Biogeochemistry最新文献

Rivers are significant sources of dissolved inorganic nitrogen, contributing to coastal eutrophication and hypoxia. While the impact of large rivers is well documented, less is known about small rivers that directly discharge into the sea after draining urban areas. Their aggregate biogeochemical significance should not be overlooked, given their potentially high loads of nitrogen. There are various kinds of human activities within a river catchment that may contribute different types and amounts of inorganic nitrogen. Identifying the relative contribution of each of these sources is important to facilitate the planning of environmental management measures that can effectively reduce riverine nutrient loading. Focusing on the subtropical Lam Tsuen River in Hong Kong, this study made use of dual nitrogen and oxygen isotopes to identify sources of nutrients within the catchment. Results indicate that nutrients were mainly released into the system in lowland areas in the form of sewage originating from anthropogenic activities. Downstream sites had a significantly higher proportion of nitrate originating from sewage than upstream sites. Nitrification generally accounted for more than 40% of the nitrite and nitrate found in the catchment. Assimilation in the river was found to be negligible, suggesting that most nutrients were transported into the seasonally hypoxic Tolo Harbour. Compared to streams worldwide that do not drain urbanised areas, the Lam Tsuen River generally discharges 10 times more inorganic nitrogen per catchment area. This nitrogen delivered to the coast ultimately consumes roughly 2% of Tolo Harbour’s oxygen. Together with four other similar-sized or even larger rivers, up to 10% of the harbour’s dissolved oxygen will be consumed. Overall, our findings highlight the need for a better accounting of the impact of these small rivers on marine nitrogen budgets.

Dissolved black carbon (DBC) is a key component of the global carbon cycle, yet its seasonal dynamics and river-to-sea transport remain poorly understood, particularly in Southeast Asia where anthropogenic pressures are intense. This study investigates the spatial and seasonal variability of DBC along with dissolved organic matter (DOM) in the main branch of the Red River (North Vietnam) based on three sampling campaigns conducted in March (dry season), June (early wet season), and September 2023 (late wet season). DBC concentrations increased from 29 μg C L⁻¹ in March to 66 μg C L⁻¹ in September, following rainfall-driven inputs. This seasonal pattern was accompanied by changes in DOM quality, as inferred from optical indices: higher SUVA₂₅₄ (specific UV absorbance at 254 nm), a_CDOM(350) (absorption coefficient of chromophoric DOM at 350 nm), and HIX (humification index) in September indicated more terrestrial and humified material, while higher BIX (biological index) in March suggested a higher contribution of fresher, autochthonous DOM. Spatial trends showed a downstream decrease in DBC in June, likely due to abiotic degradation (particularly photodegradation) and dilution. This contrasted with the increasing concentrations from Hanoi to the estuary in March and September, which may be linked to local inputs during dry-season groundwater dynamics and rainfall. DOM optical indices support a contribution of low-DBC groundwater near Hanoi in March. Estimated DBC fluxes at the estuary reached up to 20.7 Gg yr⁻¹, representing 0.11% of the global riverine DBC flux to the ocean during the wet season. These results emphasize the role of tropical rivers as dynamic conveyors of combustion-derived carbon, where seasonality and local processes, such as rainfall, photodegradation, and groundwater inflow, strongly shape DBC transport from land to sea.

As one of the major global biomes, grasslands contribute substantially to the storage of soil organic carbon (SOC) and macroelements such as nitrogen (N) and sulfur (S). However, while SOC and N storage in grassland soils have been extensively studied in the past, similar assessments for S are missing. We conducted a meta-analysis to determine which soil, climate or management parameters were most relevant in controlling S storage compared to SOC and N storage. We collected data on SOC, total N, and total S concentrations in grassland topsoils from previously published studies, along with data on mean annual temperature, mean annual precipitation, soil pH, and soil texture. We additionally classified conditions at each site according to Reference Soil Groups, Koeppen climate classes and management systems. Our data set includes a total of 248 observations from 30 studies in 15 different countries published between 1958 and 2024. We generally observed similar patterns in SOC, N, and S storage, with Reference Soil Group and Koeppen climate class as the most relevant parameters determining total element concentration or element ratios in grassland topsoils, while the type of grassland management did not consistently affect element concentrations and ratios. However, we observed that S concentration was increased especially in soils that were influenced by geogenic inputs of S from the parent material or with groundwater influx, with corresponding changes in C:S and N:S ratios. This resulted in larger variability in S storage in grassland soils compared to SOC and N storage.

Symptoms of eutrophication are increasingly evident in remote clearwater lakes. To identify the sources and dynamics of phosphorus release, we measured total phosphorus (TP) in sediments and soluble reactive phosphorus (SRP) fluxes across the sediment–water interface at 54 locations in a deep temperate lake. Once renowned for its clear waters, Lake Stechlin has experienced a fourfold increase in water column TP over the past decade. SRP fluxes from sediments generally increased with water depth across all three lake basins, although there were significant variations in SRP concentrations, up to threefold, among sampling locations at the same depth. Notably, the lake´s total mean SRP flux in June (1.12 mg m⁻² day⁻¹) was higher than that determined in October (0.74 mg m⁻² day⁻¹). This result can be attributed to the substantial contribution (about 31% of total SRP release) of shallower sediments (0–20 m), which are not affected by seasonal anoxia. Our findings highlight a notable spatial variability of SRP fluxes and underscore the importance of considering often overlooked shallow sediments when assessing P dynamics in lakes.

Internal phosphorus (P) loading often delays or prevents the recovery of eutrophic lakes. Since the biogeochemical composition of surface sediments plays a key role in sediment P release dynamics, detailed sediment chemical analyses are essential for estimating internal loading potential and implementing cost-effective lake management programs. To identify reliable methods for quantifying surface sediments’ contribution to internal P loading, sediment-chemical screening and whole-lake sonar analysis of basin morphology and sediment hardness were conducted in a shallow, 11-ha hypereutrophic lake in Denmark. Over a 50-day summer period, changes in lake water P concentrations were compared with changes in the potentially mobile P content in surface sediments at five sites. Gross sediment P release rates from intact sediment cores and P settling rates from sediment traps placed in the epilimnion were used to calculate the net P flux from the sediments to the lake water. The period’s net P flux was estimated at 62 kg (95% CI: 42.8–77.5), closely matching the calculated P accumulation in the lake water body (55 kg). Conventional sequential sediment P extractions indicated a total loss of 172 kg P. Although 70% of the potentially mobile P pool was redox-sensitive, only 11% was lost from the 0–10 cm sediment layer over the 50 days. Modified extraction procedures revealed that 46% of the sediment P was bound in non-oxygen-sensitive iron-hydroxides (e.g., vivianite), highlighting the complexity of sediment biogeochemistry and the challenges of accurately assessing changes in the potentially mobile P pool for estimating internal loading.

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.