Journal of Geophysical Research: Biogeosciences最新文献

Carbon evasion from urban river networks becomes increasingly significant as urbanization accelerates. However, there remains a limited understanding of the overall carbon emission impact integrating CO₂ and CH₄ dynamics, particularly in response to ecological restoration efforts. In this study, we investigated patterns of fluvial CO₂ and CH₄ diffusive fluxes across an urban river network in Wuxi, China. Our results reveal that water quality variables, especially dissolved oxygen (DO) and phosphorus content, predominantly influence the variability of carbon emissions. These factors exhibit a stronger correlation with CO₂ emissions compared to CH₄, indicating a net increase in carbon emissions as water quality deteriorates. Seasonally, higher water temperatures, phosphate levels, and lower DO concentrations lead to increased carbon emissions during summer months. Spatially, areas with lower carbon emissions (averaged 86 mmol m⁻² d⁻¹ CO₂ and 0.13 mmol m⁻² d⁻¹ CH₄) are primarily situated near the lake and in river sections where significant water quality improvements have been achieved through ecological restoration efforts. Cluster analysis shows that over 60% of high-carbon emission (averaged 162 mmol m⁻² d⁻¹ CO₂ and 1.21 mmol m⁻² d⁻¹ CH₄) sites in the study area have undergone ecological restoration, suggesting potential for further carbon emission reduction through enhanced restoration practices. Our findings underscore the importance of implementing carbon reduction strategies such as nutrient removal and aeration for oxygenation within water ecological restoration initiatives. Effective matching of restoration strategies holds further potential for mitigating carbon emissions from urban river networks.

Elucidating the climate feedback due to forest cover loss is critical for a comprehensive understanding of the role of forests in mitigating climate change. Current research studies predominantly focus on the impacts of permanent forest conversion, often overlooking the effects of recurrent disturbances such as fire and harvest. This study addresses this gap by examining the impact of forest cover loss caused by two distinct drivers in China over the period 2003–2020. Our analysis revealed that fire-induced forest cover loss accounted for approximately 10% of total forest cover loss in China. The immediate (i.e., 1 year after disturbance) changes in land surface temperature (ΔLST) due to fire were higher (ΔLST = 0.11°C, interquartile range (IQR): [−0.02°C–0.23°C]) compared to those caused by harvest (ΔLST = 0.04°C, IQR: [−0.01°C–0.09°C]). This finding highlights the immediate warming effect of fire-induced forest cover loss, was about triple as large as that caused by harvest. Our analysis also found that the warming effect post-fire gradually lessened but still maintained approximately 0.02°C 5 years later. Change in evapotranspiration is a primary factor influencing surface temperature changes following forest disturbances. Our study provides comprehensive insights into the differential and persistent effects of LST responses to fire and harvest, underscoring the importance of understanding the climate feedback from forest dynamics from different drivers.

Afforestation represents an effective approach for ecosystem restoration and carbon (C) sequestration. Nonetheless, it poses notable challenges concerning water depletion and soil drought in (semi)arid regions. The underlying mechanisms regulating the influence of afforestation on soil carbon-water dynamics, particularly how deep soil C reacts to afforestation-induced soil drying, remain largely unclear. This study examined the variations of soil water content (SWC), soil organic carbon (SOC), and soil inorganic carbon (SIC) in 500 cm depth along four afforestation stages: abandoned grasslands, shrublands, and 20-year and 40-year Robinia pseudoacacia forests (RP20 and RP40) in the semiarid Loess Plateau, China. The results indicated that afforestation has significantly increased SWC (+26.6%), SOC (+44.5%), and SIC (+6.5%) in the shallow layer (0–100 cm) but caused evident soil drying (−60.8%), decrease in SOC (−37.8%), and slight reduction in SIC (−0.3%) in the deep layer (300–500 cm) when compared with grasslands. The seriously decline in the coupling coordination between soil C and SWC in the middle and deep layers indicates the unsustainability of afforestation especially for RP40. Structural equation model showed that the negative impact of afforestation on deep SOC through soil water depletion (−0.38) outweighed the direct positive impact of increased aboveground biomass (AGB) (+0.33). The negative impacts of decreased SWC and increased pH on deep SIC was close to the positive impacts of AGB. Afforestation has different effects on SOC and SIC across shallow and deep layers, and its negative effects on deep soil C should be fully integrated into future forest ecosystem restoration and management efforts.

Direct comparison with in situ measurements serves as the primary method for validating TROPOMI SIF products. However, due to geometric errors in satellite data, the exact spatial extent of the nominal validation pixel may not align with the in situ site perfectly. In addressing this challenge, this study proposed, for the first time, a method to precisely identify the validation pixels matching with in situ sites. Moreover, the accuracy of the TROPOMI SIF product was reevaluated with the improved geolocation match method between the satellite pixel and the corresponding in situ site. The results indicate that ignoring the geometric errors of TROPOMI pixels can result in a 49% probability of mismatch between the validation pixel and the in situ site. The errors caused by geolocation mismatch mainly come from two aspects. One is the incorrect extraction of the validation pixel, with a maximum error of 1.385 mWm⁻² sr⁻¹ nm⁻¹. The other is the pixel-scale reference “truth,” which resulted from the improper upscaling function of in situ measurements, and the maximum of this kind of error was 0.445 mWm⁻² sr⁻¹ nm⁻¹. With this improved geolocation match method, the TROPOMI SIF product showed a RMSE of 0.58 mWm⁻² sr⁻¹ nm⁻¹, a bias of 0.19 mWm⁻² sr⁻¹ nm⁻¹, and a R² of 0.51, which indicate a better performance than without considering geometric location matching errors.

Marine sediments bury ∼160 Tg of organic carbon (OC) annually and represent an essential component of the global carbon cycle. OC burial is inherently multifactorial; however, in the past decade, the role of iron in regulating OC burial via the formation of organo-mineral associations, known as “rusty carbon sink,” has been extensively studied. Despite widespread recognition, the origin of the OC preserved within these associations and the effect of the bottom-water oxygenation on their stability are still debated. Here, we investigate the rusty carbon sink in sediments collected across transects from the head to mouth of three Swedish fjords presenting contrasting bottom-water oxygenation regimes (the oxic Hake fjord, seasonally hypoxic Gullmar fjord, and anoxic By fjord). We found that the oxygenation regimes, the intensity of benthic iron cycling or the OC origin have little to no influence on the amount of OC bound to Fe (OC – Fe). The lack of correlation with any of the parameters studied, in combination with an increase in the OC – Fe in the fjords with riverine input suggest, at least partially, an allochthonous origin of these organo-mineral associations. Our results also show that the rusty carbon sink plays a modest role in the OC burial in these fjords (∼6% OC is bound to Fe). While these fjords still represent important OC burial hotspots with an average of ∼35 g C m⁻² buried annually, the OC burial is controlled by other sedimentary processes, such as the high mass accumulation rates found in these fjord systems.

Soil carbon decomposition is primarily driven by microbial activities and is regulated by factors which stimulate or impede microbial functions. Deep podzolized carbon (DPC), found in the United States Southeastern Coastal Plain, is situated well below the soil surface in horizons isolated from active plant input. This carbon is characterized by high C:N ratios (>30) which could reflect nutrient limitation of microbial decomposition. To uncover the energy or nutrient limitation on DPC degradation, a 90-day priming experiment was performed with soils from the surface horizon and DPC horizons (i.e., Bh1 and Bh2) received the additions of ¹³C-labeled alanine and glucose. This resulted in prominent priming effects: addition of alanine increased basal decomposition of soil organic carbon by 918 ± 51% and 737 ± 7% in Bh2 and Bh1, respectively. Glucose relative priming was 505 ± 28% in Bh1 and 606 ± 77% of basal respiration in Bh2. These strong responses to substrate input highlight the susceptibility of DPC to loss when microbial carbon and nutrient constraints are alleviated. After 90 days, glucose addition increased the microbial biomass in DPC horizons relative to alanine addition, with the latter showing no difference from ultrapure-water control. The response of the microbial biomass indicates constraint by a lack of energy sources both by the paucity of labile substrates and reduced availability of organic matter as a result of podzolization. Our study has important implications for predicting the response of DPC in Coastal Plain soils in the context of land management and global change.

The fate of organic carbon (OC) in most river-dominated ocean margins (RiOMars) has undergone a noticeable transformation with the increased sediment retention engineering in watersheds. In the East China Sea (ECS), transformation in sediment and the influence of bulk OC have been broadly studied. However, the response of different mechanisms of OC protection under transformation has not been investigated, hindering our understanding of the factors that control OC deposition. In this study, we isolated different OC fractions, analyzed the basic parameters of the sediments, and compared the previous study's data to reveal how OC deposition responded to transformation. Our research indicates that transformation leads to the reduction of OC associated with minerals and sorting of OC occluded by plant debris and OC associated with minerals resulting in increased decomposition and mineralization of OC. The transformation affects the mechanism of OC binding with reactive iron (Fe_R), increasing Fe_R-protected OC content. Still, the co-precipitation mechanism and the intense redox environment in the mud deposit decrease the Fe_R-protected OC stability. Taken together, the impact of transformation is to increase the risk of OC decomposition and to weaken the OC preservation ability in RiOMars as carbon sinks. This study has implications for river-dominated passive margins subject to increased sediment retention engineering in watersheds worldwide and deserves more attention.

Coastal wetlands play a significant role in the storage of “blue carbon,” indicating their importance in the carbon biogeochemistry in the coastal zone and in global climate change mitigation strategies. We present airborne eddy covariance observations of CO₂ and CH₄ fluxes collected in southern Florida as part of the NASA BlueFlux mission during April 2022, October 2022, February 2023, and April 2023. The flux data generated from this mission consists of over 100 flight hours and more than 6,000 km of horizontal distance over coastal saline and freshwater wetlands. We find that the spatial and temporal heterogeneity in CO₂ and CH₄ exchange is primarily influenced by season, vegetation type, ecosystem productivity, and soil inundation. The largest CO₂ uptake fluxes of more than 20 μmol m⁻² s⁻¹ were observed over mangroves during all deployments and over swamp forests during flights in April. The greatest CH₄ effluxes of more than 250 nmol m⁻² s⁻¹ were measured at the end of the wet season in October 2022 over freshwater marshes and swamp shrublands. Although the combined Everglades National Park and Big Cypress National Preserve region was a net sink for carbon, CH₄ emissions reduced the ecosystem carbon uptake capacity (net CO₂ exchange rates) by 11%–91%. Average total net carbon exchange rates during the flight periods were −4 to −0.2 g CO₂-eq m⁻² d⁻¹. Our results highlight the importance of preserving mangrove forests and point to potential avenues of further research for greenhouse gas mitigation strategies.

This study evaluates global radar-derived digital elevation models (DEMs), namely the Shuttle Radar Topography Mission (SRTM), NASADEM and GLO-30 DEMs. We evaluate their accuracy over bare-earth terrain and characterize elevation biases induced by forests using global Lidar measurements from the Ice, Cloud, and Land Elevation Satellite (ICESat)'s Geoscience Laser Altimeter System (GLAS), the Global Ecosystem Dynamics Investigation (GEDI) and the ICESat-2 Advanced Topographic Laser Altimeter System (ATLAS) instruments collected on locally flat terrain. Our analysis is based on error statistics calculated for each $��1�°�\times �1�°�$ DEM tile, which are then summarized as global error percentiles, providing a regional characterization of DEM quality. We find NASADEM to be a significant improvement upon the SRTM V3. Over bare ground areas, the mean elevation bias and root mean square error (RMSE) improved from 0.68 to 2.50 m respectively to 0.00 and 1.5 m as compared to ICESat/GLAS. GLO-30 is more accurate with bare ground elevation bias and RMSE were below 0.05 and 0.55 m. Similar improvements were observed when compared to GEDI and ICESat-2 measurements. The DEM biases associated with the presence of vegetation vary linearly with canopy height, and more closely follow the $��5�0^{�t�h�}�$ percentile of Lidar Relative Height (RH50). Other factors such as canopy density, radar frequency and Lidar technology also contribute to observed elevation biases. This global analysis highlights the potential of various technologies for mapping of Earth's topography, and the need for more advanced remote sensing observations that can resolve vegetation structure and sub-canopy ground elevation.

We investigate the influence of organo-mineral associations on the dispersal of terrestrial organic matter (TerrOM) along a land-sea transect offshore the Atchafalaya river in the northern Gulf of Mexico (GoM). We analyzed bulk sediment properties, mineral surface area, and clay composition and used lipid biomarkers to distinguish plant-derived (long-chain n-alkanes and n-alcohols) freshwater aquatic (C₃₂ 1,15-diols) and soil-microbial (branched glycerol dialkyl glycerol tetraethers (brGDGTs)) OM-pools in different grain size fractions (≥250, 250–125, 125–63, 63–30, 30–10 and <10 μm) of marine surface sediments. Concentrations and mineral loadings of the targeted biomarkers were highest in the <30 μm fractions, suggesting an affinity with clay minerals. Spatially, concentrations of higher plant-derived n-alkanes remained relatively constant along the transect, whereas those of the other OM-pools rapidly decreased further offshore. This suggests that the association of plant-derived OM with mineral surfaces is better maintained than that of freshwater and soil-microbial OM. In addition, similar distributions among grain size fractions at each site for the C₃₂ 1,15-diols and brGDGTs suggest that these compounds are likely not associated with mineral surfaces in the marine realm. Furthermore, as TerrOM might be stripped from mineral surfaces upon discharge in the marine realm, the dispersal of TerrOM-pools could also represent a degradation signal, as n-alkanes are more resistant than long-chain diols and brGDGTs. Together, our results indicate that the stability of organo-mineral associations in the marine realm differs per TerrOM-pool and can lead to a differential dispersal of these pools, and thus OM sequestration patterns in the northern GoM.