Spatial distribution of soil organic carbon stocks in France

M P Martin,L Boulonne,C Jolivet,M Wattenbach,J Meersmans,D Arrouays,P Smith

doi:10.5194/bg-8-1053-2011

Abstract

Abstract. Soil organic carbon plays a major role in the global carbon budget, and can act as a source or a sink of atmospheric carbon, thereby possibly influencing the course of climate change. Changes in soil organic carbon (SOC) stocks are now taken into account in international negotiations regarding climate change. Consequently, developing sampling schemes and models for estimating the spatial distribution of SOC stocks is a priority. The French soil monitoring network has been established on a 16 km × 16 km grid and the first sampling campaign has recently been completed, providing around 2200 measurements of stocks of soil organic carbon, obtained through an in situ composite sampling, uniformly distributed over the French territory. We calibrated a boosted regression tree model on the observed stocks, modelling SOC stocks as a function of other variables such as climatic parameters, vegetation net primary productivity, soil properties and land use. The calibrated model was evaluated through cross-validation and eventually used for estimating SOC stocks for mainland France. Two other models were calibrated on forest and agricultural soils separately, in order to assess more precisely the influence of pedo-climatic variables on SOC for such soils. The boosted regression tree model showed good predictive ability, and enabled quantification of relationships between SOC stocks and pedo-climatic variables (plus their interactions) over the French territory. These relationships strongly depended on the land use, and more specifically, differed between forest soils and cultivated soil. The total estimate of SOC stocks in France was 3.260 ± 0.872 PgC for the first 30 cm. It was compared to another estimate, based on the previously published European soil organic carbon and bulk density maps, of 5.303 PgC. We demonstrate that the present estimate might better represent the actual SOC stock distributions of France, and consequently that the previously published approach at the European level greatly overestimates SOC stocks.

Highlights

Lowest soil organic carbon (SOC) stocks values were observed for vineyards and some uncultivated coastal areas
In this paper we provide a new estimate for the spatial distribution of SOC stocks in the top 30 cm of soils in France, based on the French monitoring network (RMQS)
This second estimate was 5.303 PgC, consistent with the SOC stocks published by the Joint Research Center (JRC) for European countries, and much higher than the estimate provided here, based on RMQS data

Summary

Introduction

The increasing concentration of greenhouse gases in the atmosphere has led to the need for reliable estimates of the amounts of organic carbon that might be sequestered by soils (Batjes, 1996; Eswaran et al, 1993; Lal, 2004; Paustian et al, 1997; Post et al, 1982; Saby et al, 2008a; Schlesinger, 1991).the organic matter contained in the earth’s soils is a large reservoir of carbon (C) that can act as a sink or source of atmospheric CO2. The world’s soils represent a large reservoir of C of about 1500 PgC (Batjes, 1996; Eswaran et al, 1993; Post et al, 1982) Accurate estimates of this pool are needed. Their reliability depends upon suitable data in terms of organic carbon content and soil bulk density and on the methods used to upscale point data to comprehensive spatial estimates. Several approaches involving empirical models to upscale SOC point measurements to the national level are found in the literature.

Methods

Results

Discussion

Conclusion