Journal of Geophysical Research: Biogeosciences最新文献

Urban and peri-urban forests are vital for ecological services and urban quality of life, making tree growth responses to environmental stressors key for management. In the northern Qinling Mountains near Xi'an, the largest city in northwest China, rising levels of NO_x and PM_2.5 emissions from urban development have led to elevated nitrogen deposition. However, the impact of climate, pollution, and nitrogen deposition on nearby forests remains unclear. To address this, we used tree-ring width measurements (basal area increment, wood carbon isotopes (δ¹³C) and nitrogen isotopes (δ¹⁵N), and C/N ratios to examine the effects of climate and nitrogen emissions on tree growth and intrinsic water-use efficiency (iWUE) over the 20th and early 21st centuries. By comparing two broadleaf species (chestnut, Castanea mollissima; oak, Quercus aliena) with a conifer species (pine, Pinus tabulaeformis), we examined their long-term (1922–2018 for oak and pine; 1947–2018 for chestnut) growth patterns and physiological responses. We found radial growth increased in recent decades, especially in broadleaves. Chestnut and oak growth was driven by CO₂ and nitrogen fertilization effects as expressed by increasing iWUE and decreasing δ¹⁵N over the past century. Pine showed a continuous decline in δ¹⁵N and a weaker growth response to CO₂ and nitrogen fertilization. NO_x emissions significantly promoted the growth of all three species. Our results provide new insights into species-specific long-term growth responses of broadleaves and conifers to urban expansion, elucidating the complex carbon-nitrogen interactions under rapid environmental change.

Accurate forest age data is essential for carbon stock quantification, ecosystem monitoring, and sustainable forest management. However, significant inconsistencies persist among existing forest age products in China, undermining the reliable assessment of forest dynamics and evidence-based management strategies. Leveraging China's Ninth National Forest Inventory field data (2912 plots), we conducted a comparative analysis of five forest age data sets and evaluated the accuracy of fused forest age data set. Our findings reveal that 37.9% of forested areas show substantial discrepancies and 15.7% exhibit extreme discrepancies. Although forest cover in extremely high mountain regions is limited, data set inconsistencies are particularly pronounced. Additionally, areas of significant divergence are predominantly concentrated in mid and high mountainous regions, where major natural forests are distributed. Then we evaluated the performance of random forest (RF) regression algorithms and geographically weighted regression (GWR) in synthesizing existing forest age data sets for forest age estimation. The RF-based integration approach outperformed GWR by effectively synthesizing complementary strengths of existing data sets, achieving superior model accuracy, a 47.4% improvement of R², a 19.7% decrease of RMSE, and a 75.2% decrease of Bias over the best-performing of existing products. The integrated forest age map provides enhanced reliability for national-scale carbon budget modeling, while offering critical baseline data for optimizing forest management policies and carbon neutrality roadmaps.

Biological extracellular enzymes play a pivotal role in regulating biogeochemical cycling rates in wetland ecosystems. This study investigated the spatiotemporal variability, environmental factors, and potential associations of four key extracellular enzymes — β-glucosidase (BG), N-acetylglucosaminidase (NAG), urease (UE), and polyphenol oxidase (PPO) — with greenhouse gas (GHG) emissions including methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O) in sediments of typical intertidal wetlands in the Yangtze River Estuary. Among the three gases measured, only CH₄ showed significant correlations with enzyme activities (p < 0.05), while CO₂ and N₂O exhibited no significant relationships. Seasonal field sampling, environmental monitoring, and laboratory incubation experiments were conducted across multiple sites. The results showed that the activities of all enzymes followed the seasonal pattern of autumn > summer > winter. Significant seasonal differences were observed for BG, NAG, and UE (p < 0.05), whereas PPO showed no significant seasonal variation. Enzyme activities also varied markedly among sites, reflecting spatial heterogeneity driven by local environmental conditions. Among the measured factors, redundancy and correlation analyses identified pH, total organic carbon (TOC), and extractable sediment sulfate (SSO₄²⁻) as the primary physicochemical drivers significantly regulating sediment extracellular enzyme activities. Due to combined environmental and biogeochemical controls, a significant positive correlation (p < 0.05) occurred only between BG activity and CH₄ flux under non-flooded condition. Overall, this study provides new insights into the spatiotemporal patterns of extracellular enzyme activities in estuarine wetlands, and highlighting their role in regulating GHG production and emission.

Estuaries are threatened by eutrophication due to anthropogenic nutrient loading. The Great Bay Estuary of New Hampshire and Maine has been designated as nitrogen impaired primarily due to a 65% loss in seagrass coverage between 1996 and 2023. This region has also experienced multiple consecutive years of low annual precipitation and more recent record precipitation years. The loss of seagrass, increased precipitation variability, and continued anthropogenic influence have biogeochemical consequences for the estuarine filter. Water quality budgets for a subregion of Great Bay Estuary were developed at annual timescales for nitrogen, orthophosphate (PO₄³⁻), dissolved organic carbon (DOC), and total suspended solids (TSS). Results show annual total nitrogen (TN) input loads, including tidal flux into Great Bay, (3,900 kg ha⁻¹ year⁻¹) are less than output loads whereas dissolved inorganic nitrogen (DIN) inputs (1,410 kg ha⁻¹ year⁻¹) exceed outputs, indicating net TN export and net DIN retention. PO₄³⁻ loads were nearly balanced, with a mean input and output of 257 kg ha⁻¹ year⁻¹ and 238 kg ha⁻¹ year⁻¹, respectively. Seagrass acreage negatively correlated with DOC inputs (33,300 kg ha⁻¹ year⁻¹). High TSS output loads (227,000 kg ha⁻¹ year⁻¹) resulted in net TSS export. Despite critical seagrass loss and recent point source nitrogen load reductions, the system continues to retain bioavailable forms of nutrients, suggesting the importance of other primary producers to estuarine biogeochemical cycles. Water quality budgets for Great Bay provide useful insight into the biogeochemical capacity of an estuary during a time of habitat degradation and subsequent coastal management efforts.

As a result of drought and consumptive water use, reservoirs worldwide are experiencing declines in storage. The landscapes exposed by falling water levels may be colonized by novel vegetation communities in terms of species composition and succession, and/or reworked by geomorphic processes at sub-annual to decadal timescales. Despite research on the response of downstream rivers to dam and reservoir operations, we know surprisingly little regarding the eco-geomorphic processes that shape backwaters upstream of reservoirs. Here we use a decadal-scale record of aerial imagery and support vector machine learning to classify land cover change along more than 140 km of the Colorado and San Juan Rivers within Lake Powell Reservoir in Utah. Our results reveal a consistent trajectory of land cover change, wherein declining water levels expose bare sediment, which is subsequently colonized by vegetation. With continued water level declines, vegetation exhibits defoliation consistent with water stress. Similar patterns of landscape evolution were observed with regard to exposure age of formerly inundated surfaces on both rivers, wherein older surfaces were marked by bare sediment and/or stressed vegetation. Divergence in response to reservoir drawdown was observed with regard to the downstream propagation rate of land cover changes. Specifically, on the steep-and-narrow Colorado River, the trajectory of bare sediment to green vegetation to stressed vegetation occurred more rapidly as water levels declined than on the San Juan River. The observations detailed here may guide reservoir management to promote recruitment of desirable land cover types (e.g., native and/or complex ecosystems) under shifting water supply.

Urban carbon mitigation is increasingly urgent as global greenhouse gas emissions intensify. Post-industrial shrinking cities such as Detroit face unique challenges and opportunities with widespread vacancy yet potential for extensive greenspace expansion. Here, we investigate the seasonal controls on net ecosystem exchange (NEE) of carbon dioxide (CO₂) in the post-industrial shrinking city of Detroit, Michigan from August 2019 to June 2022. We combined eddy covariance measurements with a camera-based greenness index and satellite-based normalized difference vegetation index (NDVI) to parse daily and seasonal flux variability. We hypothesized that growing-season vegetation uptake partially offsets emissions but is overshadowed by dominant anthropogenic sources. Results show Detroit is a persistent net carbon source with dormant-season fluxes driven by fossil-fuel heating and industrial plumes far exceeding the modest carbon mitigation provided by urban greenspaces in summer. A 1% increase in greenspace coverage was associated with a median NEE decrease of 0.21 gCO₂ m⁻² d⁻¹. Feature importance analyses confirm that heating and traffic emissions are the primary drivers of daily flux variability. The COVID-19 restrictions led to only brief emissions dips highlighting persistent anthropogenic sources. Although urban reforestation can partially offset emissions in the growing season, our findings show that large wintertime fluxes from heating and industrial processes keep Detroit in net source territory. Consequently, sustained emission reductions beyond greenspace expansion are essential for achieving substantial CO₂ mitigation in post-industrial shrinking cities.

Dissolved organic matter (DOM) plays a vital role in lakes, but its behavior in winter is poorly understood. This study examined the differences in DOM between lake ice and the upper water column across 18 sites in the Laurentian Great Lakes, integrating in situ sampling and remotely sensed ice data to create a mass budget model to estimate basin-scale DOM storage and release from ice. We found that the composition of the DOM pool in ice varied based on ice thickness, water DOM composition, nutrients, and dissolved organic carbon concentrations. Calculations of protein-like, microbial humic-like, and terrestrial-like DOM storage in ice under different ice cover scenarios revealed considerable contributions to the upper water column following ice melt, especially for protein-like DOM which, during extensive ice cover years, contributed an average of 17.7% to the protein-like DOM pool in spring. This ice-derived DOM may be an important source of labile carbon for microbial communities, but projected reductions in winter ice cover and duration under climate change may alter DOM dynamics, potentially impacting this important spring carbon subsidy.

Buried paleosols can store large quantities of organic carbon (C), much of which persists for millennia due to isolation from surface processes that promote decomposition. Subsoil organic matter (SOM) persistence is often enhanced by mineral associations and ionic conditions—particularly high clay content and polyvalent cations—that limit microbial degradation and leaching. However, the vulnerability of these deep C stocks under erosion or environmental change remains poorly understood. This study investigates controls on SOM stabilization in the Brady paleosol and overlying modern soils across contrasting geomorphic settings in the Great Plains of Nebraska where Late Quaternary loess deposition and erosion created a sequence of buried and exposed paleosols. We sampled soils along burial and erosional toposequences and analyzed their physicochemical properties and radiocarbon-based persistence of occluded particulate organic matter (oPOM) and mineral fractions (MF). Brady paleosol showed greater persistence (lower Fm) of oPOM and MF than modern soils, particularly under burial. This was linked to higher silt and clay content, elevated electrical conductivity, and increased exchangeable calcium and magnesium content, supporting roles for organo-mineral interactions, flocculation, and carbonate cementation. In modern soils, SOM persistence and C content were more strongly tied to pH and cation exchange capacity. Erosional exposure reduced SOM stability and promoted geochemical convergence toward modern surface soils. These findings show that burial enhances SOM persistence via multiple stabilization mechanisms, while erosion increases subsoil C vulnerability. Our results underscore the importance of geomorphic and geochemical context in predicting soil C stability under environmental change.

Carbon dioxide (CO₂) evasion and downstream export of carbon (C) from headwater streams represent important fluxes in the global C cycle. Yet, these fluxes are generally studied in isolation, leaving gaps in the understanding of the overall role of streams in the C cycle. In this study, we carried out high resolution measurements of dissolved inorganic and organic C to estimate CO₂ evasion and C export along a 400 m reach of a boreal headwater stream to assess the magnitude and control of the C evasion:export ratio. Higher downstream C export (3.1–74.0 kg C d⁻¹) compared to CO₂ evasion rates (0.53–2.56 kg C d⁻¹) for the full stream network over the open water season resulted in an average C evasion:export ratio of 0.23, which corresponds to a 17% loss of C entering the stream through CO₂ evasion. The temporal variation in C evasion:export ratios (0.03–0.60) was mainly driven by stream discharge, largely through its strong influence on downstream C export. Further, CO₂ evasion showed high spatial variability, and we demonstrate that using only data of a subset of the stream reach would lead to a wide range in the overall C evasion:export ratios upscaled to the whole stream network. Resolving the processes controlling spatial and temporal variation in C fluxes and understanding the importance of discharge for the fate of C routed through streams is crucial for predicting the terrestrial C sink capacity at high latitudes under a changing climate.