Increased Fluvial Dissolved Organic Carbon Fluxes over 130 Years of Land-Use Change in the Thames Basin

Abstract:

This study investigates drivers of changing dissolved organic carbon (DOC) export in the UK’s River Thames basin between 1881 and 2011. Specifically, we consider how impacts of land-use change drive increases in DOC concentrations and fluxes at the basin outlet. First, we estimate soil organic carbon (SOC) stocks in the Thames basin for the period. Second, SOC losses due to land-use change are partitioned into DOC lost to surface waters through runoff, DOC leached into deeper soils and groundwater, and losses to the atmosphere as CO₂.

SOC stocks for each year are calculated from a large database of typical SOC levels for land-uses present in the Thames basin and are combined with literature values of transition times for SOC to adjust to a new level following land-use change. We also account for climate change effects on SOC stock due to temperature increases, which reduces SOC stocks as soil organic matter turnover rates increase. Soil carbon fluxes are calculated as the inter-annual change in SOC. We use a 130 year record of DOC concentration in the Thames, and parameters from previous long-term nitrate modeling, to constrain estimates of fluvial DOC rises caused by SOC losses.

We developed a sewage model to evaluate the relative contribution of point and diffuse sources to the total DOC flux. The results show that sewage effluent point sources do not contribute to DOC concentration at the monitoring point, except for isolated periods of exceptionally low flow.

Our work shows for the majority of years, diffuse sources are the main contributor to annual DOC loads. Moreover even though there are many small inter-annual variations in DOC concentration, the major change in both estimated SOC storage and fluvial DOC export occurred during WWII due to substantial changes in land-use, the legacy of which continues to date.