in terms of the time since last burn is a key aspect of understanding its ecology, as it helps reveal post-fire succession stages and vegetation recovery patterns. Although satellite remote sensing supports biodiversity monitoring, its relatively coarse resolution limits its utility in capturing fine-scale vegetation dynamics. To address this, we employed high-resolution unmanned aerial system (UAS) multispectral imagery to classify vegetation of different post-fire ages and map species diversity in the Fynbos biome. Our methodology, grounded in the Spectral Variation Hypothesis (SVH), leverages optimal spectral and textural features derived from UAS imagery to distinguish between vegetation of different post-fire ages and estimate alpha diversity at fine scales within Fynbos. We used sequential feature selection (SFS) to identify key predictors, achieving high classification performance with a support vector machine (SVM) classifier (overall accuracy: 97%; F1 score: 97.47%). We employed a similarity metric, Euclidean distance to map alpha diversity across vegetation of different post-fire ages within the Fynbos biome, by utilizing optimal features and the Shannon diversity index from ground truth samples. This study highlights the role of advanced remote sensing and ecological research in supporting biodiversity monitoring in regions like the GCFR.

Salt marshes are ecologically and socio-economically valuable intertidal ecosystems that have suffered rapid losses worldwide. As a blue carbon ecosystem (BCE), they are efficient sinks and long-term reservoirs of organic carbon (OC), with vital roles in the global carbon cycle. To accurately characterize salt marsh carbon sequestration dynamics, knowledge of halophyte below- and aboveground biomass (BGB, AGB) and their relation to the subsurface organic carbon content (OC) is fundamental, yet only vaguely defined. Herein, the biomass and subsoil dry bulk density (DBD), organic matter (OM), and OC of plots pertaining to seven different halophyte species were analyzed. The volume and partitioning of biomass were found to be species-specific. The difference between the mean AGB in the lower marsh (425 ± 217 g m⁻²) and the upper marsh (1,616 ± 807 g m⁻²) was highly statistically significant. BGB, OM, and OC values peaked in the middle marsh zone (5,890 g m⁻²; 14.5%; 5.4%). DBD was negatively correlated with OM. The BGB, BGB/AGB ratio, OM and OC all had their maxima coinciding with Sea Lavender. As a nexus of unique attributes, Sea Lavender has remarkable potential as a keystone halophyte capable of enhancing and maintaining blue carbon reservoirs. Overall, these results affirm that the observed distribution of biomass and OC is an artifact of biogeomorphic feedbacks where phytomorphic and physiologic traits interplay with underlying soil chemistry, sediment supply, geomorphology, hydrodynamics and external forcings. This data set elucidates factors underpinning interdisciplinary frameworks focused on intertidal wetlands and the intertwined mechanisms that dictate salt marsh evolutionary trajectories.

The vegetation greenness rate (GR: green-up rate, SR: senescence rate) quantifies canopy development, reflecting the acceleration and deceleration of photosynthesis during the growing season. However, long-term changes and climatic drivers of natural vegetation greenness rates remain poorly understood globally. Here, utilizing multi-source remote sensing and climatic data sets, we examined trends in the greenness rate of global natural vegetation over the past four decades, identified primary climatic drivers, and evaluated their sensitivities. The results reveal significant global changes in greenness rates. Notably, the GR increased significantly in 24.1% of pixels, while the SR rose in 23.9% of pixels (P < 0.05). Conversely, GR and SR experienced significant decreases in 13.4% and 15.1% of pixels, respectively. Temperature was the primary driver of GR changes in 32.9% of pixels worldwide. A higher accumulated temperature rate during the green-up phase generally accelerated GR, enhancing vegetation greenness at maturity onset. Similarly, temperature influenced SR in 28.6% of pixels; however, a higher accumulated temperature rate during the senescence phase typically slowed SR, delaying dormancy onset. In contrast, moisture-related factors, solar radiation, and VPD exhibited strong regional influences, with precipitation and soil moisture exerting particularly positive effects in drylands. Additionally, advancing or delaying green-up onset significantly decreased or increased GR, subsequently affecting greenness amplitude, the senescence rate (SR), and autumn phenology. Our study highlights that the rate of vegetation greenness is a critical transitional variable linking phenological timing and vegetation productivity, and a robust indicator in assessing the climate change impacts on Earth's terrestrial ecosystems.

The erodibility of cohesive sediment is known to vary both spatially and temporally but the factors governing its variation are not well understood. We conducted a field investigation of the influence of hydrodynamic forcing, sediment properties, and benthic infauna on erodibility in the muddy shallows of San Pablo and Grizzly Bays in northern San Francisco Bay in summer 2019 and winter 2020. An erosion rate parameter $��M_c�$ was determined from regressions between near-bed vertical turbulent sediment flux, as a proxy for erosion, and bed shear stress due to currents. During each 2-month study period, we measured benthic infauna abundance and dry bulk density, particle size distribution, percent organic carbon, chlorophyll a, pheophytin a, and carbohydrates carbon concentrations of surficial bed sediments five or six times. $��M_c�$ increased with bed shear stress due to waves in both embayments. In San Pablo Bay, erodibility was approximately 50% lower during the winter than the summer deployment, whereas in Grizzly Bay, there was no significant difference. The factor most strongly related to the decrease in $��M_c�$ in San Pablo Bay was increased abundance of the amphipod Ampelisca abdita. The observed reduction in erodibility may occur in many muddy estuaries because A. abdita is broadly distributed in the coastal waters of North America. Erodibility was also directly related to biomass of the invasive clam Potamocorbula amurensis. Erodibility did not depend on dry bulk density: bulk density did not vary seasonally in San Pablo Bay and was lower in winter than summer in Grizzly Bay.

Fluvial export of dissolved carbon plays an important role in watershed-scale biogeochemistry. Predicted changes in climate are expected to impact watershed hydrologic regimes, and in turn, the sources and export of dissolved carbon from watersheds. Here, we utilize high resolution measurements of discharge and dissolved carbon concentration to examine how concentration-discharge (CQ) relationships vary seasonally and during high flow events over the main runoff season (May–October) in a temperate forested watershed in Southeast Alaska. Concentration-discharge relationships for dissolved organic carbon (DOC) and alkalinity demonstrated strong seasonal patterns, with more linear relationships in May and June versus other months. Changing power law model slopes (b values; the exponent in a power law regression between runoff and carbon yields) indicated potentially shifting watershed sources (biogenic vs. geologic) and contrasting dominant flowpaths (shallow vs. deeper groundwater) for DOC and alkalinity over the sampling period. During the largest storm event of the study, DOC and alkalinity b values shifted from an overall pattern of transport (mean b = 1.58 values >1.0 indicate transport limitation) and source limitation (mean b = 0.48, values <1.0 indicate source limitation) to chemostatic (DOC, b = 0.99; alkalinity, b = 1.019). In June through August, patterns in hysteresis index suggest that CQ relationships were altered when storms followed in close succession to each other. Together, these findings indicate that seasonal and antecedent flow conditions play a role in dissolved carbon export from forested watersheds. Understanding these dynamics, particularly during winter months, will become increasingly important as changes to hydroclimate impact riverine carbon export.

Crop phenology is a key indicator for assessing agroecosystem responses to environmental changes and informing adaptive management practices. Accurate detection of phenology dynamics is crucial for understanding agroecosystem function and structure at regional scales and supporting policy-making aimed at sustainable management. This study fused Landsat and MODIS observations with a newly developed Gap Filling and Savitzky-Golay filtering (GF-SG) method to derive 30 m major phenological metrics (planting, corn silking, soybean blooming, corn maturity, soybean dropping-leaves, and harvesting) in the US Midwest from 2000 to 2022. The 30 m metrics were evaluated against PhenoCam observations and ground surveys at field and state scales. Our results suggested that the growth seasons of corn (from planting to maturity date) and of soybeans (from planting to dropping-leaves date) were lengthened by 0.19 days year⁻¹ (p = 0.03) and 0.08 days year⁻¹ (p = 0.20), respectively. While planting dates historically advanced in response to rising temperatures, satellite observations indicated a recent trend toward delayed planting in certain regions, shifting more time into the reproductive period. Further analysis suggests that these delays were associated with increased cumulative pre-season precipitation (p < 0.0001 for corn and p < 0.0001 for soybeans). Our 30 m phenological metrics can strengthen yield gap analysis and regional agricultural assessments and provide science-based information to facilitate the development of tailored adaptation strategies.