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

Christopher A Yates,Penny J Johnes,David L Jones,Charlotte E M Lloyd,Helen C Glanville,Christopher D Evans,Francesca L Brailsford,Miles R Marshall,Tim Jickells,Alun T Owen,Richard P Evershed

doi:10.1002/lno.11186

Abstract

AbstractDissolved 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.

Highlights

Global flux estimates of dissolved organic carbon (DOC) suggest that river networks are responsible for the transport of 0.25 Pg C yr−1, the largest transfer of reduced carbon from the
Dominant land classifications sampled during this study include: blanket peatland (BP), improved grassland (IG), coniferous woodland (CW), acid grassland (AG), low-productivity grassland (LPG), arable and horticultural (AH), and calcareous grassland (CG)
When sites are ranked according to total dissolved nitrogen (TDN) (Fig. 3a) and total dissolved phosphorus (TDP) (Fig. 3b) concentration, a clear pattern emerges in the proportion of the TDN and TDP present in the water column in the form of dissolved organic nitrogen (DON)

Summary

Introduction

Global flux estimates of dissolved organic carbon (DOC) suggest that river networks are responsible for the transport of 0.25 Pg C yr−1, the largest transfer of reduced carbon from the. Is DOM concentration known to vary in relation to catchment character (Palviainen et al 2016) and a wide range of studies have observed compositional differences in DOM related to specific catchment sources (Mattsson et al 2005; Spencer et al 2007; Hernes et al 2008; Yates et al 2016) These differences reflect the influence of land use and management, soil type, and hydrological function in controlling the rates of microbial decomposition, nutrient cycling, uptake within the soil, and the net flux of DOM from different soil horizons to adjacent waters (Austnes et al 2010). Autochthonous production through both autotrophic and heterotrophic pathways is a significant source contributing N- and P-rich organic compounds to the aquatic DOM pool (Roberts and Mulholland 2007; Lutz et al 2012; Evans et al 2017)

Methods

Results

Discussion

Conclusion