Global soil organic carbon removal by water erosion under climate change and land use change during AD 1850–2005

Victoria Naipal,Kristof Van Oost,Ronny Lauerwald,Philippe Ciais,Yilong Wang,Bertrand Guenet

doi:10.5194/bg-15-4459-2018

Abstract

Abstract. Erosion is an Earth system process that transports carbon laterally across the land surface and is currently accelerated by anthropogenic activities. Anthropogenic land cover change has accelerated soil erosion rates by rainfall and runoff substantially, mobilizing vast quantities of soil organic carbon (SOC) globally. At timescales of decennia to millennia this mobilized SOC can significantly alter previously estimated carbon emissions from land use change (LUC). However, a full understanding of the impact of erosion on land–atmosphere carbon exchange is still missing. The aim of this study is to better constrain the terrestrial carbon fluxes by developing methods compatible with land surface models (LSMs) in order to explicitly represent the links between soil erosion by rainfall and runoff and carbon dynamics. For this we use an emulator that represents the carbon cycle of a LSM, in combination with the Revised Universal Soil Loss Equation (RUSLE) model. We applied this modeling framework at the global scale to evaluate the effects of potential soil erosion (soil removal only) in the presence of other perturbations of the carbon cycle: elevated atmospheric CO2, climate variability, and LUC. We find that over the period AD 1850–2005 acceleration of soil erosion leads to a total potential SOC removal flux of 74±18 Pg C, of which 79 %–85 % occurs on agricultural land and grassland. Using our best estimates for soil erosion we find that including soil erosion in the SOC-dynamics scheme results in an increase of 62 % of the cumulative loss of SOC over 1850–2005 due to the combined effects of climate variability, increasing atmospheric CO2 and LUC. This additional erosional loss decreases the cumulative global carbon sink on land by 2 Pg of carbon for this specific period, with the largest effects found for the tropics, where deforestation and agricultural expansion increased soil erosion rates significantly. We conclude that the potential effect of soil erosion on the global SOC stock is comparable to the effects of climate or LUC. It is thus necessary to include soil erosion in assessments of LUC and evaluations of the terrestrial carbon cycle.

Highlights

Carbon emissions from land use change (LUC), recently estimated as 1.0±0.5 Pg C yr−1, form the second largest anthropogenic source of atmospheric CO2 (Le Quéré et al, 2016)
In total 7183 ± 1662 Pg of soil and 74 ± 18 Pg of soil organic carbon (SOC) is mobilized across all plant functional types (PFTs) by erosion during the period 1850– 2005, which is equal to approximately 46 %–74 % of the total net flux of carbon lost as CO2 to the atmosphere due to LUC over the same period estimated by our study (S1–S2)
In this study we introduced a 4-D modeling approach where we coupled soil erosion to the carbon cycle of ORCHIDEE and analyzed the potential effects of soil erosion, without sediment deposition or transport, on the global SOC stocks over the period 1850–2005

Summary

Introduction

Carbon emissions from land use change (LUC), recently estimated as 1.0±0.5 Pg C yr−1, form the second largest anthropogenic source of atmospheric CO2 (Le Quéré et al, 2016). Their uncertainty range is large, making it difficult to constrain the net land–atmosphere carbon fluxes and reduce the biases in the global carbon budget (Goll et al, 2017; Houghton and Nassikas, 2017; Le Quéré et al, 2016). V. Naipal et al.: Global soil organic carbon removal by water erosion is stabilized and buried for decades to millennia (Hoffmann et al, 2013a, b; Wang et al, 2017). Together with dynamic replacement of removed SOC by new litter input at the eroding sites, and the progressive exposure of carbon-poor deep soils, this translocated and buried SOC can lead to a net carbon sink at the catchment scale, potentially offsetting a large part of the carbon emissions from LUC (Berhe et al, 2007; Bouchoms et al, 2017; Harden et al, 1999; Hoffmann et al, 2013a; Lal, 2003; Stallard, 1998; Wang et al, 2017)

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Conclusion