The fluvial flux of particulate organic matter from the UK: Quantifying in-stream losses and carbon sinks

Abstract

Summary This study considers records of fluvial suspended sediment concentration and its organic matter content from across the United Kingdom from 1974 to 2010. Suspended sediment, mineral concentration and river flow data were used to estimate the particulate organic matter (POM) concentration and flux. Median annual POM flux from the UK was 1596 ktonnes/yr. The POM concentration significantly declined after the European Commission’s Urban Wastewater Directive was adopted in 1991 although the POM flux after 1992 was significantly higher. Estimates of POM flux were compared to a range of catchment properties to estimate the flux of particulate organic carbon (POC) and particulate organic nitrogen (PON) as they entered rivers and thus estimate the net catchment losses. The total fluvial flux of N from the soil source to rivers was 2209 ktonnes N/yr with 814 ktonnes N lost at the tidal limit, and so leaving 1395 ktonnes N/yr loss to atmosphere from across UK catchments - equivalent to an N2O flux from UK rivers of between 33 and 154 ktonnes (N2O)/yr. The total fluvial flux of carbon from the soil source to rivers for the UK was 5020 ktonnes C/yr; the flux at the tidal limit was 1508 ktonnes C/yr, equivalent to 6.5 tonnes C/km2/yr. Assuming that all the net catchment loss goes into the atmosphere, then the impact of rivers on the atmosphere is 3512 ktonnes C/yr, equivalent to 15.2 tonnes C/km2/yr. The loss of POM from the UK suggests that soil erosion in the UK prevents soil being a net sink of CO2 and is instead a small net source to the atmosphere.