CE-DYNAM (v1): a spatially explicit process-based carbon erosion scheme for use in Earth system models

Abstract

Abstract. Soil erosion by rainfall and runoff is an important process behind the redistribution of soil organic carbon (SOC) over land, thereby impacting the exchange of carbon (C) between land, atmosphere, and rivers. However, the net role of soil erosion in the global C cycle is still unclear as it involves small-scale SOC removal, transport, and redeposition processes that can only be addressed over selected small regions with complex models and measurements. This leads to uncertainties in future projections of SOC stocks and complicates the evaluation of strategies to mitigate climate change through increased SOC sequestration. In this study we present the parsimonious process-based Carbon Erosion DYNAMics model (CE-DYNAM) that links sediment dynamics resulting from water erosion with the C cycle along a cascade of hillslopes, floodplains, and rivers. The model simulates horizontal soil and C transfers triggered by erosion across landscapes and the resulting changes in land–atmosphere CO2 fluxes at a resolution of about 8 km at the catchment scale. CE-DYNAM is the result of the coupling of a previously developed coarse-resolution sediment budget model and the ecosystem C cycle and erosion removal model derived from the Organising Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE) land surface model. CE-DYNAM is driven by spatially explicit historical land use change, climate forcing, and global atmospheric CO2 concentrations, affecting ecosystem productivity, erosion rates, and residence times of sediment and C in deposition sites. The main features of CE-DYNAM are (1) the spatially explicit simulation of sediment and C fluxes linking hillslopes and floodplains, (2) the relatively low number of parameters that allow for running the model at large spatial scales and over long timescales, and (3) its compatibility with global land surface models, thereby providing opportunities to study the effect of soil erosion under global changes. We present the model structure, concepts, limitations, and evaluation at the scale of the Rhine catchment for the period 1850–2005 CE (Common Era). Model results are validated against independent estimates of gross and net soil and C erosion rates and the spatial variability of SOC stocks from high-resolution modeling studies and observational datasets. We show that despite local differences, the resulting soil and C erosion rates, as well as SOC stocks from CE-DYNAM, are comparable to high-resolution estimates and observations at subbasin level. We find that soil erosion mobilized around 66±28 Tg (1012 g) of C under changing climate and land use over the non-Alpine region of the Rhine catchment over the entire period, assuming that the erosion loop of the C cycle was nearly steady state by 1850. This caused a net C sink equal to 2.1 %–2.7 % of the net primary productivity of the non-Alpine region over 1850–2005 CE. This sink is a result of the dynamic replacement of C on eroding sites that increases in this period due to rising atmospheric CO2 concentrations enhancing the litter C input to the soil from primary production.

The soil carbon erosion paradox reconciled

“…For example, for European cropland subjected to a recent erosional disturbance 105 of c. 2 decades associated with mechanized tillage, a sink-term representing only 26 % of the eroded C was found (Van Oost et al, 2007). In contrast, for cropland subjected to >100 yr of continued water erosion, replacement rates of 58-100 % were found (Dymond, 2010;Li et al, 2015;Naipal et al, 2020). Thus, both observation-and model-based studies support the notion that the fraction of the eroded C that is replaced, and hence the erosion-induced sink term, increases with the duration of the erosional disturbance (Fig.…”

Modelling of land nutrient cycles: recent progress and future development

“…For global cropland area, estimates range from 1 to 26 Tg/year 7 . Representations of the transport and deposition of particulate organic matter by water for use in land surface models are being developed but have not been adapted for nutrient cycles nor included in earth system models 25 . The inclusion of these processes in nutrient models will likely affect the simulated evolution of phosphorus fluxes in the recent past and for future scenarios due to land uses and management.…”

Estimating the lateral transfer of organic carbon through the European river network using a land surface model

“…Global land surface models (LSMs) are important tools to simulate the feedbacks between terrestrial carbon cycle, increasing atmospheric CO2, and climate and land use change. However, the lateral carbon transfer, especially for the particulate organic carbon (POC), is still missing or incompletely represented in existing LSMs (Lauerwald et al, 2017;Lauerwald et al, 2020;Lugato et al, 2016;Naipal et al, 2020;Nakhavali et al, 2021;Tian et al, 2015). It has been hypothesized that the exclusion of lateral carbon transfer in LSMs implies a significant bias in the simulated global land carbon budget (Ciais et al, 2013;Ciais et al, 2021;Janssens et al, 2003).…”

“…However, the erosioninduced transport of POC, which is maybe even more important than the DOC transport in terms of lateral carbon flux (Lal., 2003;Tian et al, 2015;Tan et al, 2017), is still not or poorly represented in LSMs. The explicit simulation of the complete transport process of POC at large spatial scales is still a major challenge, due to the complexity of the processes involved, including erosion-induced sediment and POC delivery to rivers, deposition of sediment and POC in river channels and floodplains, re-detachment of the previously deposited sediments and POC, decomposition and transformation of POC in riverine and flooding waters, as well as the changes of soil profile caused by erosion and deposition (Doetterl et al, 2016;Naipal et al, 2020;Zhang et al, 2020).…”

“…However, it does not represent the POC deposition in floodplains, nor the impacts of soil erosion and floodplain deposition on the vertical profiles of soil organic carbon (SOC). The Carbon Erosion DYNAMics model (CE-DYNAM, Naipal et al, 2020) simulates erosion of SOC and its re-deposition on the toe-slope or floodplains, transport of POC along river channels, as well as the impact on SOC dynamics at the eroding and deposition sites.…”