Assessing the source and delivery processes of organic carbon within a mixed land use catchment using a combined n-alkane and carbon loss modelling approach

peer reviewedPurpose: Understanding fluxes of soil organic carbon (OC) from the terrestrial to aquatic environments is crucial to evaluate their importance within the global carbon cycle. Sediment fingerprinting (SF) is increasingly used to identify land use-specific sources of OC, and, while this approach estimates the relative contribution of different sources to OC load in waterways, the high degree of spatial heterogeneity in many river catchments makes it challenging to precisely align the source apportionment results to the landscape. In this study, we integrate OC SF source apportionment with a carbon loss model (CLM) with the aim of: (i) reducing ambiguity in apportioning OC fluxes when the same land use exists in multiple locations within a catchment; and (ii) identifying factors affecting OC delivery to streams, e.g., buffer zones. Methods: Two main approaches were used in this study: (i) identification of the sources of freshwater bed sediment OC using n-alkane biomarkers and a Bayesian-based unmixing model; and (ii) modelling and analysis of spatial data to construct a CLM using a combination of soil OC content modelling, RUSLE soil erosion modelling and a connectivity index. The study was carried out using existing OC and n-alkane biomarker data from a mixed land use UK catchment. Results: Sediment fingerprinting revealed that woodland was the dominant source of the OC found in the streambed fine sediment, contributing between 81 and 85% at each streambed site. In contrast, CLM predicted that arable land was likely the dominant source of OC, with negligible inputs from woodland. The areas of the greatest OC loss in the CLM were predicted to be from arable land on steeper slopes surrounding the stream channels. Results suggest extensive riparian woodland disconnected upslope eroded soil OC and, concomitantly, provided an input of woodland-derived OC to the streams. It is likely the woodland contribution to streambed OC is derived from litter and leaves rather than soil erosion. Conclusion: This study demonstrates how location-specific OC sources and delivery processes can be better determined using sediment fingerprinting in combination with CLM, rather than using sediment fingerprinting alone. It highlights that, although wooded riparian buffer strips may reduce the impact of upslope, eroded soil OC on waterways, they could themselves be a source of OC to stream sediments through more direct input (e.g., organic litter or leaf debris). Characterising this direct woodland OC as a separate source within future fingerprinting studies would allow the contributions from any eroded woodland soil OC to be better estimated

Assessing the source and delivery processes of organic carbon within a mixed land use catchment using a combined n-alkane and carbon loss modelling approach

Purpose: Understanding fluxes of soil organic carbon (OC) from the terrestrial to aquatic environments is crucial to evaluate their importance within the global carbon cycle. Sediment fingerprinting (SF) is increasingly used to identify land use-specific sources of OC, and, while this approach estimates the relative contribution of different sources to OC load in waterways, the high degree of spatial heterogeneity in many river catchments makes it challenging to precisely align the source apportionment results to the landscape. In this study, we integrate OC SF source apportionment with a carbon loss model (CLM) with the aim of: (i) reducing ambiguity in apportioning OC fluxes when the same land use exists in multiple locations within a catchment; and (ii) identifying factors affecting OC delivery to streams, e.g., buffer zones. Methods: Two main approaches were used in this study: (i) identification of the sources of freshwater bed sediment OC using n-alkane biomarkers and a Bayesian-based unmixing model; and (ii) modelling and analysis of spatial data to construct a CLM using a combination of soil OC content modelling, RUSLE soil erosion modelling and a connectivity index. The study was carried out using existing OC and n-alkane biomarker data from a mixed land use UK catchment. Results: Sediment fingerprinting revealed that woodland was the dominant source of the OC found in the streambed fine sediment, contributing between 81 and 85% at each streambed site. In contrast, CLM predicted that arable land was likely the dominant source of OC, with negligible inputs from woodland. The areas of the greatest OC loss in the CLM were predicted to be from arable land on steeper slopes surrounding the stream channels. Results suggest extensive riparian woodland disconnected upslope eroded soil OC and, concomitantly, provided an input of woodland-derived OC to the streams. It is likely the woodland contribution to streambed OC is derived from litter and leaves rather than soil erosion. Conclusion: This study demonstrates how location-specific OC sources and delivery processes can be better determined using sediment fingerprinting in combination with CLM, rather than using sediment fingerprinting alone. It highlights that, although wooded riparian buffer strips may reduce the impact of upslope, eroded soil OC on waterways, they could themselves be a source of OC to stream sediments through more direct input (e.g., organic litter or leaf debris). Characterising this direct woodland OC as a separate source within future fingerprinting studies would allow the contributions from any eroded woodland soil OC to be better estimated.Natural Environment Research Council (NERC): NE/R010218/1. Biotechnology and Biological Sciences Research Counci

Assessing branched tetraether lipids as tracers of soil organic carbon transport through the Carminowe Creek catchment (southwest England)

Soils represent the largest reservoir of organic carbon (OC) on land. Upon mobilization, this OC is either returned to the atmosphere as carbon dioxide (CO2) or transported and ultimately locked into (marine) sediments, where it will act as a long-term sink of atmospheric CO2. These fluxes of soil OC are, however, difficult to evaluate, mostly due to the lack of a soil-specific tracer. In this study, a suite of branched glycerol dialkyl glycerol tetraethers (brGDGTs), which are membrane lipids of soil bacteria, is tested as specific tracers for soil OC from source (soils under arable land, ley, grassland, and woodland) to sink (Loe Pool sediments) in a small catchment located in southwest England (i.e. Carminowe Creek draining into Loe Pool). The analysis of brGDGTs in catchment soils reveals that their distribution is not significantly different across different land use types (p > 0:05) and thus does not allow land-use-specific soil contributions to Loe Pool sediments to be traced. Furthermore, the significantly higher contribution of 6-methyl brGDGT isomers in creek sediments (isomerization ratio (IR) D 0:48 ± 0:10, mean ± standard deviation (SD); p < 0:05) compared to that in catchment soils (IR D 0:28±0:11) indicates that the initial soil signal is substantially altered by brGDGT produced in situ. Similarly, the riverine brGDGT signal appears to be overwritten by lacustrine brGDGTs in the lake sedimentary record, indicated by remarkably lower methylation of branched tetraethers (MBT05ME D 0:46 ± 0:02 in creek bed sediments and 0:38 ± 0:01 in lake core sediments; p < 0:05) and a higher degree of cyclization (DC D 0:23±0:02 in creek bed sediments and 0:32±0:08 in lake core sediments). Thus, in this small catchment, brGDGTs do not allow us to trace soil OC transport. Nevertheless, the downcore changes in the degree of cyclization and the abundance of isoprenoid GDGTs produced by methanogens in the Loe Pool sediment do reflect local environmental conditions over the past 100 years and have recorded the eutrophication history of the lake. © 2020 Author(s)

Tracing of particulate organic C sources across the terrestrial-aquatic continuum, a case study at the catchment scale (Carminowe Creek, southwest England)

Soils deliver crucial ecosystem services, such as climate regulation through carbon (C) storage and food security, both of which are threatened by climate and land use change. While soils are important stores of terrestrial C, anthropogenic impact on the lateral fluxes of C from land to water remains poorly quantified and not well represented in Earth system models. In this study, we tested a novel framework for tracing and quantifying lateral C fluxes from the terrestrial to the aquatic environment at a catchment scale. The combined use of conservative plant-derived geochemical biomarkers n-alkanes and bulk stable δ13C and δ15N isotopes of soils and sediments allowed us to distinguish between particulate organic C sources from different land uses (i.e. arable and temporary grassland vs. permanent grassland vs. riparian woodland vs. river bed sediments) (p < 0.001), showing an enhanced ability to distinguish between land use sources as compared to using just n-alkanes alone. The terrestrial-aquatic proxy (TAR) ratio derived from n-alkane signatures indicated an increased input of terrestrial-derived organic matter (OM) to lake sediments over the past 60 years, with an increasing contribution of woody vegetation shown by the C27/C31 ratio. This may be related to agricultural intensification, leading to enhanced soil erosion, but also an increase in riparian woodland that may disconnect OM inputs from arable land uses in the upper parts of the study catchment. Spatial variability of geochemical proxies showed a close coupling between OM provenance and riparian land use, supporting the new conceptualization of river corridors (active river channel and riparian zone) as critical zones linking the terrestrial and aquatic C fluxes. Further testing of this novel tracing technique shows promise in terms of quantification of lateral C fluxes as well as targeting of effective land management measures to reduce soil erosion and promote OM conservation in river catchments

Assessing n-alkane and neutral lipid biomarkers as tracers for land-use specific sediment sources

Sediment fingerprinting (SF) methods using taxonomic-specific biomarkers such as n-alkanes have been successfully used to distinguish sediment sources originating from different land uses at a catchment scale. In this study, we hypothesise that using a combination of soil biomarkers of plant, fungal and bacterial origin may allow greater discrimination between land uses in SF studies. Furthermore, we assess if the inclusion of short chain (shorter than C22) neutral lipid fatty acids (SC-NLFA) improves land use discrimination, considering the Loch Davan catchment (34 km2) in Scotland as a case study. Fatty acids are commonly used to measure abundance and diversity of soil microbial and fungal communities. The spatial distribution of these soil communities has been shown to depend mainly on soil properties and, therefore, soil types and land management practices. The n-alkane and SC-NLFA concentrations and their compound specific stable isotope signatures (CSSI) in four land cover classes (crop land, pasture, forest, and moorland) were determined and their contribution to six virtual sediment mixture samples was modelled. Using a Bayesian un-mixing model, the performance of the combined n-alkane and SC-NLFA biomarkers in distinguishing sediment sources was assessed. The collection of new empirical data and novel combinations of biomarkers in this study found that land use can be distinguished more accurately in organic sediment fingerprinting when combining n-alkanes and SC-NLFA or using SC-NLFA and their CSSI alone. These results suggest that fingerprinting methods using the output of unmixing models could be improved by the use of multiple tracer sets if there is a commensurate way to determine which tracer set provides the “best” capacity for land use source discrimination. This new contribution to the organic sediment fingerprinting field highlights that different combinations of biomarkers may be required to optimise discrimination between soils from certain land use sources (e.g., arable-pasture). The use of virtual mixtures, as presented in this study, provides a method to determine if addition or removal of tracers can improve relative error in source discrimination. Combining biomarkers from different soil communities could have a significant impact on the identification of recent sources of sediment within catchments and therefore on the development of effective management strategies.Natural Environment Research Council (NERC): Biotechnology and Biological Sciences Research Council: NE/M009106/

Eliciting expert judgements to underpin our understanding of faecal indicator organism loss from septic tank systems

Septic tank systems (STS) in rural catchments represent a potential source of microbial pollution to watercourses; however, data concerning the risk of faecal indicator organism (FIO) export from STS to surface waters are scarce. In the absence of empirical data, elicitation of expert judgements can provide an alternative approach to aid understanding of FIO pollution risk from STS. Our study employed a structured elicitation process using the Sheffield Elicitation Framework to obtain expert judgements on the proportion of FIOs likely to be delivered from STS to watercourses, based on 36 scenarios combining: (i) septic tank effluent movement risk, driven by soil hydro-morphological characteristics; (ii) distance of septic tank to watercourse; and (iii) degree of slope. Experts used the tertile method to elicit a range of values representing their beliefs of the proportion of FIOs likely to be delivered to a watercourse for each scenario. The experts judged that 93 % of FIOs would likely be delivered from an STS to a watercourse under the highest risk scenario that combined (i) very high STS effluent movement risk, (ii) STS distance to watercourse 25 %. Under the lowest risk scenario, the proportion of FIOs reaching a watercourse would likely reduce to 5 %. Expert confidence was high for scenarios that represented extremes of risk, while uncertainty increased for scenarios depicting intermediate risk conditions. The behavioural aggregation process employed to obtain a consensus among the experts proved to be useful for highlighting both areas of strong consensus and high uncertainty. The latter therefore represent priorities for future empirical research to further improve our understanding of potential pollution risk from septic tanks and in turn enable better assessments of potential threats to water quality in rural catchments throughout the world where decentralised wastewater systems are common

Twenty-three unsolved problems in hydrology (UPH) – a community perspective

This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through on-line media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focussed on process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come

What is the deal with the Green Deal: Will the new strategy help to improve European freshwater quality beyond the Water Framework Directive?

Agricultural land use covers almost half of the EU territory and reducing nutrient and pesticide losses to freshwaters is central to existing EU policy. However, the progress of improving freshwater quality and reducing eutrophication is slow and lags behind targets. The Green Deal is a key element of the EU plans to implement the United Nation's Sustainable Development Goals. Here, we discuss the opportunities that the Green Deal and associated strategies may provide for the achievement of the water quality goals of the Water Framework Directive in agricultural landscapes. We welcome Green Deal's aspirational stated goals. However, the reliance of mitigation of diffuse agricultural pollution on the reform of the Common Agricultural Policy represents grave risks for practical implementation and the achievement of the Green Deal objectives. We also argue that the new strategies should be targeted at tackling and understanding the sources of water quality problems along the full pollution continuum. To maximise the opportunities for tackling diffuse pollution from agricultural land use and achieving the delayed water quality targets, we stress that a range of targeted new instruments will be needed to close the gaps in the pollution continuum ‘from source to impact’. These gaps include: (I) smart and standardised monitoring of the impacts of proposed eco-schemes and agri-environment-climate measures, (ii) active restoration of agricultural streams and ditches and their floodplains to reduce secondary pollution sources, (iii) options to draw down nutrient levels to or below the agronomic optimum that reduce legacy sources, (iv) integrating farm-scale and catchment-scale analysis of trade-offs in reducing different pollutants and their combined effects, and finally (v) accounting for emerging pressures to freshwater quality due to climate change. Incorporation of the pollution continuum framework into tackling diffuse agricultural pollution will ensure that the European water-related policy goals are achieved

A Framework for Assessing Concentration-Discharge Catchment Behavior From Low-Frequency Water Quality Data

Effective nutrient pollution mitigation measures require in-depth understanding of spatio-temporal controls on water quality which can be obtained by analyzing export regime and hysteresis patterns in concentration-discharge ((Formula presented.)) relationships. Such analyses require high-frequency data (hourly or higher resolution), hampering the assessment of hysteresis patterns in widely available low-frequency (monthly, biweekly) regulatory water quality data. We propose a reproducible classification of (Formula presented.) relationships considering export regime (dilution, constancy, enrichment) and long-term average hysteresis pattern (clockwise, no hysteresis, anticlockwise) applicable to low-frequency water quality data. The classification is based on power-law (Formula presented.) models with separate parametrization for low and high discharge and rising and falling hydrograph limb, enabling a better representation of (Formula presented.) dynamics. The classification has been applied to a 30-years record of daily streamflow and monthly spot samples of solute concentrations in 45 Scottish catchments with contrasting characteristics in terms of topography, climate, soil and land cover. We found that (Formula presented.) classification is solute- and catchment-specific and linked to upland versus lowland catchments and streamflow variability. However as the relationship between solute behavior and catchment characteristics is variable, we propose that future typologies should integrate both water quality response, that is, (Formula presented.) classification, and catchment characteristics. The data-driven (Formula presented.) classification allows us to increase the information content of low-frequency water quality data and thus inform mitigation measures, monitoring strategies, and modeling approaches. Such approaches open up an ability to characterize processes and best management for a wider number of catchments, subject to regulatory surveillance and outside of research catchments. © 2021. The Authors