Dissolved organic carbon dynamics in a UK podzolic moorland catchment: linking storm hydrochemistry, flow path analysis and sorption experiments

M I Stutter,D G Lumsdon,S M Dunn

doi:10.5194/bg-9-2159-2012

Abstract

Abstract. Better knowledge of spatial and temporal delivery of dissolved organic carbon (DOC) in small catchments is required to understand the mechanisms behind reported long-term changes in C fluxes from some peatlands. We monitored two storms with contrasting seasons and antecedent conditions in a small upland UK moorland catchment. We examined DOC concentrations and specific UV absorbance (SUVA at 285 nm), together with solute concentrations required to undertake end-member mixing analyses to define dominant flow paths contributing to streamflow. This was combined with laboratory soil-solution equilibrations. We aimed to resolve how seasonal biogeochemical processing of DOC and flowpath changes in organo-mineral soils combine to affect DOC exported via the stream. An August storm following a dry period gave maximum DOC concentration of 10 mg l−1. Small DOC:DON ratios (16–28) and SUVA (2.7–3.6 l mg−1 m−1) was attributed to filtration of aromatic compounds associated with up to 53% B horizon flow contributions. This selective filtration of high SUVA DOC was reproduced in the experimental batch equilibration system. For a November storm, wetter antecedent soil conditions led to enhanced soil connectivity with the stream and seven times greater DOC stream-load (maximum concentration 16 mg l−1). This storm had a 63% O horizon flow contribution at its peak, limited B horizon buffering and consequently more aromatic DOC (SUVA 3.9–4.5 l mg−1 m−1 and DOC:DON ratio 35–43). We suggest that simple mixing of waters from different flow paths cannot alone explain the differences in DOC compositions between August and November and biogeochemical processing of DOC is required to fully explain the observed stream DOC dynamics. This preliminary evidence is in contrast to other studies proposing hydrological controls on the nature of DOC delivered to streams. Although our study is based only on two storms of very different hydrological and biogeochemical periods, this should promote wider study of DOC biogeochemical alteration in headwaters so that this be better incorporated in modelling to predict the impacts of changes in DOC delivery to, and fate in, aquatic systems.

Highlights

The loss of dissolved organic carbon (DOC) from soils to waters is a critical process in C cycling within upland environments and influences geochemical and biological conditions in receiving aquatic ecosystems
In a recent paper Morel et al (2009) give three main reasons why examining the delivery of DOC to streams from their catchments during storm events is a critical part of furthering these explanations: (i) that the majority of DOC is exported during storm events, (ii) that event-duration changes in DOC concentrations gives insight into DOC delivery, and (iii) enables understanding and models of DOC sources and flowpaths to be developed
There was no apparent “first flush” effect of DOC release that is sometimes observed when a previously dried soil is initially rewet. In this discussion we focus on the evidence presented of concentration change over the events, concentration – discharge (C-Q) hysteresis form, hydrograph separation by end-member mixing approach (EMMA) and laboratory experimentation of soil – DOC interactions to answer three questions: 1. Do the contrasting antecedent hydrological conditions between the two storms bring differences in the sources and timing of DOC delivery from this small catchment to its stream?

Summary

Introduction

The loss of dissolved organic carbon (DOC) from soils to waters is a critical process in C cycling within upland environments and influences geochemical and biological conditions in receiving aquatic ecosystems. In a recent paper Morel et al (2009) give three main reasons why examining the delivery of DOC to streams from their catchments during storm events is a critical part of furthering these explanations: (i) that the majority of DOC is exported during storm events, (ii) that event-duration (i.e. hourly) changes in DOC concentrations gives insight into DOC delivery, and (iii) enables understanding and models of DOC sources and flowpaths to be developed. Stutter et al.: Dissolved organic carbon dynamics of the DOC in the stream and may provide an additional parameter discriminating DOC sources and interactions during transport

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