Journal of Geophysical Research: Biogeosciences最新文献

Recent advancements in remote sensing imagery classification have greatly improved monitoring of land use/land cover (LULC) dynamics, deepening our understanding of their effects on ecosystems and terrestrial nutrient cycling. Forecasting LULC change remains challenging because it is strongly influenced by socioeconomic drivers and biogeochemical processes linked to land management and climate change. To address this complexity, a wide range of models has been developed, from process-based to statistical approaches. Yet, comparisons at regional and global scales reveal large discrepancies, underscoring the need for more consistent calibration and validation with historical observations. Here, we leverage the increasing availability of annual LULC maps to evaluate the temporal performance of two independent data-driven approaches: ARIMA time-series forecasting and a deterministic Lotka–Volterra ecological-inspired model, across the Río de la Plata Grasslands, a threatened South American ecosystem. Both methods outperformed memoryless Markov chain models in capturing annual LULC transitions without requiring time-consuming processing spatial inputs. These results demonstrate that incorporating long-term annual LULC histories can substantially improve predictive skill and provide a robust framework for model intercomparison, with clear implications for linking land-cover change to ecosystem and Earth system modeling.

Reliable paleotemperature proxies are essential for reconstructing past climate. To refine interpretation of the MBT'_5ME index, based on bacterial brGDGT lipids, a year-long study was conducted in Rotsee, Switzerland, a seasonally stratified lake with a 4–21°C temperature range. Suspended particulate matter was collected monthly from the epilimnion and the hypolimnion, complemented by surface sediments and surrounding soils. Both intact polar (IPL) and core lipid (CL) brGDGTs were analyzed alongside 16S rRNA gene data to disentangle environmental (temperature, dissolved oxygen, and pH) and biological (microbial community) controls on brGDGT compositions. In the stratified epilimnion, MBT'_5ME values showed a muted response to summer warming (r = 0.59, p < 0.1), whereas the isomer ratio (IR) correlated more strongly with temperature (r = 0.68, p < 0.05). MBT'_5ME and IR were also significantly correlated (r = 0.93, p < 0.0001), providing a novel diagnostic tool to identify sedimentary GDGTs derived from surface waters. In the seasonally anoxic hypolimnion, MBT'_5ME correlated with pH (r = 0.79, p < 0.01) and IR with dissolved oxygen (r = −0.65 and p < 0.05). Microbial DNA analysis revealed low Acidobacterial abundances (<0.4% of reads), suggesting MBT'_5ME patterns are not solely driven by this phylum. Instead, hypolimnion IPL-brGDGTs correlated with gene abundance of several other bacteria, indicating broader microbial contributions. Surface sediments reflected an integrated water column signal, while also showing evidence for additional in situ IPL-brGDGT production. Overall, findings demonstrate that stratification onset drives MBT'_5ME variability, while epilimnion temperature exerts stronger control on IR, refining their application in paleoclimate proxies.

Wetlandscapes—networks of hydrologically connected wetlands—can influence the transport and transformation of nutrients across watersheds. As climate change and human activity reshape wetland extent and connectivity, these landscape-scale processes are being altered in ways that may intensify eutrophication in downstream lakes. We used Landsat-derived inundation data (1984–2020) to evaluate how long-term changes in wetlandscape properties have affected nutrient loading and phytoplankton bloom dynamics in Lake Winnipeg, Canada. Over this period, wetlands generally increased in number and size and exhibited greater connectivity to rivers and the lake but with declines observed after ∼2015. These changes coincided with periods of substantial increases in the magnitude and spatial extents of phytoplankton blooms followed by declines in 2015 in the North Basin. Sub-watersheds with shorter runoff travel distances to the lake showed stronger relationships between wetland connectivity and bloom metrics (p ≤ 0.10), suggesting reduced opportunity for nutrient retention and transformation. Incorporating runoff travel distance into wetlandscape assessments improved correlations with nutrient inputs and bloom extent. Rising surface temperatures further contributed to bloom intensification. These findings highlight how climate-driven changes in wetland connectivity influence lake nutrient dynamics and demonstrate the potential for globally available satellite data to support spatially targeted water quality management.

Crown-group eukaryotes diversified rapidly in the late Mesoproterozoic (ca. 1.1–1.0 Ga), but their evolution was slowed in the early Neoproterozoic (ca. 1.0–0.8 Ga), with a significant episode of diversification occurring again around 0.8 Ga. Previous studies suggest nutrient (nitrogen and phosphorus) scarcity in the seawater may have delayed eukaryotic evolution during this period. However, the influence of marine redox conditions on the evolution, either directly or indirectly, remains unclear. Applying integrated approaches of sedimentology, mineralogy and geochemistry, we analyze the shallow marine carbonates from the Zhaowei, Niyuan, and Jiudingshan formations of the Huaibei Group (ca. 1.0–0.95 Ga) in the southeastern North China to constrain the redox conditions of the early Neoproterozoic seawater. Microscopic observations revealed abundant water-column carbonate mud (“whiting”) precipitates in these formations, indicating moderately oxygenated surface waters. Geochemical results show that the I/(Ca + Mg) values of the carbonates are mostly (98%) below the Precambrian background baseline (0.5 μmol/mol), without significant negative Ce anomaly (0.82 ± 0.06, n = 44). These findings support low-oxygen conditions in the shallow waters. The combination of low oxygen conditions and nutrient scarcity likely delayed the Neoproterozoic diversification of early crown-group eukaryotes. This study provides valuable insights into marine redox conditions in Earth's middle age and their potential impact on early eukaryotic evolution.

Rapidly melting Arctic glaciers deliver increasing amounts of allochthonous material to the coastal ocean, altering carbon cycling and promoting heterotrophy. As key factors influencing the activity of heterotrophic microbes, the quantity and quality of Arctic coastal organic carbon warrant closer examination. We investigated the molecular composition of dissolved organic matter (DOM) in two rivers and surface waters of Young Sound, NE Greenland—a high Arctic fjord where glacial runoff contributes to low primary productivity and increasing heterotrophy. Using ultra-high-performance liquid chromatography–tandem mass spectrometry (UHPLC-qTOF-MS), we conducted a non-targeted analysis of solid-phase extracted DOM (SPE-DOM). We expected DOM composition to differ between the two studied rivers (Tyroler and Zackenberg), which contrast in length, glacial water source, and catchment characteristics, and to reflect the salinity gradient in fjord waters. Both rivers carried a strong glacial imprint, with DOM enriched in aliphatic constituents typically associated with higher bioavailability, yet the proglacial Zackenberg River also exhibited unique compositional features that were more unsaturated and aromatic in character. Comparisons along a salinity gradient, from river plumes to outer fjord and open sea, revealed limited contrasts beyond the most glacially influenced section, with SPE-DOM composition showing high similarity across sites. Although multiple factors may contribute to this similarity, dilution and rapid processing of glacially derived DOM are likely to play a role. While further research is needed to understand carbon cycling in high Arctic fjords, our findings offer relevant insight into the molecular characteristics and potential ecological roles of DOM in this environment.

Knowledge of the distribution of soil thermal properties is important for understanding subsurface hydrological and biogeochemical processes. This study describes and evaluates quick thermal profiling (QTP), a new measurement technique aimed at providing rapid, depth-resolved measurements of soil thermal inertia at numerous locations across the landscape. A cylindrical probe with temperature sensors at multiple depths is quickly inserted into the ground, and soil thermal inertia is estimated from how quickly the probe temperature equilibrates with the soil. To this end, a finite volume heat transfer model is used to generate temperature equilibration time series across combinations of controlling factors, and a gridded search inversion approach is applied to infer soil thermal inertia. Field tests in the Arctic indicate that QTP measurements have a minimum uncertainty of 0.14 J m⁻² K⁻¹ s^−1/2 and covary with dual-probe heat pulse thermal analyzer measurements (concordance correlation coefficient = 0.56) with a root-mean-square error of 0.40 J m⁻² K⁻¹ s^−1/2. Besides demonstrating the value of QTP for estimating thermal inertia, this study identifies various sources of measurement uncertainty, particularly probe-soil contact resistance and frictional heating. Further, analysis of soil samples indicates that thermal inertia can be used to estimate thermal conductivity and dry bulk density in the studied area, although such inferences are highly site-specific. Overall, the QTP method holds promise to generate thermal inertia data products and to complement other characterization approaches for advancing understanding of soil properties across far more locations than is currently possible.

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.