Variation in dissolved organic matter (DOM) stoichiometry in U.K. freshwaters: Assessing the influence of land cover and soil C:N ratio on DOM composition

Abstract

Dissolved organic matter (DOM) plays an important role in freshwater biogeochemistry. To investigate the influence of catchment character on the quality and quantity of DOM in freshwaters, 45 sampling sites draining subcatchments of contrasting soil type, hydrology, and land cover within one large upland‐dominated and one large lowland‐dominated catchment were sampled over a 1‐yr period. Dominant land cover in each subcatchment included: arable and horticultural, blanket peatland, coniferous woodland, and improved, unimproved, acid, and calcareous grasslands. The composition of the C, N, and P pool was determined as a function of the inorganic nutrient species (NO3−, NO2−, NH4+, and PO43−) and dissolved organic nutrient (dissolved organic carbon [DOC], dissolved organic nitrogen [DON], and dissolved organic phosphorus [DOP]) concentrations. DOM quality was assessed by calculation of the molar DOC : DON and DOC : DOP ratios and specific ultraviolet absorbance (SUVA254). In catchments with little anthropogenic nutrient inputs, DON and DOP typically composed > 80% of the total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) concentrations. By contrast, in heavily impacted agricultural catchments DON and DOP typically comprised 5–15% of TDN and 10–25% of TDP concentrations. Significant differences in DOC : DON and DOC : DOP ratios were observed between land cover class with significant correlations observed between both the DOC : DON and DOC : DOP molar ratios and SUVA254 (rs = 0.88 and 0.84, respectively). Analysis also demonstrated a significant correlation between soil C : N ratio and instream DOC : DON/DOP (rs = 0.79 and 0.71, respectively). We infer from this that soil properties, specifically the C : N ratio of the soil organic matter pool, has a significant influence on the composition of DOM in streams draining through these landscapes.

Untargeted characterisation of dissolved organic matter contributions to rivers from anthropogenic point sources using direct‐infusion and high‐performance liquid chromatography/Orbitrap mass spectrometry

“…These data reveal little difference in the DOC concentrations and the SPE extracts of the water samples from the sewage outfall and the river. The DOC concentrations were similar for all samples at ca 3 mg C L −1 , sitting within the range of variation previously reported for UK rivers, including in this study, which ranged from 0.76 mg C L −1 in chalk catchments to >26 mg C L −1 in peat catchments 51. These data emphasise the ineffectiveness of DOC concentrations in revealing differences in the composition of the DOM pool, where markedly different compound mixes can share a similar DOC concentration.…”

Nutrient enrichment induces a shift in dissolved organic carbon (DOC) metabolism in oligotrophic freshwater sediments

“…Carbon (C), nitrogen (N) and phosphorus (P) are the nutrients which most limit primary production and microbial growth in freshwater ecosystems (Hill et al 2014). For dissolved organic nutrients in particular, the C, N and P cycles are inextricably linked as they can constitute parts of the same compound, however, there is still limited information on the composition of these molecules and how these cycles interact (Creamer et al 2014;Swenson et al 2015;Yates et al 2019). Defined as the compounds that pass through a 0.45 µm filter, dissolved organic matter (DOM) can be a key transport mechanism for nutrients in terrestrial environments and a source of energy for aquatic communities in low-nutrient status waters (Thurman 1985 autochthonous DOC production in rivers, this may not necessarily lead to an increase in labile C due to the enhancement of microbial growth and rates of organic matter degradation (Stanley et al 2011).…”

“…Spatial and temporal shifts in nutrient inputs to aquatic systems will affect the instream stoichiometry of the DOM pool (Yates et al 2019). This is likely to have a particular impact on river sediments, as the primary interface between the water column, hyporheic and groundwater flows, where the majority of nutrient and water exchange takes place (Boano et al 2014).…”

Determining the Impact of Riparian Wetlands on Nutrient Cycling, Storage and Export in Permeable Agricultural Catchments

“…In order to provide a full assessment of the cycling of nitrogen (N) and phosphorus (P) within and through the wetland, all water samples collected from the wetland soil water, gravels and chalk were analysed to determine the concentrations of dissolved inorganic N (NO 3 -N, NO 2 -N, NH 4 -N), dissolved organic N (DON), and soluble reactive P (SRP, measured as PO 4 -P), dissolved organic P (DOP), and dissolved organic C (DOC) as Non-Purgeable Organic Carbon (NPOC), using standard operating procedures and quality assurance protocols [23,25]. Samples were collected over a period of 4 h at each sampler nest on a weekly basis and returned to the laboratory at 4 • C in the dark.…”

“…Reported rates of nutrient 'removal' following the installation of riparian wetlands or 'buffer strips' are then erroneously low for catchment managers [8], or highly variable over the hydrological year [15]. Where a high frequency sampling programme has been undertaken which adopts an holistic approach to assessing C, N and P cycling and flux [23][24][25], very often different conclusions are drawn, and the capacity of wetlands to attenuate nutrient flux from catchment sources, to transform inorganic nutrient influx to organic nutrient forms stored within the wetland, and to export accumulated dissolved organic nutrient stores during flushing events becomes apparent [5,23,[26][27][28]. The best environmental policy is underpinned by sound and holistic science, but the resources required to undertake such holistic assessment are significant and often prohibitive even for single-site investigations.…”