Human impact on long‐term organic carbon export to rivers

Valentina Noacco,Thorsten Wagener,Nicholas J K Howden,Fred Worrall,Tim P Burt

doi:10.1002/2016jg003614

Abstract

AbstractAnthropogenic landscape alterations have increased global carbon transported by rivers to oceans since preindustrial times. Few suitable observational data sets exist to distinguish different drivers of carbon increase, given that alterations only reveal their impact on fluvial dissolved organic carbon (DOC) over long time periods. We use the world's longest record of DOC concentrations (130 years) to identify key drivers of DOC change in the Thames basin (UK). We show that 90% of the long‐term rise in fluvial DOC is explained by increased urbanization, which released to the river 671 kt C over the entire period. This source of carbon is linked to rising population, due to increased sewage effluent. Soil disturbance from land use change explained shorter‐term fluvial responses. The largest land use disturbance was during the Second World War, when almost half the grassland area in the catchment was converted into arable land, which released 45 kt C from soils to the river. Carbon that had built up in soils over decades was released to the river in only a few years. Our work suggests that widespread population growth may have a greater influence on fluvial DOC trends than previously thought.

Highlights

Global rivers transport, turn over, or store around 2.8 Pg of terrestrial organic carbon per year, a large fraction of the net terrestrial production [Cole et al, 2007; Battin et al, 2009a; Regnier et al, 2013]
We show that 90% of the long-term rise in fluvial dissolved organic carbon (DOC) is explained by increased urbanization, which released to the river 671 kt C over the entire period
We develop a model linking basin properties and DOC release to the river by adapting a UK nation-scale DOC export model [Worrall et al, 2012] and combine it with a model of the effects of land use and land use change (LU and LUC) on soil organic carbon (SOC) release [Bell et al, 2011] to estimate the relative influence of drivers of land use, climate, and population on DOC dynamics for the Thames basin since 1884

Summary

Introduction

Turn over, or store around 2.8 Pg of terrestrial organic carbon per year, a large fraction of the net terrestrial production [Cole et al, 2007; Battin et al, 2009a; Regnier et al, 2013]. Fluxes of carbon to world rivers have increased by 1.1 Pg C yrÀ1 since the preindustrial era (with a similar amount released from them), and previous studies suggest soils as the main source [Regnier et al, 2013; Leach et al, 2016]. These fluxes are a net C transfer between two of the world’s major carbon stores, the terrestrial biosphere and the ocean, so it is important to elucidate the physical mechanisms that underlie this significant change [Hope et al, 1994; Cole et al, 2007; Jansen et al, 2014] for informed decision making, policy development, and insight into future trends. There has been little focus to quantify fluvial organic C transfers from terrestrial ecosystems to the oceans in lowland basins that host both intensive agricultural activity, and large and growing human (predominantly urban) populations, mainly due to the absence of long-term data to compare the relative influence of different drivers (past and present) of natural and anthropogenic activity in such areas

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