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

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, forty-five 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 one-year period. Dominant land cover in each subcatchment included: arable and horticultural, blanket peatland, coniferous woodland, 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+, PO43-) and dissolved organic nutrient (DOC, DON and 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 comprised >80% of the TDN and 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.