Journal of Geophysical Research: Biogeosciences最新文献

Phosphorus (P), a crucial element for all life forms on Earth, is often insufficiently available in terrestrial ecosystems. The interaction and feedback between plants and soil microorganisms are crucial links integrating above- and below-ground ecosystems. However, changes in soil P fractions in response to plant-microbe interactions during vegetation succession remain poorly understood. This study investigated the trends and relationships between plant resorption, microbial properties, and soil P fractions in the transformation interface soil layer (TIS) and underlying topsoil layer (UTS) during grassland vegetation succession. The results point to a combination of soil properties, alongside microbial and plant factors driving soil P changes. However, the TIS and UTS layers differ, with phosphorus dynamics in the TIS layer primarily influenced by microorganisms. Microorganisms are co-limited by C and P in both the TIS and UTS layers. Under microbial P-limitation, microorganisms produce alkaline phosphatases (AP), while C deficiency stimulates the production of C-acquiring enzymes, subtly regulating soil P dynamics through organic matter decomposition. Plant P resorption efficiency and microbial P-limitation exhibit synergistic variations, reaching their lowest levels during the mixed Bothriochloa ischaemum and Stipa bungeana Trin (Bo.I + St.B) stage. This study emphasizes that P cycling is influenced by plant-microbe-soil interactions and feedback. Plants and soil microorganisms jointly regulate soil nutrient effectiveness and partitioning in the ecosystem.

Nitrous oxide (N₂O) is a potent greenhouse gas and is depleting the stratospheric ozone layer. Diazotrophic N₂O assimilation to biomass represents a novel biological N₂O consumption pathway in addition to canonical denitrification. Thermodynamically, N₂O assimilation is more favorable than dinitrogen (N₂) fixation in natural environments, especially under higher N₂O concentration and cooler conditions. Via isotopic tracing experiments, N₂O assimilation was detected on cultured diazotrophs Crocosphaera and Trichodesmium with specific rates from 1.27 ± 0.16 × 10⁻⁴ to 2.00 ± 0.25 × 10⁻⁴ hr⁻¹ under elevated [N₂O]/[N₂] conditions (0.0005–0.01) within 24-hr incubation. The rates of N₂O assimilation during the light and dark periods were statistically insignificant compared with N₂ fixation activity. In a eutrophic estuary, N₂O assimilation was not detected in the absence of diazotrophic activity. A competitive substrate kinetic model with experimentally calibrated parameters successfully quantified rate ratios of N₂O assimilation and N₂ fixation in varying substrate concentrations. The low [N₂O]/[N₂] ratio in natural conditions leads to N₂O assimilation rate being <0.1% of N₂ fixation rate, rendering negligible impact of N₂O assimilation. The model was also used to predict the time required for experimental detection of N₂O assimilation in isotopic tracing experiments under varying [N₂O]/[N₂] ratios. This study enhances the mechanistic understanding of N₂O assimilation by diazotrophs, broadening the microbial nitrogen cycle by a potential N₂O sink and nitrogen source for production.

Extreme wildfire events and cyclones are on the rise across tropical regions in response to climate change. Despite assumptions about their impact on phytoplankton through nutrient supplies, field evidence is lacking, and their combined effects remain unclear. In an on-site microcosm experiment conducted in the Xisha Islands, South China Sea (SCS) after Typhoon Noru, we observed enhanced phytoplankton growth in response to exposure to total suspended particulates (TSP) from wildfires (2 mg/L and 6 mg/L) under wind-driven upwelling conditions. Upwelled nutrients had a limited effect on Chl-a concentration due to phosphate depletion, by contrast, wildfire TSP contributed nutrients enriched in nitrogen and phosphate, resulting in a 3.30–5.61-fold increase in Chl-a. However, upwelled nutrients increased the diatom-to-dinoflagellate ratio from the initial 11.0 to 12.7, TSP at low and high levels reduced the ratio to 0.3–0.8 and significantly altered the communities, with 61.8% of species, including two dominant diatoms, negatively correlated with N and/or P supplies. Species diversity declined significantly at high TSP levels. These findings suggest that enhanced primary productivity by wildfires may come at the cost of an altered phytoplankton community. This field study improves understanding of the effects of simultaneous occurrences of multiple extreme climate events on marine ecosystems.

Peatland carbon storage is increasingly threatened by the combination of land-use change and climate variability, though carbon losses from land-use changes that span centuries are difficult to quantify, particularly in systems where little undisturbed area remains. Here we use a combination of vegetation change, fire history, and calculations of excess ash mass to quantify carbon loss in the Great Dismal Swamp National Wildlife Refuge (GDS NWR), USA, a highly impacted oligotrophic temperate peat swamp. Our results indicate that ditch construction that began in the Colonial Era in the late 1700s and continued into the mid-20th century across the swamp resulted in shifts from cypress-tupelo swamps to a combination of maple-gum and pine pocosin forests, consistent with drying surface conditions. Two large smoldering fires (2008, 2011) that were exacerbated by surface drainage, shifted vegetation from swamp to marsh, consumed peat over 25 km², and caused losses of 1.05–1.34 Tg C due to peat burning. Across the Refuge as a whole, up to 48.2 Tg C has been lost to peat oxidation since ditch construction. Both stocks and rates of carbon loss remain higher than post-disturbance accumulation across most of GDS NWR, suggesting that existing efforts to block drainages to elevate water tables may not be enough to offset carbon losses. Rewetting heavily impacted surface peats may reduce peat oxidation and carbon loss, and shift vegetation toward hydrologic conditions preferred by pre-disturbance cypress-tupelo swamps.

Carbonate-associated sulfate (CAS) δ³⁴S values (δ³⁴S_CAS) are generally assumed to reflect S isotopic composition of paleo-seawater and have been extensively used to reconstruct secular variations in seawater sulfate concentrations during the geological past. However, it has often been documented that δ³⁴S_CAS records are incompatible with seawater sulfur isotopes (20.9 ± 0.1%, 2σ) determined from other archives, such as sulfate evaporites and barite (both of which may also display inconsistencies). A possible explanation for this discrepancy is that δ³⁴S_CAS values can be easily altered by atmospheric sulfate and sulfide re-oxidation. However, the specific influence of biological factors (vital effects, common in biogenic carbonates) on CAS S isotopic composition remains unresolved, particularly at microscale levels. To elucidate these effects on δ³⁴S_CAS, S isotopic profiles were analyzed across two skeletal transects of two modern deep-sea corals (gorgonia) using a novel secondary-ion mass spectrometry method. Strong S isotopic fractionation was observed in calcitic skeletons from the most ³⁴S-depleted center (δ³⁴S = ∼19‰), increasing outward to a relatively constant 22.5‰ in gorgonia sp. coral and 21.6‰ in bamboo coral, suggesting that vital effects are much larger than previous estimated (∼±1‰ fractionation from seawater). Oxygen isotopic and Mg, S, O elemental compositions, and Raman spectral and crystal morphological features indicate that processes such as pH control, Rayleigh fractionation, and organic effects are precluded as causes of such fractionation. Instead, vital effects associated with kinetic processes related to surface entrapment seem plausible as controls on S isotopic fractionations in the coral. This novel method is significant for gaining insights into vital effects, assessing the reliability of biogenic carbonates as high-resolution environmental archives of S isotopes, and understanding the fundamental mechanisms governing biomineralization.

The globally increasing reactive N richness affects even remote ecosystems such as the tropical montane forests in Ecuador. We tested whether the δ¹⁵N values of total dissolved N (TDN), measured directly in solution with a TOC-IRMS, can be used to help elucidate N sources and sinks along the water path and thus might be suitable for ecosystem monitoring. From 2013 to 2016, the δ¹⁵N values of TDN in bulk deposition showed the most pronounced temporal variation of all ecosystem solutions (δ¹⁵N values: 1.9–5.9‰). In throughfall (TF), TDN was on average ¹⁵N-depleted (−1.8 ± s.d. 0.4‰) relative to rainfall (3.4 ± 0.9‰), resulting from net retention of isotopically heavy N, mainly as NH₄⁺. Simultaneously, N-isotopically light NO₃⁻-N and dissolved organic nitrogen (DON) with a δ¹⁵N value between NO₃⁻-N and NH₄⁺-N were leached from the canopy (leaves: −3.5 ± 0.5‰). The increasing δ¹⁵N values in the order, TF < stemflow (SF, 0.1 ± 0.6‰)< litter leachate (LL, 1.3 ± 0.7‰) concurred with an increasing DON contribution to TDN reflecting the δ¹⁵N value of the organic layer (1.9 ± 0.9‰). The lower δ¹⁵N value of the mineral soil solution at the 0.15 m soil depth (SS15, −1.5 ± 0.3‰) than in LL can be explained by the retention of DON and NH₄⁺ and the addition of NO₃⁻ from mineralization and nitrification. The increasing δ¹⁵N values in the order, SS15 < SS30 (−0.6 ± 0.2‰) < streamflow (ST, 0.5 ± 0.6‰) suggested gaseous N losses because of increasing denitrification. There was no seasonality of the δ¹⁵N values. Our results demonstrate that the δ¹⁵N values of TDN in ecosystem solutions help identify N sources and sinks in forest ecosystems.

Soil iron (Fe)-associated carbon (C) (Fe-OC) plays a vital role in the soil C cycle due to its high stability, but vegetation restoration might alter the composition and quantity of Fe-OC by introducing a large amount of plant-derived C and affecting soil properties. However, how vegetation restoration affects soil Fe-OC remains unclear. Herein, plant and topsoil samples from grasslands, shrublands, and forestlands across three soil types (loam, loess, and sandy soil) since cropland conversions were collected to address this issue. The results showed soil Fe-OC content decreased in loam soil but increased in loess and sandy soil following vegetation restoration. Additionally, the Fe-OC accumulation efficiency induced by vegetation restoration increased with the coarser soil texture. Vegetation restoration promoted the accumulation of Fe-OC by increasing soil microbial biomass C, dissolved organic C, aromatic-C, and citric acid, but also disrupted the combination of Fe oxides and C by introducing oxalic acid, reducing Fe oxide content and iron trivalent (Fe(III)). There were two-sided effects of vegetation restoration on Fe-OC, but the overall effect depends on the soil types. Moreover, isotopic evidence indicated that microbial source C is the main source of Fe-OC, but Fe oxides preferentially adsorbed dissolved organic matter (DOM) and root deposits from plants rather than microbial residues and metabolites following vegetation restoration. In addition, Fe oxides preferentially adsorbed aromatic-C compared to other functional group components. These findings indicated that vegetation restoration in coarser-texture soils, coupled with selecting species that increase soil microbial biomass, produce more root deposits, and enhance DOM, contribute to the accumulation of soil Fe-OC.

The seasonal behavior of fluvial dissolved silica (DSi) concentrations, termed DSi regime, mediates the timing of DSi delivery to downstream waters and thus governs river biogeochemical function and aquatic community condition. Previous work identified five distinct DSi regimes across rivers spanning the Northern Hemisphere, with many rivers exhibiting multiple DSi regimes over time. Several potential drivers of DSi regime behavior have been identified at small scales, including climate, land cover, and lithology, and yet the large-scale spatiotemporal controls on DSi regimes have not been identified. We evaluate the role of environmental variables on the behavior of DSi regimes in nearly 200 rivers across the Northern Hemisphere using random forest models. Our models aim to elucidate the controls that give rise to (a) average DSi regime behavior, (b) interannual variability in DSi regime behavior (i.e., Annual DSi regime), and (c) controls on DSi regime shape (i.e., minimum and maximum DSi concentrations). Average DSi regime behavior across the period of record was classified accurately 59% of the time, whereas Annual DSi regime behavior was classified accurately 80% of the time. Climate and primary productivity variables were important in predicting Average DSi regime behavior, whereas climate and hydrologic variables were important in predicting Annual DSi regime behavior. Median nitrogen and phosphorus concentrations were important drivers of minimum and maximum DSi concentrations, indicating that these macronutrients may be important for seasonal DSi drawdown and rebound. Our findings demonstrate that fluctuations in climate, hydrology, and nutrient availability of rivers shape the temporal availability of fluvial DSi.

Peat particulate organic matter (POM) in the anoxic subsurface of peatlands is increasingly recognized as an important terminal electron acceptor (TEA) in anaerobic respiration. While POM reduction has been demonstrated in laboratory peat-soil incubations and (electro-) chemical reduction assays, direct demonstration of POM reduction in peat soils under in situ, field conditions involving quantification of transferred electrons remain missing. Herein, we demonstrate that deployment of an oxidized reference POM in the anoxic, methanogenic subsurface of three ombrotrophic bogs, followed by one year incubation, resulted in the transfer of approximately 150–170 μmol of electrons per gram POM to the deployed reference POM. The capacity of this reduced POM to accept electrons was partially restored upon subsequent exposure to dissolved oxygen. These findings provide direct evidence for POM acting as regenerable and sustainable TEA for anaerobic respiration in temporarily anoxic parts of peat soils. Based on the number of electrons transferred to POM and thermodynamic considerations, we estimate that anaerobic respiration to POM may largely suppress methanogenesis in peat soils, particularly close to the oxic-anoxic interface across which POM is expected to undergo redox cycling.

Model representation of carbon uptake and storage is essential for accurate projection of the response of the arctic-boreal zone to a rapidly changing climate. Land model estimates of LAI and aboveground biomass that can have a marked influence on model projections of carbon uptake and storage vary substantially in the arctic and boreal zone, making it challenging to correctly evaluate model estimates of Gross Primary Productivity (GPP). To understand and correct bias of LAI and aboveground biomass in the Community Land Model (CLM), we assimilated the 8-day Moderate Resolution Imaging Spectroradiometer (MODIS) LAI observation and a machine learning product of annual aboveground biomass into CLM using an Ensemble Adjustment Kalman Filter (EAKF) in an experimental region including Alaska and Western Canada. Assimilating LAI and aboveground biomass reduced these model estimates by 58% and 72%, respectively. The change of aboveground biomass was consistent with independent estimates of canopy top height at both regional and site levels. The International Land Model Benchmarking system assessment showed that data assimilation significantly improved CLM's performance in simulating the carbon and hydrological cycles, as well as in representing the functional relationships between LAI and other variables. To further reduce the remaining bias in GPP after LAI bias correction, we re-parameterized CLM to account for low temperature suppression of photosynthesis. The LAI bias corrected model that included the new parameterization showed the best agreement with model benchmarks. Combining data assimilation with model parameterization provides a useful framework to assess photosynthetic processes in LSMs.