Context
Nitrogen (N) and phosphorus (P) exports from rural landscapes can cause eutrophication of inland and coastal waters. Few studies have investigated the influence of the spatial configuration ...of nutrient sources—i.e. the spatial arrangement of agricultural fields in headwater catchments—on N and P exports.
Objectives
This study aimed to (1) assess the influence of the spatial configuration of nutrient sources on nitrate (NO
3
−
) and total phosphorus (TP) exports at the catchment scale, and (2) investigate how relationships between landscape composition (% agricultural land-use) and landscape configuration vary depending on catchment size.
Methods
We analysed NO
3
−
and TP in 19 headwaters (1–14 km², Western France) every two weeks for 17 months. The headwater catchments had similar soil types, climate, and farming systems but differed in landscape composition and spatial configuration. We developed a landscape configuration index (LCI) describing the spatial organisation of nutrient sources as a function of their hydrological distance to streams and flow accumulation zones. We calibrated the LCI’s two parameters to maximise the rank correlation with median concentrations of TP and NO
3
−
.
Results
We found that landscape composition controlled NO
3
−
exports, whereas landscape configuration controlled TP exports. For a given landscape composition, landscape spatial configuration was highly heterogeneous at small scales (< 10 km
2
) but became homogeneous at larger scales (> 50 km
2
).
Conclusions
The spatial configuration of nutrient sources influences TP but not NO
3
−
exports. An ideal placement of mitigation measures to limit diffuse TP export should consider both the hydrological distance to streams and flow accumulation zones.
Understanding how water and solutes enter and propagate through freshwater landscapes in the Anthropocene is critical to protecting and restoring aquatic ecosystems and ensuring human water security. ...However, high hydrochemical variability in headwater streams, where most carbon and nutrients enter river networks, has hindered effective modelling and management. We developed an analytical framework informed by landscape ecology and catchment hydrology to quantify spatiotemporal variability across scales, which we tested in 56 headwater catchments, sampled periodically over 12 years in western France. Unexpectedly, temporal variability in dissolved carbon, nutrients and major ions was preserved moving downstream and spatial patterns of water chemistry were stable on annual to decadal timescales, partly because of synchronous variation in solute concentrations. These findings suggest that while concentration and flux cannot be extrapolated among subcatchments, periodic sampling of headwaters provides valuable information about solute sources and subcatchment resilience to disturbance.
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.
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•Assessment of in-stream alterations of C-N-P dynamics in a mesoscale catchment•Repeated synoptic sampling of headwaters and stochastic landscape mixing modelling•In-stream processes influence C-N-P dynamics greatly but annual loads little.•Streamflow is the key control of in-stream processes.
It is well established that shallow tillage (ST) and no-tillage (NT) of cultivated soils can have higher risk of dissolved phosphorus (P) loss during runoff than conventional tillage (CT). However, ...quantitative descriptors of how differences in tillage practices influence soil P sorption properties and how these properties in turn influence dissolved P loss remain lacking. Moreover, the influence of tillage practices on the forms of dissolved P, particularly molybdate unreactive dissolved P (MUDP), has rarely been investigated. Thus, we simulated runoff using pilot soils that had experienced three tillage practices (CT, ST, and NT) and two types of fertilization (pig and poultry manure) for 20 years. The results indicated that shifting from CT to ST or NT changed the soil’s capacity to bind P and increased Olsen-P, Dyer-P, equilibrium P concentration (EPC0), water soluble P (WSP), and the degree of P saturation (DPSOX). The experiment confirmed that switching from CT to ST or NT could increase P loss during runoff. The increase was evident for the two fertilizations (pig manure and poultry manure) with an amplified effect for the poultry manure modality. Switching from CT to ST or NT increased the loss of MUDP, which again was higher for poultry manure fertilization, for which the percentage of MUDP increased to more than 40% of TDP. Correlations between the risk of dissolved P loss and risk assessment indicators were positive, but DPSOX was opposite predicted results to other indicators, indicating that DPSOX was not suitable for predicting the risk of dissolved P loss. Our results suggest that MUDP should be considered when assessing the risk of dissolved P loss when adopting ST and NT, since MUDP is bioavailable and thus can contribute to the eutrophication of surface water.
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•No-tillage increased the risk of loss of three fractions of dissolved P.•No-tillage exacerbated P lost as molybdate unreactive dissolved P (MUDP).•High organic matter content contributed most to the increased loss of MUDP.
The long‐term evolution of nutrient dynamics in rivers under changing external forcings, termed hereafter trajectory, is influenced by local human activities and regional climatic variations. Here we ...investigate nitrogen (N) and phosphorus (P) dynamics in seven mesoscale agricultural catchments (median size 800 km2) of western France from seasonal to multidecadal time scales (1970–2016). Results show that, in these catchments dominated by shallow groundwater, long‐term nitrate exports responded to variations of the agricultural N surplus with time lags of approximately 10 years. Presence of legacy N storage, related to the catchments' denitrification capacity, was found to increase response times. In contrast, P trends were predominantly controlled by decreasing point source emissions during the study period, and P dynamics were influenced by in‐stream retention/remobilization processes that hampered precise quantification of land‐to‐river diffuse transport processes. Occurrence of interannual climate variations during three 5‐ to 10‐year dry‐wet cycles, influenced by the North Atlantic Oscillation, affected N and P dynamics with persistent interannual hysteresis patterns among catchment and years. Thus, water quality assessment programs should cover at least five years to decipher the effect of mitigation measures from climate variations.
Key Points
Long‐term N and P trends disentangled from effect of interannual climate variability
N and P trends responded to changes in agricultural N surplus and point source P inputs, respectively
Time lags in N load response to variations in inputs influenced by legacy storage in catchments
In wetlands, large quantities of dissolved organic matter (DOM) are solubilized under reducing conditions. Controlled incubations of a wetland soil were performed under oxic and anoxic conditions to ...investigate the extent to which the following processes account for this phenomenon: i) production of organic metabolites by microbes during soil reduction; ii) release of organic matter (OM) from Mn- and Fe-oxyhydroxides that undergo reductive dissolution; and iii) desorption of OM from soil minerals due to pH changes. Anaerobic incubation releases 2.5% of the total soil organic carbon (OC) as dissolved organic carbon (DOC), and is accompanied by a pH rise from 5.5 to 7.4 and by the soil Mn- and Fe-reduction. The three processes above all take place. However, anaerobic incubation at a constant pH of 5.5 (preventing OM desorption) releases only 0.5% of the total soil OC, while aerobic incubation at pH 7.4 (preventing Mn- and Fe-reduction) releases 1.7% of the total soil OC. By contrast, aerobic incubation at pH 5.5 (preventing both Mn- and Fe-reduction and pH rise) does not solubilize any DOC. The DOC released is markedly aromatic, indicating little contribution from microbial metabolites, but, rather, the presence of microbes leading to OM mineralization. The pH rise is the key factor controlling OM solubilization under reducing conditions. This rise of pH accounts for >
60% of the total released DOC, which is not due to reductive dissolution as such.
Aerobic and anaerobic incubation experiments on a wetland soil samples were used to assess the respective roles of organic matter (OM) release, Fe-oxyhydroxides reduction and redox/speciation changes ...on trace metal mobility during soil reduction. Significant amounts of Cu, Cr, Co, Ni, Pb, U, Th and Rare Earth Elements (REE) were released during anaerobic incubation, and were accompanied by strong Fe(II) and dissolved organic matter (DOM) release. Aerobic incubation at pH 7 also resulted in significant trace metal and DOM release, suggesting that Fe-oxyhydroxide reduction is not the sole mechanism controlling trace metal mobility during soil reduction. Using these results and redox/speciation modeling, four types of trace metal behavior were identified: (i) metals bound to organic matter (OM) and released by DOM release (REE); (ii) metals bound to both OM and Fe-oxyhydroxides, and released by the combined effect of DOM release and Fe(III) reduction (Pb and Ni); (iii) metals bound solely to soil Fe-oxyhydroxides and released by its reductive dissolution (Co); and (iv) metals for which release mechanisms are unclear because their behavior upon reduction is affected by changes in redox state and/or solution speciation (Cu, Cr, U and Th). Even though the process of soil Fe-oxyhydroxide reduction is important in controlling metal mobility in wetland soils, the present study showed that the dominant mechanism for this process is OM release. Thus, OM should be systematically monitored in experimental studies dedicated to understand trace metal mobility in wetland soils. Due to the fact that the process of OM release is mainly controlled by pH variations, the pH is a more crucial parameter than Eh for metal mobility in wetland soils.
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.
Highlights
Wetlands soils in western France show high variability in their phosphorus properties.
There is a high‐risk of dissolved P losses from wetland soils to watercourses.
The risk can remain high even in soils that are currently not cultivated
Degree of phosphorus saturation method to assess dissolved P release risk could underestimate it.
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