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 km2 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.
Key Points
We found high spatial persistence of water chemistry, despite high spatiotemporal variability in water chemistry
Spatial persistence of water chemistry is primarily determined by the ratio between spatial and temporal variability
A single synoptic sampling during the high‐flow season allows efficient identification of source and sink subcatchments
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.
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.
Display omitted
•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.
Landscape organized (or structured) heterogeneity influences hydrological and biogeochemical patterns across space and time. We developed landscape indices that describe the spatial configuration of ...nutrient sources and sinks as a function of their hydrological distance to the stream (lateral dimension) or to the outlet (longitudinal dimension) and their intersection with flow‐accumulation areas. Using monthly nitrate, total phosphorus (TP), soluble reactive phosphorus (SRP) and daily discharge (Q) data from 221 rural catchments (1–300 km2) from 2010–2020, we observed higher variability in flow‐weighted mean concentrations in smaller catchments than in larger ones. The variability in landscape configurations also decreased with increasing catchment size. A landscape configuration index, calculated as mean arable land use weighted by spatial data on hydrological distance and flow accumulation, improved prediction of TP and SRP, but not nitrate, compared to the unweighted mean arable land use. We conclude that landscape configuration influences phosphorus transfer more than nitrate transfer, and that flow‐accumulation zones and riparian areas are critical source areas for TP and SRP, respectively. By contrast, landscape spatial configuration in the lateral (upslope–downslope) and longitudinal (upstream–downstream) dimensions did not have an identifiable influence on nutrients temporal dynamics. The indices developed in this study can help design landscapes that minimize diffuse phosphorus losses to streams and show that landscape management is not a first order control for nitrate losses.
We developed landscape indices that describe the spatial configuration of nutrient sources and sinks as a function of their hydrological distance to the stream or to the outlet and their intersection with flow‐accumulation areas. Landscape configuration influences phosphorus transfer but not nitrate transfer.
•N2O emissions from temperate agricultural soils were measured for 6 years.•Rainfall/soil moisture were strong controlling factors.•Cumulative N2O crops emissions ranked as follows: Legume< maize < ...cereals.•High emissions could occur in fallow periods and need to be investigated.
Nitrous oxide (N2O) emissions from agricultural soil were studied during 6 consecutive years on three plots of two real farms. We analyzed a total of 37 agricultural sequences: 20 sequences with crops including wheat, maize, and legumes and 17 fallow sequences with cover crops, volunteers or bare fallow soils. Controlling factors of N2O emissions were disentangled by using three approaches: (i) applying a random forest algorithm then extracting the most important predictors; (ii) analyzing all sequences separately as a function of the rain and soil mineral nitrate (SMN) content in soils; and (iii) experimentally, by watering soils with different SMN contents. The results showed that rainfall (and hence soil moisture) exerted the greatest control over N2O emissions, followed by daily maximum temperature and SMN. Regarding crop sequences, 6-year N2O cumulative emissions rates (g N2ON ha−1) ranked as follows: legumes (967) < maize (1192) < cereals (1375). Lowest cumulative N2O emissions at the Derrier conventional, no-till, farming plot (∼ 6000 kg N ha−1) was observed compared to ∼ 15000 kg N2ON ha−1 measured at the tilled organic farming Chantemerle and Barrière plots. Further, cumulative N2O emissions were rather important during fallow periods with or without cover crops. This study thus supports that N2O emissions need to be measured all year round, including both cropping and fallow periods to grasp the full rotation cycles. This long-term study of N2O emissions gathered a large number of data on N2O emissions over a variety of crops and fallow management and helped to initiate a typology of N2O emissions by crop.
Display omitted
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.
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.
Display omitted
•High dissolved reactive P concentrations in natural wetland soil waters were found•Conventional soil P tests fail to accurately predict the dissolved P release risk•The dissolved P release potential is dependent on soil hydromorphic class•It is imperative to take measures to avoid the accumulation of P in wetlands
The search for statistical techniques and forms of graphical representation that can explain the most relevant correlations among limnological variables can help interpret phenomena in a body of ...water. The objective of the article was to propose a graphical representation of the correlations among limnological variables applied in the contributing basin of the Dr. Joao Penido reservoir, in Juiz de Fora, Minas Gerais state, Brazil. Six sections were monitored monthly from May 2012 to April 2014, analysing 15 water quality parameters and their statistical correlations. The correlations were represented graphically with the program Gephi 0.8.2-beta. The influence of organic matter (of natural and anthropogenic origin resulting from pasture runoff and sewage) on water quality was verified, with an observed increase in water quality parameters especially nitrogen and phosphorous, oxygen consumed, chemical oxygen demand, turbidity and total suspended solids. It is concluded that the correlation chart assists in the understanding of the dynamics of the water quality parameters at the different sites analysed.
The search for statistical techniques and forms of graphical representation that can explain the most relevant correlations among limnological variables can help interpret phenomena in a body of ...water. The objective of the article was to propose a graphical representation of the correlations among limnological variables applied in the contributing basin of the Dr. Joao Penido reservoir, in Juiz de Fora, Minas Gerais state, Brazil. Six sections were monitored monthly from May 2012 to April 2014, analysing 15 water quality parameters and their statistical correlations. The correlations were represented graphically with the program Gephi 0.8.2-beta. The influence of organic matter (of natural and anthropogenic origin resulting from pasture runoff and sewage) on water quality was verified, with an observed increase in water quality parameters especially nitrogen and phosphorous, oxygen consumed, chemical oxygen demand, turbidity and total suspended solids. It is concluded that the correlation chart assists in the understanding of the dynamics of the water quality parameters at the different sites analysed. Key words: water resources, limnological variables, hydrographic basin A busca de tecnicas estatisticas e/ou formas de representacao grafica que consigam explicar as correlacoes mais relevantes entre as variaveis limnologicas podem auxiliar na interpretacao dos fenomenos atuantes naquele corpo d'agua. O objetivo do artigo e propor uma representacao grafica das correlacoes entre variaveis limnologicas aplicada na bacia de contribuicao da represa Dr. Joao Penido, em Juiz de Fora, Minas Gerais, Brasil. Foram monitoradas seis secoes mensalmente de maio de 2012 a abril de 2014, sendo analisados quinze parametros de qualidade da agua e estes correlacionados estatisticamente. As correlacoes foram entao representadas graficamente atraves do Programa Gephi 0.8.2-beta. Verificou-se a influencia da materia organica na qualidade da agua, de origem natural e antropica, oriundos de escoamento superficial das pastagens e do esgoto domestico, observando uma piora nos parametros de qualidade da agua com destaque para o nitrogenio e fosforo, oxigenio consumido, demanda quimica de oxigenio, turbidez e solidos suspensos totais. Conclui-se que o grafico de correlacao auxilia no entendimento da dinamica entre os parametros de qualidade da agua do local analisado. Palavras-chave: recursos hidricos, variaveis limnologicas, bacia hidrografica
PDF
