Biogeochemistry最新文献

Internal phosphorus (P) loading is a main cause for persistent eutrophication of shallow freshwater systems and can delay restoration for decades. Iron (Fe) amendment is often used to enhance P binding in the sediment and reduce benthic P fluxes. However, sufficient dosing using Fe salts is challenging due to acidification. Fe-rich water treatment residuals (Fe-WTR) are an attractive alternative, but their behavior in aquatic sediments is poorly studied. In this field study, a ditch in a peat polder was treated with ~ 2.5 kg Fe/m² using Fe-WTR. Sediment porewater and solid phase analyses, including sequential Fe extraction, showed that the added Fe-WTR significantly increased the reactive Fe reservoir of the surface sediment. Sediment incubation experiments and surface water monitoring for one year indicated an efficient reduction of internal P loading. Redox cycling was found to redistribute the added Fe both laterally across the ditch and vertically towards the sediment surface. Reactive Fe phases were thus continuously replenished in the surface sediment and available for P retention via co-precipitation and adsorption, potentially increasing the longevity of the treatment. Loss of the added Fe to sulfidation was limited due to the large excess of available Fe. However, the initial P-content of the Fe-WTR also increased the sediment P reservoir by ~ 10%, potentially enhancing future internal P loading. This study shows that Fe-WTR are viable for freshwater restoration, however, in spite of detailed knowledge of the system, to judge longevity of the treatment remains challenging and long-term monitoring after treatment remains necessary.

Nitrification and denitrification are two important biological processes producing N₂O in soils, but their contributions to N₂O emissions are not well understood, hindering precise mitigation measures. Here, we developed process-based models (PBM) with and without transport (T) to partition N₂O sources by tracking nitrogen flows (NF) through different reaction pathways. The model with transport (PBM-T-NF) well predicted N₂O production from nitrification and denitrification in two different repacked soils with a shallow depth of 8 mm under moisture conditions ranging from 40 to 100% water-filled pore space (WFPS), demonstrating its robustness and reliability. In comparison, the model without transport (PBM-NF) failed to capture the N₂O dynamics and the relative contribution of denitrification to N₂O production (({C}_{D})), highlighting the need of including mass transport in predicting N₂O dynamics. The PBM-T-NF model was further employed to investigate the effects of soil properties on N₂O emissions and sources. Increased NH₄⁺ concentration significantly decreased ({C}_{D}) under relatively low moisture conditions, while increased NO₃⁻ slightly promoted ({C}_{D}) over different moisture contents, emphasizing the importance of substrate availability and moisture conditions in controlling ({C}_{D}). Furthermore, the PBM-T-NF model was used to quantify N₂O sources from an artificial soil core of 80 mm depth. Soil depth was shown to be important in mediating ({C}_{D}) by controlling O₂ diffusivity, which is highly dependent on moisture content. Given the long-standing challenge in experimental quantification of N₂O sources from soils, our developed model provides a novel way to estimate N₂O production from different nitrogen processes, which is key for accurately targeting mitigation of N₂O emissions from soils.

Efforts to increase soil organic carbon (SOC) storage and predict its responses to climate change demand enhanced understanding of the interrelationships of controls on SOC storage and their dependence on environmental context. To this end, we use structural equation modeling to test a hypothesized structure of controls that includes the mediating influences of plant productivity and soil pH together with the direct effects of climate and soil properties on two contrasting SOC components, particulate (POC) and mineral-associated organic carbon (MAOC), using > 1000 topsoils from across the USA for which POC and MAOC were directly measured or predicted using mid-infrared spectroscopy. We find that separating systems into arid and humid systems by AI (0.65 cutoff) improves understanding controls on POC and MAOC storage, as the relationships between predictors and their effects on POC and MAOC differ between arid and humid systems based on the multigroup structural equation model and random forest models. Net primary productivity is more important for predicting POC and MAOC storage in arid than humid systems, while base cations, pH, and texture are more important in humid than arid systems. Reactive metals (oxalate-extractable Al and Fe) together are the most important predictor of topsoil POC and MAOC storage regardless of climate. We find the negative relationship between MAOC and potential evapotranspiration is stronger than that for POC, suggesting that for the mineral topsoils studied here, MAOC may be more sensitive than POC to increasing aridity. Our results support the concept that SOC storage in arid systems is more constrained by plant inputs than in humid systems, where microbial inhibition via pH and association with minerals and metals are stronger constraints, and point to the sensitivity of MAOC formation to drought. Overall, these results help to clarify the context-dependence of SOC storage and show how representing aridity as an overarching influence over the controls on SOC formation and loss processes can inform its stewardship under climate change.

Recent climate trends in the Colorado Mineral Belt have intensified acid mine drainage (AMD) impacts, increasing the importance to understand trace metal and rare earth element (REE) cycling in affected watersheds. This diel study investigated biogeochemical and photochemical controls on metal and REE mobility in an AMD-impacted wetland below a large, abandoned mine. Daily photochemical cycling of H₂O₂ and iron species drove complex metal mobility patterns for both trace metals and REEs, with Cu, Cd, and Pb increasing during peak daylight hours (30%, 9%, and 113% respectively), while Zn, Mn, and Al decreased by 9%, 14% and 19%, respectively. REE concentrations frequently exceeded 100 µg/L for Ce, Nd, and Y, with both light REEs (LREEs) and heavy REEs (HREEs) exhibiting photochemically-driven diel fluctuations. Ce, Nd, Gd, Pr, and La concentrations increased by 3–10% during daylight hours, while Y and Dy decreased slightly (2–4%), and Sm decreased by 20%. Cerium anomaly calculations revealed distinct spatial patterns across the wetland-groundwater-creek continuum, with values ranging from 0.73 to 0.90, indicating ongoing oxidative processing of REEs throughout the system driven by retention time. These findings demonstrate that AMD-impacted wetlands are not simple flow-through systems, but rather complex environments where photochemical processes influence the cycling of both trace metals and REEs, with important implications for water quality management.

Complex interactions controlling carbon (C), nitrogen (N), and inorganic nutrients: calcium (Ca), magnesium (Mg), potassium (K), phosphorus (P), in forest soils are difficult to tease apart due to covarying factors (e.g., soil parent material) and reductionist approaches can miss potential synergistic effects. We evaluated if increasing mean annual temperature (MAT), decreased organic horizon development, shallow tree rooting, and accumulation of C, N, and inorganic nutrients. We transplanted 144 mineral soil columns across six temperate forests from Virginia to New Hampshire and collected them 1-year and 4-years later. Our results show that organic horizon C, N, and nutrient pools were negatively associated with MAT with 4 × to 5 × greater pools at the coldest sites than the warmest sites. Since five-years of inputs from litterfall and throughfall monitoring show similar or increasing fluxes with MAT, differences were likely due to faster mineralization and transport from the columns. Transplanted mineral soil C, N, Ca, and P pools did not vary with MAT nor with root-access or root biomass, showing roots and organic horizon masses did not have consistent effects. Mineral soil root and MAT effects may still be developing or impacted by other variables not evaluated. Lastly, we found increases of organic phase Ca, Mg, K, and P from Year 0 to Year 1in the mineral soil across all six sites using Scanning Electron Microscopy- Energy-Dispersive X-ray Spectroscopy (SEM–EDS) imaging but only a significant effect of MAT or root-access for K. Our study highlights that MAT, organic horizon development, and nutrient accumulation and storage are linked but not in the mineral soil.

Export of dissolved organic carbon (DOC) from freshwater systems has been the focus of many studies owing to its pivotal role in regulating global carbon fluxes and ecosystem function. Both the flux and composition of dissolved organic matter (DOM) are critical for understanding its ecological impact, as similar compositions can have vastly different consequences depending on the magnitude of input and hydrological context. However, very little data exists on the composition of DOM export fluxes to downstream ecosystems. Here we investigate the interaction of water temperature and discharge on DOC and DOM composition export fluxes in two streams draining contrasting watersheds (agriculture versus forested) in southern Ontario, Canada across seasons. Using Generalized Additive Models, we observed that both stream discharge and water temperature significantly affected DOM composition, and the proportion of terrestrial humic-like DOM exhibited strong positive relationship with discharge. Although DOC loads were comparable between the two streams, the export loads and fluxes of DOM composition (in terms of fluorescent loads and fluxes) differed significantly. These patterns of DOM composition fluxes in both streams remained consistent across seasons, suggesting that watershed characteristics and nutrient availability primarily govern DOM dynamics and export, while seasonal drivers such as discharge and temperature further modulate these patterns. Export loads and fluxes of DOM components were higher in spring and winter months compared to summer and autumn in both streams, while fluxes also increased at medium (Q10-Q90) and high flow (> Q10) at a variable extent in the contrasting streams. Temperature and discharge regulated export of DOM can be further affected with changing climate and increasing frequency of extreme events and alter the processing and delivery of DOM to downstream ecosystems.

Climate change is rapidly redefining the biogeochemical dynamics of our planet, particularly in relation to soil organic carbon (SOC) storage and loss. Also, most existing soil warming studies have focused on nutrient-rich soils in temperate and arctic/boreal regions, limiting predictions for the many nutrient-poor tropical/subtropical soils that store a substantial fraction of global soil C. To address this gap, we evaluated the influence of temperature and substrate (C and nutrient) availability on soil C cycling in a nutrient-poor (substrate-limited) subtropical forest, where previous field research suggested mixed warming responses. We aimed to isolate confounding elements and elucidate the principal mechanisms underpinning SOC dynamics under diverse environmental scenarios: warming (ambient at 25° C, + 1.5 °C at 26.5 °C, and + 2.5 °C at 27.5° C), nutrient addition (nitrogen and phosphorus) and carbon addition treatments. Samples were collected from a low-latitude soil warming experiment with subtropical Typic Kanhapludults soil (Whitehall Forest, Athens, Georgia). Under laboratory conditions, we incubated soil samples for 22 days at the temperatures recorded during sample collection in the field. We looked at key elements of the soil C cycle, including particulate and mineral-associated organic C, microbial biomass C, and microbial necromass C. We also examined important processes like soil microbial respiration and enzyme kinetics. Our systematic evaluations helped us distinguish between the direct and indirect effects of warming (i.e., inherent and apparent temperature sensitivity) on SOC formation and loss. Our laboratory incubations showed that warming alone did not produce a sustained increase in microbial respiration or microbial biomass, underscoring the dominant role of C limitation in regulating microbial metabolism. In contrast, adding labile C alone or in combination with nutrients (N + P + C) significantly boosted microbial metabolism, supporting a co-limitation framework in which nutrient amendments became impactful only after alleviating C scarcity. Enzymatic assays further indicated that substrate depletion, rather than enzyme denaturation, constrained any prolonged warming effect. These findings underscore the need for continued research into SOC dynamics and microbial adaptation in nutrient-poor ecosystems, which remain underrepresented in Earth system models.

Temporal patterns in chemistry of headwater streams reflect responses of water and elemental cycles to perturbations occurring at local to global scales. We evaluated multi-scale temporal patterns in up to 32 y of monthly observations of stream chemistry (ammonium, calcium, dissolved organic carbon, nitrate, total dissolved phosphorus, and sulfate) in 22 reference catchments within the northern temperate zone of North America. Multivariate autoregressive state-space (MARSS) models were applied to quantify patterns at multi-decadal, seasonal, and shorter intervals during a period that encompassed warming climate, seasonal changes in precipitation, and regional declines in atmospheric deposition. Significant long-term trends in solute concentrations within a subset of the catchments were consistent with recovery from atmospheric deposition (e.g., calcium, nitrate, sulfate) and increased precipitation (e.g., dissolved organic carbon). Lack of evidence for multi-decadal trends in most catchments suggests resilience of northern temperate ecosystems or that subtle net effects of simultaneous changes in climate and disturbance regimes do not result in directional trends. Synchronous seasonal oscillations of solute concentrations occurred across many catchments, reflecting shared climate and biotic drivers of seasonality within the northern temperate zone. Despite shared patterns among catchments at a seasonal scale, multi-scale temporal patterns were statistically distinct among even adjacent headwater catchments, implying that local attributes of headwater catchments modify the signals imparted by atmospheric phenomena and regional disturbances. To effectively characterize hydrologic and biogeochemical responses to changing climate and disturbance regimes, catchment monitoring programs could include multiple streams with contributing areas that encompass regional heterogeneity in vegetation, topography, and elevation. Overall, detection of long-term patterns and trends requires monitoring multiple catchments at a frequency that captures periodic variation (e.g., seasonality) and a duration encompassing the perturbations of interest.