Changes in soil organic carbon in a mountainous French region, 1990–2004 N. P. A. Saby 1 , D. A rrouays 1 , V. A ntoni 2 , B. L emercier 3 , S. F ollain 4 , C. W alter 3 & C. S chvartz 4 1 Centre de Recherches d’Orle ´ans, Unite´ InfoSol, US 1106, BP 20619 Ardon, 45166 Olivet, France, 2 Institut Franc ¸ ais de l’Environnement, 5, Route d’Olivet, BP 16105, 45061 Orle´ans Cedex 2, France, 3 UMR SAS, INRA-Agrocampus Rennes, CS84215, 35042 Rennes, France, and 4 ISA, Laboratoire Sols et Environnement, 48 Boulevard Vauban, F-59046 Lille cedex, France Abstract This paper reports spatial and temporal changes at the regional level in soil organic carbon (SOC) using a soil-test database. A total of 23 329 SOC test values recorded between 1990 and 2004 by certi- fied commercial laboratories and collected in a mountainous French region (Franche-Comte´) were integrated in a database. Results show a strong trend in organic carbon content, mainly related to ele- vation. A large loss in SOC was observed over the survey period. This loss correlated with baseline SOC content with greater loss from soils with higher carbon content. This loss is likely to be due to both changes in land use from permanent grassland to cultivation and to an increase in temperature during the survey period. Our study demonstrates that past soil-test results which were not originally intended for monitoring can provide an alternative method for detecting changes in SOC. Keywords: Soil organic carbon, soil-test database, soil monitoring, spatial analysis, losses in organic carbon, France Introduction One consequence of the rise in atmospheric CO 2 has been increased research on organic carbon (OC) in soils (Schle- singer, 1986; Eswaran et al., 1993; Batjes, 1996; Paustian et al., 1997; Post et al., 1998; Smith et al., 2000; Lal, 2004; Sippola & Yli-Halla, 2005; Smith, 2005). The view that cli- mate change is being reinforced by increased CO 2 emission from soils is highly controversial (Jenkinson et al., 1991; Cao & Woodward, 1998; Cox et al., 2000; Davidson et al., 2000; Giardina & Ryan, 2000; Bellamy et al., 2005; Knorr et al., 2005; Powlson, 2005). Temperate climates favour OC accu- mulation in soils (Eswaran et al., 1993; Howard et al., 1995; Batjes, 1996; Arrouays et al., 2001; Jones et al., 2005) and a major concern for such regions is the change that may take place in the large SOC stocks as temperatures rise. Several studies have shown that when SOC is lost, the rate of loss increases with the initial OC content (Mann, 1986; Jolivet et al., 2003; Bellamy et al., 2005). In Europe, the largest SOC stocks occur at high latitudes and in high elevation areas (Jones et al., 2005). Marked contrasts in elevation in mountainous areas mean that it is possible to investigate cli- matic effects on SOC levels and on any changes within small regions. Assessments of long-term trends in SOC content based on measurements over large areas are very few because long-term resampling schemes are quite rare (e.g. Bellamy et al., 2005; Lettens et al., 2005). Low rates of change and large spatial variation in SOC require very high sampling density and sufficient duration to demonstrate changes (Hun- gate et al., 1996; Zar, 1996; Garten & Wullscheleger, 1999; Conant & Paustian, 2002a,b; Conen et al., 2003, 2004; Saby & Arrouays, 2004; Smith, 2004). In France, analytical results from samples of agricultural topsoil are stored in the national ‘Soil-test’ database. All data have been produced using standardized protocols by commercial soil-testing laboratories, certified by the French Ministry of Agriculture. The large number of analytical results stored in the database since 1990 and the even spatial distribution of sample locations at the canton level allow spatial analysis without data transformation or interpolation (Schvartz et al., 1997; Walter et al., 1997). This study focuses on changes in SOC for a French region with variable relief (Franche-Comte´ ) in the Jura Mountains and adjacent piedmont, and aims to demonstrate the spatial and temporal S U M 1 5 9 B Dispatch: 29.5.08 Journal: SUM CE: Krishna Sarma Journal Name Manuscript No. Author Received: No. of pages: 9 PE: Radhika Correspondence: N. Saby. E-mail: nicolas.saby@orleans.inra.fr Received October 2007; accepted after revision May 2008 Editor: Donald Davidson Soil Use and Management doi: 10.1111/j.1475-2743.2008.00159.x ª 2008 The Authors. Journal compilation ª 2008 British Society of Soil Science 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54