Potassium (K+) fertilization is essential for crop production, yet its efficacy is often hindered by rapid soil fixation. Traditionally, this fixation is attributed to the interlayer spaces of 2:1-type clay minerals. However, emerging evidence suggests that amorphous silica (ASi) significantly influences soil nutrient dynamics. This study investigated the relationship between soil ASi fractions and K+ pools in the agricultural soils (n = 49) of the Jianghan Plain, China. We operationally fractionated ASi into exchangeable (ASiexch), active (ASiactive), non-active (ASin-active), and biogenic (BSi) fractions and quantified their associated K+ pools (K+exch, K+active, K+n-active, and BK+). Results demonstrated that all ASi fractions were significantly correlated with their corresponding K+ pools (p < 0.05). The cumulative K+ associated with ASi fractions (1.45 ± 0.23 g kg-1) significantly exceeded the conventionally defined readily available and slow-available K+ pools, suggesting that current agronomic assessments may underestimate soil K+ fertility. The strength of correlations between the associated K+ and ASi followed the order: ASiexch > ASiactive > ASin-active > BSi. Furthermore, Strong positive correlations (r = 0.81–0.94, p < 0.001) were observed bewteen ASiactive, ASin-active, and Fe/Al, suggesting that these metal oxides are key mediators in K+-ASi interactions. Dynamic extraction revealed sustained co-release of Si and K+, with cumulative K+ extraction reaching up to 17.5% of total K, while X-ray diffraction analysis confirmed minimal alteration of crystalline K-bearing minerals, indicating that K+ release primarily originated from amorphous phases. The findings demonstrate ASi constitutes a substantial, previously underappreciated reservoir of labile K+ in agricultural soils, with its liberation kinetics governed by synergistic interactions with Fe/Al oxides.

Sediment reducing capacity (RC) regulates benthic habitat function and the biogeochemical cycling of carbon, nutrients, and pollutants. However, the compositional complexity and dynamic responses of RC to redox alternation remain inadequately characterized. Through laboratory stimulation experiments of redox alternation with 20 freshwater sediments from four water bodies with distinct trophic status in China (Honghu Lake: HL, n = 3; Xiashan Reservoir: XR, n = 6; Yudong Reservoir: YR, n = 7; and Zhanghe Reservoir: ZR, n = 4), we monitored the dynamics of RC and its fractions (RCpH7.0, RCpH2.0), and quantified the responses of major reducing components (reducing organic substances (RedOr), Fe(II), and sulfides (Sn, -2 ≤ n < 0)). Redox alternation triggered asynchronous RC responses, with depletion during oxic phases and recovery under anoxia. RC fluctuation amplitude exhibited sediment-type dependence, with RedOr-Sn sediments (HL/XR) showing greater variability than RedOr-Fe(II) systems (YR/ZR). Component-specific contributions differed significantly between sediment types (p < 0.05). RedOr contributed most to RC variation in RedOr-Sn sediments (45.9-71.2% vs 35.7-52.2% in RedOr-Fe(II)), whereas Fe(II) contributed 45.8-61.5% in RedOr-Fe(II) sediments (18.9-26.6% in RedOr-Sn), and Sn accounted for 8.73-27.5% in RedOr-Sn sediments (1.62-2.62% in RedOr-Fe(II)). These differences reflected anoxia-driven differences in component reactivity and abundance. RedOr-Sn sediments exhibited concurrent enrichment of RedOr, Fe(II), and Sn, whereas RedOr-Fe(II) sediments showed mainly enhanced Fe(II) reactivity with limited RedOr/Sn changes. Mechanistically, the observed RC patterns were consistent with distinct Fe cycling pathways. In RedOr-Sn sediments, limited labile Fe(III) was associated with greater apparent involvement of magnetite, whereas easily reducible/reducible Fe(III) fractions were more prominent in RedOr-Fe(II) systems. Our findings demonstrate the high lability of eutrophic sediment RC and establish redox alternation as an efficient diagnostic tool for identifying reductive sediment types and pinpointing regulatory targets.

Elevated internal loading of bioavailable phosphorus (P) from sediments sustains aquatic eutrophication globally, impeding restoration of diverse water bodies. Establishing a unified method to assess sediment P multi-level risk is critical for effective aquatic ecosystem management. This study examined the feasibility of using 0.1 M HCl-extracted sediment P (dHCl-P) as a novel indicator for assessing internal P release risk. Sediment samples were collected from four water bodies: Honghu Lake (HL, n = 6), Xiashan Reservoir (XR, n = 18), Zhanghe Reservoir (ZR, n = 5), and Yudong Reservoir (YR, n = 30) in China. The dependences of dHCl-P on reductive sediment types (RedOr-Fe(II) sediment and RedOr-Sn sediment; RedOr: reducing organic substances), sediment matrix types (Ca2+/Mg2+-dominant and Fe/Al-dominant), total P (TP), and P fractions were evaluated. Statistical relationships between dHCl-P and both total dissolved P (TDP) and soluble reactive P (SRP) in overlying water were characterized. The results showed that dHCl-P content was primarily controlled by reductive sediment types rather than sediment matrix types and TP. RedOr-Sn sediments (HL, XR; mean TP of 574 ± 183 mg kg-1DW) exhibited significantly higher dHCl-P (mean 314 ± 147 mg kg-1DW) than RedOr-Fe(II) sediments (ZR, YR; mean TP of 1260 ± 354 mg kg-1DW) with an average value of 165 ± 182 mg kg-1DW (p < 0.01). The proportion of dHCl-P in TP (dHCl-P/TP) increased linearly with the reduction degree of iron oxides (ratio of Fe(II) to total Fe, Fe(II)/TFe). P fraction components of dHCl-P consisted apparently of exchangeable P (Ex-P), P bound to Fe(II) (Fe(II)-P), and P bound to Ca2+ (Ca-P). Sediment reduction promoted the transformation of P bound to Al oxides (Al-P), P bound to Fe oxides (CDB-P), and organic P (O-P) to dHCl-P, resulting in its enrichment. Statistical analysis confirmed the significant linear correlations between dHCl-P in sediments, and TDP and SRP in overlying water (p < 0.01). Based on reductive sediment types and the relationship between dHCl-P/TP and Fe(II)/TFe, our findings indicate that dHCl-P can serve as an effective indicator for establishing an operational framework, enabling four-level assessment of internal P risks across different water bodies.

Long-term grassland-crop rotations are widely promoted for enhancing soil fertility and resilience; however, their ability to sustain phosphorus (P) and potassium (K) availability in the absence of fertilization remains poorly understood. This study investigates the temporal dynamics of soil P and K over a 13-year grassland-crop rotation experiment in Lusignan, France, under contrasting management practices. Soil available P and K were monitored every three years from 2005 to 2017 in three soil layers (0-30, 30-60, and 60-90 cm), together with assessments of biomass export and nutrient leaching. Results revealed significant depletions of available P and K in the topsoil (0-30 cm), with reductions of 43% and 62%, respectively. Total system losses of P and K occurred mainly through biomass export and were two to three times greater than the decline in soil available P and K stocks, suggesting an increasing reliance on less mobile soil nutrient pools to maintain available forms over time. Systems including a grassland phase combined with full N fertilization reached agronomic threshold levels for available P and K more rapidly, after approximately 14-16 years and 10-11 years, respectively, compared with 19-20 years for both nutrients under continuous cropping systems. These findings highlight the urgent need for targeted nutrient management strategies to sustain soil fertility and ensure the long-term sustainability of lowinput agroecosystems. To the best of our knowledge, this is the first study to investigate the long-term dynamics of soil available P and K in unfertilized grassland-crop rotation systems.

Long-term straw returning is widely promoted to improve soil quality in intensive cropping systems, yet its combined effects with mineral fertilization on soil quality and phosphorus (P) loss risks remain unclear. Using a 16-year field experiment in a rice-wheat rotation system, we evaluated the impacts of straw returning (S), NPK fertilization (NPK), their combination (NPK + S), and an unfertilized control (CK) on soil properties, soil quality index (SQI), and P sorption indicators. Straw-return treatments improved soil chemical and biological properties, including soil organic carbon (SOC), total nitrogen (TN), microbial biomass carbon, and available potassium, but had little effect on physical properties. NPK + S markedly increased the degrees of P saturation (DPS) and equilibrium P concentrations (ECP0), indicating elevated P loss risk. A minimum data set (MDS) identified silt, SOC, N:P ratio, and total P as key indicators. SQI derived from the MDS (SQI(MDS)) was highest in NPK + S and strongly correlated with both SQI from the total dataset and rice yield (r > 0.92, p < 0.01). Silt was the dominant soil quality factor, contributing 56.7%-64.8% to SQI(MDS). Both NPK and NPK + S increased SQI(MDS) and P-related risk indicators relative to CK, with NPK + S showing the greatest improvement in soil quality but also the greatest P loss risk. Overall, long-term straw returning with NPK fertilization enhances soil fertility and productivity but increases soil P saturation and leaching potential. Sustainable residue management should therefore integrate straw returning with reduced or controlled P inputs to balance agronomic benefits and environmental protection.

The silicon (Si) biogeochemical cycle in ecosystems is tightly linked with other elemental cycles and plays a key role in addressing ecological challenges such as water quality deterioration, climate warming, and biodiversity reduction. As transitional zones between aquatic and terrestrial ecosystems, riparian wetlands possess unique eco-environmental characteristics that enable them to regulate the flow and forms of terrestrial Si into aquatic ecosystems. This paper systematically reviews the characteristics of the Si biogeochemical cycle in riparian wetlands, emphasizing the influence of environmental factors on Si transformation. Additionally, it highlights key knowledge gaps in the Si cycle within riparian wetlands that warrant further research. Si is considered “quasi-essential” for plant growth. During growth, plants not only assimilate CO2 from the atmosphere but also convert dissolved Si into biogenic silicon (BSi). Enhancing the ability of plants to assimilate CO2 through Si uptake is regarded as an effective approach to mitigating climate warming. BSi plays a dominant role in Si fluxes from terrestrial to aquatic ecosystems, with riparian wetlands serving as primary sites for BSi formation. The distinct hydrological characteristics of riparian wetlands have significant impacts on Si movement and transformation. Additionally, factors such as vegetation composition, soil physicochemical properties, and human activities further influence the Si cycle. This review summarizes the characteristics of riparian wetlands, as well as the forms and distribution of Si within these ecosystems. It then emphasizes the biogeochemical processes of Si, the characteristics of Si cycle, and the factors that influence it. This review also identifies knowledge gaps and outlines priorities for future research.

Subsurface leaching is a major pathway of phosphorus (P) loss from agricultural landscapes and soil P indices (SPIs) have been developed to assess P leaching risk. However, most SPIs are developed for well-drained agricultural soils, their ability to predict P leaching risk in hydromorphic soils remains underexplored. Using soil water P concentration data from a 3-year field monitoring (weekly or biweekly) and the SPIs from the same hydromorphic soils in an agricultural catchment in Western France, we tested the ability (Pearson r2) of SPIs to predict soil water P concentrations and assessed the influence of hydrological conditions on these relationships. Eight SPIs were tested including Dyer P, Olsen P, total P, water-extractable P (WEP), equilibrium soil P concentration (ECPo), and three degree of P saturation indices (DPS_Dyer, DPS_Olsen, and DPS_Pöthig). Results showed high temporal variations in the positive correlations between the SPIs and molybdate-reactive P (MRP) and total dissolved P (TDP), with mean r2 ranging 0.36-0.64 for MRP and 0.34-0.58 for TDP. The prediction effectiveness of most SPIs showed no significant difference among hydrological years, while those of Olsen P, ECPo, DPS_Olsen, DPS_Pöthig, and TP were significantly lower in winter waterlogging seasons compared to rewetting/drying seasons. This suggests a greater influence of intra-annual hydrological conditions on SPIs' prediction effectiveness, particularly for MRP. This study highlights the need to consider the resilience of SPIs to hydrological variations and proposes that aggressive indicators like Dyer P and DPS_Dyer should be prioritized when developing P leaching risk indicators for hydromorphic agricultural soils.

Internal phosphorus (P) loading is a key driver of waterbody eutrophication. Various sediment P indexes are developed to assess sediment P risks by linking them to water column P, but their seasonal reliability remains underexplored. This study evaluated, for the first time, sediment P status in the Xiashan reservoir, a large shallow reservoir in northern China serving 9.4 million people. The ability of three P indexes, including exchangeable P (Ex-P), Olsen P (Olsen-P), and diluted HCl-extractable P (HCl-P), to predict water column P concentrations was tested across February, May, and August. Sediments in the Xiashan reservoir exhibited moderate total P levels (531–650 mg kg−1) but high P availability, with Ex-P, Olsen-P, and HCl-P in ranges of 19–35, 58–101, and 327–444 mg kg−1, respectively, likely due to sandy composition. Water column P concentrations significantly correlate with August sediment P indexes (r = 0.42–0.81) but not with February and May sediments, highlighting the ability of August sediment P indexes to predict water column P across seasons. Sampling in August is recommended to efficiently identify critical zones for internal P loading, with Ex-P as the preferred indicator given its simple extraction and strong correlation with water column P (r = 0.81).

Terrestrial silicon (Si) from biogeochemically weathered rocks and soils into oceans must pass through several water bodies, resulting in some Si immobilized. Hence, the knowledge on Si distribution characteristics in different water bodies at a basin scale is helpful to understand Si immobilization. A total of 65 surface sediments and corresponding overlying water samples were sampled from six water bodies (Dianchi Lake, DL; Dadu River, DR; Tuojiang River, TR; Honghu Lake, HL; Donghu Lake, DhL; Taihu Lake, TL) in the Yangtze River Basin of China, total dissolved Si (TDSi) in overlying water and exchangeable Si (Ex-Si), active non-biogenic Si (NBSi), and total acid dissolved Si (TADSi) in sediments were analyzed. Water chemical parameters (pH, EC, and TDP) and sediment components (LOI, TN, TP, and TADFe) showed that the water environment characteristics of six water bodies differed. TDSi differed among regions and between lakes and rivers, significantly higher in water bodies in the upper reaches and rivers than the middle or lower reaches and lakes (p < 0.05), respectively. Ex-Si in sediments in the upper reaches was significantly higher than in the middle or lower reaches (p < 0.05), except for DhL, whose Ex-Si was the highest. Mean TADSi and active NBSi were significantly higher in lakes than rivers (p < 0.05). Oxidation of sediments significantly increased TDSi in overlying water and active NBSi in sediments (p < 0.01). Si forms in six water bodies significantly depended on components of the sediments (e.g. active Ca2+, Mg2+, Fe, and Al3+) and water chemical parameters (p < 0.05). Our results suggest that immobilization of Si in water bodies in the Yangtze River Basin depends on the types of water bodies and sediments, lakes and Fe-Al dominated sediments have a high potential to immobilize Si, but anthropogenic interference should not be ignored.

Reducing substances are a mixture of different forms and types and play extremely important roles in manipulating the redox status of sediments, benthic habitats, and substance exchanges at the sediment-water interface in aquatic ecosystems. However, little is known about their abundance, forms, and reducibility in sediments. In this study, the procedures were developed to sequentially fractionate sediment reducing capacity (RC) fractions with the pH dependence of KMnO4 oxidability. The procedures were then applied to 60 sediments from 2 lakes and 3 reservoirs, generating an RCpH7.0 fraction (oxidized at ~0.48 V [reference: SHE]) and an RCpH2.0 fraction (oxidized at ~0.95 V [reference: SHE]), and the component of each fraction was characterized. The RCpH7.0 fraction amounted to 45.4 ± 25.9 cmol e-·kg-1 DW, and the RCpH2.0 fraction amounted to 42.8 ± 22.9 cmol e-·kg-1 DW; fraction sizes depended greatly on sediment origin. Reducing organic substances (ROS) were the main contributors to the RC fractions, with mean value of 30.0 ± 24.1 and 38.5 ± 22.2 cmol e-·kg-1 DW in RCpH7.0 (% contribution: 68.0 ± 5.3 % of RCpH7.0) and RCpH2.0 (90.0 ± 1.5 % of RCpH2.0), respectively. The next contributor was Fe(II), with mean value of 13.5 ± 8.2 and 3.8 ± 3.7 cmol e-·kg-1 DW in RCpH7.0 (28.3 ± 5.2 %) and in RCpH2.0 (9.9 ± 8.6 %), respectively. The smallest component was sulfide (Sn), which had a mean of 2.0 ± 3.1 cmol e-·kg-1 DW in RCpH7.0 and was essentially negligible in RCpH2.0. The number of electrons lost per mole of reducing substances (Ni) differed between the two RC fractions and among sediments of different origins. NROS was lower in the RCpH7.0 fraction (0.22 ± 0.09) compared to the RCpH2.0 fraction (0.31 ± 0.12) and significantly related to levels of active Fe(III) and sulfides (Sn) (p < 0.05). The opposite pattern was seen for NFe(II) and NSn. Based on the compositive reducing capacity (CRC) for the RCpH7.0 fraction, sediment redox status could be classified as ROS-Fe(II) (3.8 ± 1.7 cmol e-·kg-1 DW) or ROS-Sn (10.1 ± 4.8 cmol e-·kg-1 DW) (weaker vs. stronger, respectively; p < 0.01). The RC-based index provides a more comprehensive perspective on characterizing sediment redox status compared to the Eh.

Researches have proved that agricultural phosphorus (P) loss contributes significantly to surface water eutrophication. Various soil test P (STP) methods have been developed to assess the P loss risk from agricultural soils. In the intensively-cultivated Brittany region of Western France, hydromorphic soils in wetland domains exhibit high risks of leaching and transferring dissolved P -the most bio-available form of P -to surface waters. It remains unclear whether STP conventionally developed for well-drained soils can accurately predict the risk of dissolved P release from these hydromorphic soils. In this study, we measured the dissolved reactive P (DRP) concentrations in soil solutions sampled in situ from 26 hydromorphic soils in the Brittany region and examined their relationship with several STPs available on the corresponding soils, such as the degree of soil P saturation, the equilibrium soil P concentration, or the soil Olsen P, Dyer P, and water extractable P contents. DRP concentrations ranged from 0.01 to 0.310 mg P l- 1 (mean = 0.075 mg P l- 1), highlighting the potential of hydromorphic soils as hotspots for DRP release in agricultural landscapes. Correlations between DRP concentrations and STPs were relatively weak (0.09 < r2 < 0.64), indicating that conventional STPs are generally unable to accurately predict the DRP release risks in hydromorphic soils. Tentatively, Olsen P showed promises as a useful risk indicator, with a relatively high r2 value of 0.6 and wide inclusion in the current STP database, especially in the Brittany region. Nevertheless, this hypothesis requires further evaluation with additional data. This study confirms the high risk of dissolved P release from hydromorphic soils in agricultural wetland domains and emphasizes the need for developing specific risk assessment tools to these hydromorphic soils.

China has recently seen a substantial increase in integrated rice-crayfish (Procambarus clarkii) culture (IRCC). However, grain yield reductions (GYR) with rice plants having normal vegetative growth but failed filling are becoming common in IRCC, and the driving factors remain unclear. Here, we tried to identify the mechanisms behind this GYR mode in the Jianghan Plain, China, by investigating the soil properties, grain yield parameters, and plant mineral content in five IRCC plots experiencing GYR and an adjacent paddy-upland rotation plot (CK). The results showed slight to extreme GYR (11-82%) in the IRCC plots compared to those in CK. The translocation of phosphorus from shoot to grain during the productive stage of rice plants was inhibited in IRCC plots (26-71%) compared to CK (75%). The grain yield and phosphorus translocation coefficient correlated strongly (r > 0.95) with zinc uptake in aboveground rice organs which was significantly lower in IRCC than in CK. We conclude that this GYR in IRCC plots is not due to decreased macronutrients availability or heavy metals toxicity in IRCC soils, but the insufficient uptake of zinc by IRCC rice plants. We recommend Zn foliar spray or introducing Zn efficient rice cultivars as potential mitigation measures.

Increasing concerns over water eutrophication due to agricultural phosphorus (P) loss have led to the development of indicators to assess the risk of P release from agricultural soils. Recently, a logarithmic equation linking the degree of phosphorus saturation (DPS) to the simple water-soluble P (WSP) content of soils has been proposed as a universal method to assess this risk based, however, mainly on the analysis of well-drained soils. Here, we studied the P sorption properties and DPS values of 69 hydromorphic soils from cultivated and uncultivated wetland zones located in Brittany, Western France, to test whether the method could also apply to poorly-drained soils. The bulk soil analysis showed that P contents of the studied hydromorphic soils were 30% to 80% higher than P contents normally found in Brittany soils, evidencing a possible P enrichment process. Adsorption isotherms revealed a surprisingly high variability in the P sorption properties as a function of the location of the soil (maximum P adsorption capacity ranging from 500 to 1850 mg kg(-1)), which is caused by variations in the phases controlling P sorption in soil (from clay to organic matter and/or iron and aluminium oxides, depending on the soil location). Distinct relationships between DPS and WSP values were also obtained depending on the location of the soils. The obtained DPS versus WSP relationships showed that the P saturation threshold above which the risk of dissolved P release increases markedly is 30% lower on average for hydromorphic soils than for well-drained soils. Hydromorphic soils appear to be more at risk of releasing dissolved P at the same DPS values than well-drained soils. The present study indicates an underestimation of the P release risk from hydromorphic soils by the existing method developed for well-drained soils and calls for the development of specific risk assessment tools for hydromorphic soils, especially given on the strong spatial heterogeneity of their P sorption properties.

Growing evidence has demonstrated the influence of internal nitrogen (N) and phosphorus (P) on harmful algae blooms in eutrophic freshwater ecosystems. However, the main controlling factors for internal N and P release risks, and whether these factors vary as environmental conditions change, remains poorly understood. We evaluated potential release risks of N and P from sediments in two freshwater reservoirs in Beihai City, southern China, by evaluating apparent nutrient fluxes during simulated static incubation experiments at two temperatures (15 °C and 25 °C). Sediments were analyzed to determine their basic properties as well as N and P fractions. Results showed that the main controlling factors of the apparent fluxes in dissolved total P, soluble reactive P, total N, and ammonium were related to sediment adsorption properties, redox properties, and microbial-mediated properties (e.g., water-extractable P, total inorganic N, redox-sensitive P, total organic carbon, organic P). The primary controlling factors for apparent N and P fluxes were dependent on the form of N and P and changed with temperature. The results suggest that care should be taken when simply using total N and P contents in sediments to evaluate their internal nutrient release risks.

Protecting water quality at catchment scales is complicated by the high spatiotemporal variability in water chemistry. Consequently, determining pollutant sources requires costly monitoring strategies to diagnose causes and guide management solutions. However, recent studies have shown that spatial patterns in water chemistry can be persistent at catchment scales, potentially allowing identification of pollution sources and sinks with just a few sampling campaigns. Here, we tested a new method to quantify spatial persistence (SP) of water chemistry patterns with data from synoptic samplings in 22 headwater subcatchments within a 375 km 2 catchment in western France (March 2018 to July 2019). This new method to quantify SP reduces dependence on long‐term metrics such as flow‐weighted concentrations, which are usually uncertain or unavailable. We applied the method to 16 ecologically relevant water quality parameters, including soluble reactive phosphorus, nitrate, and dissolved organic carbon. The results showed an average SP of 0.68 among parameters during the study period. For most parameters, SP was higher during the high‐flow winter period but lower and more variable during the low‐flow summer period. We found that the SP ultimately depended on the ratio between the temporal and spatial coefficients of variation (variance explained: 70%) rather than the temporal synchrony among subcatchments (variance explained: 4%). These results demonstrate that in these temperate catchments, synoptic sampling during the high‐flow winter period allows efficient identification of source and sink subcatchments, while more frequent samplings are needed to characterize ecological conditions at low flow.

Acid-base reactivity is a fundamental property of sediments and is responsible for sediments' multiple roles in aquatic ecosystems. However, little information currently exists about the composition, magnitude, and change of the available acid consumption capacity (AACC) of sediments. To optimize reaction conditions, we developed operational procedures to determine AACC using base titration to recover surplus acid in suspensions. We characterized the sediment AACC of Dianchi Lake (DL), Daduhe River (DR), Tuojiang River (TR), Honghu Lake (HL), Wuhan Donghu Lake (DhL), and Taihu Lake (TL) in the Yangtze River Basin, China. The procedure demonstrated that reacting 40 mL 0.1 M HCl with fresh sediments equivalent to 1.0 g dry weight for 4 h and recovering surplus acid in the suspension by NaOH titration to an endpoint pH of 3.0 could determine sediment AACC. Sediment AACC in the Yangtze River Basin had high regional variability. The mean magnitude of AACC among sites was ranked DL > DR > DhL > TR > HL > TL, which is extremely similar to their geographical location from the upper to lower reaches of the Yangtze River Basin. Qualitative results from acid titration curves showed that more components contributed to AACC in DL, DR, TR, and DhL sediments than to those in HL and TL sediments. The correlation between AACC and the total amount of multivalent cations released indicated that AACC depended significantly on labile acid-soluble minerals that contain multivalent cations (Fe3+, Fe2+, Ca2+, Al3+, Mg2+, and Mn2+) (p < 0.01). Based on the contribution percentages of multivalent cations to AACC, sediment AACC of six water bodies were divided into two types: Ca-Mg dominated (DL, DR, and TR) and Fe-Al dominated (HL, DhL, and TL). We suggest that sediment AACC complexing with pH can contribute to a better description of the acid-base characteristics of sediments.

The majority of freshwater ecosystems worldwide suffer from eutrophication, particularly because of agriculture-derived nutrient sources. In the European Union, a discrepancy exists between the scale of regulatory assessment and the size of research catchments. The Water Framework Directive sets water quality objectives at the mesoscale (50-500 km2), a scale at which both hillslope and in-stream processes influence carbon (C), nitrogen (N) and phosphorus (P) dynamics. Conversely, research catchments focus on headwaters to investigate hillslope processes while minimising the influence of river processes on C-N-P dynamics. Because hillslope and river processes have common hydro-climatic drivers, the relative influence of each on C-N-P dynamics is difficult to disentangle at the mesoscale. In the present study, we used repeated synoptic sampling throughout the river network of a 300 km2 intensively farmed catchment, spatial stochastic modelling and mass balance calculations to analyse this mesoscale conundrum. The main objective was to quantify how river processes altered C-N-P hydrochemical dynamics in different flow, concentration and temperature conditions. Our results show that flow was the main control of alterations of C-N-P dynamics in the river network, while temperature and source concentration had little or no influence. The influence of river processes peaked during low flow, with up to 50% of dissolved organic carbon (DOC) production, up to 100% of nitrate (NO3) retention and up to 50% of total phosphorus (TP) retention. Despite high percentages of river processes at low flow, their influence on annual loads was low for NO3 (median of -10%) and DOC (median of +25%) but too variable to draw conclusions for TP. Because of the differing river alteration rates among carbon and nutrients, stoichiometric ratios varied greatly from headwaters to the outlet, especially during the eutrophication-sensitive low-flow season.

Quantifying nutrient attenuation at watershed scales requires long-term water chemistry data, water discharge, and detailed nutrient input chronicles. Consequently, nutrient attenuation estimates are largely limited to long-term research areas or modeling studies, constraining understanding of the ecological characteristics controlling nutrient attenuation and complicating efforts to protect or restore water quality in developed and developing regions. Here, we combined long-term data and a broad suite of biogeochemical parameters from 49 watersheds in northwestern France to test how well instantaneous measurements can predict nitrogen (N) and phosphorus (P) attenuation at watershed scales. We evaluated 13 biogeochemical and 12 hydrological proxies of hydrological flowpaths, residence time, and biogeochemical transformation. Across the 49 watersheds, nutrient attenuation ranged from 88 to -2% for N and 99-96% for P. The strongest biogeochemical proxies of N attenuation were NO3- isotopes, rare earth elements (REEs), radon, and turbidity, together explaining 75% of observed variation. For P attenuation, REEs, NO3- isotopes, molecular weight of dissolved organic matter, and radon were the strongest proxies, but only explained 27% of observed variation. However, a single hydrological parameter-annual runoff-explained 91% of N attenuation and the relative abundance of schist bedrock explained 56% of P attenuation. We discuss how runoff both controls and reflects watershed hydrology, biogeochemistry, and nutrient attenuation. For example, runoff was correlated with long-term decreases in nutrient concentration, demonstrating how leakier watersheds recover more quickly from nutrient saturation. Given the immense fertilization capacity of modern society, we propose that eutrophication can only be solved by reducing nutrient inputs, though hydrochemical proxies can provide valuable information on where to carry out essential food production activities.

Colloids (1-1,000 nm) are important phosphorus (P) carriers in agricultural soils. However, most studies are based on colloids from soil waters extracted in the laboratory, thus limiting the understanding of the natural transfer of colloidal P along the soil-to-stream continuum. Here, we conducted a field study on the colloidal P in both natural soil waters and their adjacent stream waters in an agricultural catchment (Kervidy-Naizin, western France). Soil waters (10-15 cm, Albeluvisol) of two riparian wetlands and the adjacent stream waters were sampled monthly during wet seasons of the 2015-2016 hydrological year (seven dates in total). Ultrafiltration at three pore sizes (5 kDa, 30 kDa, and 0.45 µm) was combined with inductively coupled plasma mass spectrometry (ICP-MS) to investigate variability in colloidal P concentration and its concomitant elemental composition. Results showed that colloidal P represented, on average, 45 and 30% of the total P (<0.45 µm) in the soil waters and stream waters, respectively. We found that colloidal P was preferentially associated with (a) organic carbon in the fine nanoparticle fraction (5-30 kDa) and (b) iron-oxyhydroxides and organic carbon in the coarse colloidal fraction (30 kDa-0.45 µm). The results confirmed that colloidal P is an important component of total P in both soil waters and stream waters under field conditions, suggesting that riparian wetlands are hotspot zones for the production of colloidal P at the catchment scale, which has the potential to be transported to adjacent streams.