Journal of Geophysical Research: Biogeosciences最新文献

Temperature and water stress are important factors limiting the gross primary productivity (GPP) in terrestrial ecosystems, yet the extent of their influence across ecosystems remains uncertain. This study examines how surface air temperature, soil water availability (SWA) and vapor pressure deficit (VPD) influence ecosystem light use efficiency (LUE), a critical metric for assessing GPP, across different ecosystems and climatic zones at 80 flux tower sites based on in situ measurements and data assimilation products. Results indicate that LUE increases with temperature in spring, with higher correlation coefficients in colder regions (0.79–0.82) than in warmer regions (0.68–0.78). LUE reaches a plateau earlier in the season in warmer regions. LUE variations in summer are mainly driven by SWA, exhibiting a positive correlation indicative of a water-limited regime. The relationship between the daily LUE and daytime temperature shows a clear seasonal hysteresis at many sites, with a higher LUE in spring than in fall under the same temperature, likely resulting from younger leaves being more efficient in photosynthesis. Drought stress influences LUE through SWA in all ranges of water availability; VPD variation under moderate conditions does not have a clear influence on LUE, but extremely high VPD (exceeding the threshold of 1.6 kPa, often observed during extreme drought-heat events) causes a dramatic reduction of LUE. Our findings provide insight into how ecosystem productivities respond to climate variability and how they may change under the influence of more frequent and severe heat and drought events projected for the future.

Svalbards permafrost is thawing as a direct consequence of climate change. In the Low Arctic, vegetation has been shown to slow down and reduce the active layer thaw, yet it is unknown whether this also applies to High Arctic regions like Svalbard where vegetation is smaller, sparser, and thus likely less able to insulate the soil. Therefore, it remains unknown which components of High Arctic vegetation impact active layer thaw and at which temporal scale this insulation could be effective. Such knowledge is necessary to predict and understand future changes in active layer in a changing Arctic. In this study we used frost tubes placed in study grids located in Svalbard with known vegetation composition, to monitor the progression of active layer thaw and analyze the relationship between vegetation composition, vegetation structure and snow conditions, and active layer thaw early in summer. We found that moss thickness, shrub and forb height, and vascular vegetation cover delayed soil thaw immediately after snow melt. These insulating effects attenuated as thaw progressed, until no effect on thaw depth was present after 8 weeks. High Arctic mosses are expected to decline due to climate change, which could lead to a loss in insulating capacity, potentially accelerating early summer active layer thaw. This may have important repercussions for a wide range of ecosystem functions such as plant phenology and decomposition processes.

Reservoirs are a significant source of carbon (C) to the atmosphere, but their emission rates vary in space and time. We compared C emissions via diffusive and ebullitive pathways at several stations in six large hydropower reservoirs in the southeastern US that were previously sampled in summer 2012. We found that carbon dioxide (CO₂) diffusion was the dominant flux pathway during 2012 and 2022, with only three exceptions where methane (CH₄) diffusion or CH₄ ebullition dominated. CH₄ diffusion rates were positively associated with water temperature. However, we found no clear predictors of CH₄ ebullition, which had extremely high variability, with rates ranging from 0 to 739 mg C m⁻² day⁻¹. For CO₂ diffusion, the direction of the flux shifted between 2012 and 2022, where all but three stations across all reservoirs emitted CO₂ in summer 2012, but every station sequestered CO₂ in summer 2022. Here, indicators of greater algal production were associated with CO₂ sequestration, including surface chlorophyll-a concentration, surface dissolved oxygen saturation, and pH. Additional sampling campaigns outside the summer season highlighted the importance of seasonal phenology in primary production on the direction of CO₂ diffusive fluxes, which shifted to positive CO₂ fluxes by the end of August as productivity decreased. Our results demonstrate the importance of capturing CO₂ sequestration in field and modeling measurements and understanding the seasonal drivers of these estimates. Measuring C emissions from multiple pathways in reservoirs and understanding their spatiotemporal responses and variability are vital to reducing uncertainties in global upscaling efforts.

Coastal erosion mobilizes large quantities of organic matter (OM) to the Arctic Ocean where it may fuel greenhouse gas emissions and marine production. While the biodegradability of permafrost-derived dissolved organic carbon (DOC) has been extensively studied in inland soils and freshwaters, few studies have examined dissolved OM (DOM) leached from eroding coastal permafrost in seawater. To address this knowledge gap, we sampled three horizons from bluff exposures near Drew Point, Alaska: seasonally thawed active layer soils, permafrost containing Holocene terrestrial and/or lacustrine OM, and permafrost containing late-Pleistocene marine-derived OM. Samples were leached in seawater to compare DOC yields, DOM composition (chromophoric DOM, Fourier transform ion cyclotron resonance mass spectrometry), and biodegradable DOC (BDOC). Holocene terrestrial permafrost leached the most DOC compared to active layer soils and Pleistocene marine permafrost. However, DOC from Pleistocene marine permafrost was the most biodegradable (33 ± 6% over 90 days), followed by DOC from active layer soils (23 ± 5%) and Holocene terrestrial permafrost (14 ± 3%). Permafrost leachates contained relatively more aliphatic and peptide-like formulae, whereas active layer leachates contained relatively more aromatic formulae. BDOC was positively correlated with nitrogen-containing and aliphatic formulae, and negatively correlated with polyphenolic and condensed aromatic formulae. Using estimates of eroding OM, we scale our results to estimate DOC and BDOC inputs to the Alaska Beaufort Sea. While DOC inputs from coastal erosion are relatively small compared to rivers, our results suggest that erosion may be an important source of BDOC to the Beaufort Sea when river inputs are low.

We examined data from the American Geophysical Union (AGU), the world's largest earth and space science society, to characterize demographics of multiple milestones in a biogeoscientists' career. Geoscientists of color and White women make up a smaller proportion of those participating in activities critical to transitioning from student to professional (submitting manuscripts, getting published, and receiving review invitations) in comparison to White men. Gender parity for biogeoscientists appears within reach at earlier career stages, with 37% of AGU Biogeosciences members and 41% of Biogeosciences attendees at the Fall Meeting identifying as women in 2020. The demographics of those successfully completing activities required for career advancement, that is, publishing and reviewing with Journal of Geophysical Research and Global Biogeochemical Cycles, two biogeoscience journals, illustrate more progress is needed. A large majority of manuscripts were submitted by men (73%), many of which have no co-authors that identify as women or non-binary geoscientists. Further, our communities' bias of who we suggest as reviewers, results in 85% of the reviewer invites going to White geoscientists and 63% going to men. Thus, while representation of diverse communities has improved in some areas, barriers to publishing result in authorship not reflective of society: only 25% of accepted manuscripts were led by self-identified geoscientists of color and fewer than 5% were led by women geoscientists of color. It is critical that efforts to diversify move beyond gender, to ensure that scientists of color are also afforded the access and opportunity needed to thrive as biogeoscientists.

Although the Arctic Ocean is small, its shallow shelves and highly productive coastal waters make it an important component of global biogeochemical cycling, especially of nitrogen (N). Because inorganic forms of dissolved N exist in so many different oxidation states, the cycling of N can be quite complex. In this review, we describe the current understanding of the major conduits bringing N into Arctic surface waters as well as the key physical and biological processes that are responsible for the transformation from one form of N to another. We also discuss the environmental factors that are currently controlling these transformations and how this may change in a future Arctic.

The hysteresis response of tree sap flux (SF) to its main driving factor of incoming short-wave radiation (Rsi) has been widely reported, affecting the accuracy of sap flux and transpiration estimates in forest ecosystems. The diurnal cycle of SF usually lags the Rsi cycle by certain hours, thereby generating a closed counterclockwise hysteresis pattern. However, a few studies have reported that diurnal SF cycle may advance Rsi cycle, and such a response pattern has not been fully explored. In this study, we reported a rarely seen crossed hysteresis response pattern of SF to Rsi in 1/3 trees of a young temperate pine forest. We found that the diurnal SF cycle advances Rsi cycle especially in the morning induced by the early stomatal closure, thereby generating the crossed hysteresis response of SF to Rsi. We also proposed a method to quantify the magnitude of hysteresis (A_hys) for both the crossed and closed hystereses. Our analysis suggests that a lower A_hys of two time series results in (a) a larger crossing degree of hysteresis, and (b) a stronger linear correlation between the two time series. The seasonal variation of soil water content can explain the variation in A_hys for the hysteresis response of SF to Rsi, and the crossed hysteresis of SF is more likely to occur under water stress conditions. This study contributes to advancing our understanding of forest transpiration and how forests may respond to drought stress, which are expected to become more frequent and longer under future climate change.

Nocturnal water use (Q_night) is an important component of the eucalyptus water budget, but it has always been under-appreciated and poorly understood. To improve the accuracy of water balance estimates and the understanding of the nocturnal water use process in eucalypts plantations, we conducted a 3-year study to investigate the characteristics of Q_night and its components in an E. urophylla × E. grandis plantation in southern China. The results showed that the Q_night of E. urophylla × E. grandis was substantial and the ratio of nocturnal to daily water use (R_night) was on average 12.35%, with higher R_night (14.97%) in the dry season than in the wet season (9.50%). The Q_night includes two components, nocturnal transpiration (Tn) and nocturnal refilling (Re), which are driven by different factors. Nocturnal Re-Tn dynamics were controlled by a combination of nocturnal environmental factors that drive Tn and corresponding daytime environmental factors that drive daytime transpiration. Therefore, the compositional ratios of Tn and Re differed between weather conditions and months. We developed a novel method to distinguish between Re and Tn and quantified the dynamics of their ratios. We found that on a 3-year average, the Q_night of E. urophylla × E. grandis was mainly used for Tn (58.63%). Our results highlight the non-ignorability of Q_night and the high variability of the compositional ratios of Re and Tn, and suggest that Q_night and its components should be accurately quantified and considered when studying the water balance in eucalyptus stands.

The Arctic is experiencing rapid warming, which among other processes results in increasing erosion of coastal permafrost and the release of ancient organic carbon (OC) into the Arctic Ocean, which in turn might result in greenhouse gas emissions following its decomposition. Supply of terrigenous organic matter to the ocean affects near-shore nutrient concentrations and the composition of microbial communities—highlighting the need to understand the fate of permafrost-derived carbon in this fragile ecosystem. We incubated material from coastal Yedoma permafrost for 85 days in seawater collected during the Arctic Century expedition. During this experiment, 2.8 ± 1.4% of OC from coastal Yedoma was respired to CO₂. Radiocarbon analysis revealed that 88 ± 15% of the released CO₂ originated from ancient material (∼40,000 years), indicating that degradation of permafrost OC reintroduces old carbon into the short-term carbon cycle. Hence, the permafrost climate feedback may be enhanced in the coming decades when coastal erosion accelerates. Additionally, 0.9 ± 0.3% of Yedoma OC was leached as dissolved OC. The observed net production of inorganic nitrogen during the incubation could potentially provide a negative feedback by stimulating primary production. Bacterial community analysis showed a succession of primary responders to biolabile substrates (e.g., Psychrobacter and Colwellia) followed by secondary consumers of less biolabile substrates (e.g., Maribacter and Pseudohongiella), plus a potential establishment of permafrost associated-bacteria on particles. Overall, our data show that OC input from thawing permafrost stimulates bacterial dynamics, with likely implications for regional biogeochemical cycles and the Earth's climate.

Wetlands play an essential role in the global greenhouse gas budget via carbon dioxide sequestration as well as methane emission. In recent decades, solar-induced chlorophyll fluorescence (SIF) has been recognized as a remotely sensed proxy of gross primary productivity (GPP), which generates substrates for methane production. To examine the suitability of SIF for estimation of these two fluxes, we conducted ground tower-based SIF observation with an ultrafine-resolution spectroradiometer in conjunction with eddy covariance measurement in a cool-temperate bog. The daily SIF retrieved in the red (687 nm) and far-red (760 nm) bands (SIF_red and SIF_far-red, respectively) increased nonlinearly with GPP and linearly with absorbed photosynthetically active radiation (APAR). The relatively weak correlation between apparent SIF yield (ΦSIF = SIF/APAR) and light use efficiency implied that both APAR and plant physiology constrained the SIF emission in this wetland. The SIF_red/SIF_far-red ratio showed a significant negative relationship with vegetation greenness indices, and the similar seasonal variation in SIF_red and SIF_far-red indicated that the SIF_red reabsorption effect only weakly influenced the SIF_red–GPP relationship. Episodic temporal reduction in the water table did not distinctly influence SIF and ΦSIF. Estimation of the methane emission rate was subtly improved by incorporating SIF, which was substituted for GPP as the methanogenesis substrate, in a multivariable regression analysis together with two environmental factors: soil temperature and water table depth. This study illustrates the potential of both SIF_red and SIF_far-red to monitor GPP and to predict methane emission in wetlands.