Assessing carbon stock changes in French top soils in croplands and grasslands: comparison of fixed depth and equivalent soil mass.

International audience

إنجليزي. There is consensus that equivalent soil mass (ESM) approaches are preferred over fixed-depth (FD) procedures for monitoring changes in soil organic carbon (SOC) stocks. However, ESM is not often implemented, even in cases in which land use induces changes in bulk density. In this presentation, we use data from the first and second campaign of the French soil quality monitoring network (RMQS) to compare SOC changes based on ESM and FD in top soils (~30 cm depth) in croplands and permanent pastures. We will illustrate a methodology to calculate ESM, and we will discuss cases where C stock changes are overestimated by FD calculations. The sites were selected based on the availability of data of the second RMQS campaign (ongoing sampling). In addition, we selected only the sites in which bulk density (BD) and coarse fragments (CF) were measured by the volumetric ring method in both campaigns (42 sites in croplands and 15 in pastures). At each site in both campaigns, a composite soil sample for carbon analysis was taken from a 400 m2 quadrangular area by combining soil cores taken every 5 m in a regular grid. A soil pit was dug adjacent to the quadrangular area, the soil profile was described up to 1 m depth, and three rings of 500 cm3 each were used estimate BD and CF in each horizon. The time between campaigns was in average 14 years for all the sites. The preliminary results show that in the second campaign, the difference between SOC stocks calculated based on ESM and FD was not significant. However, the linear regression between SOC stocks of the first and second campaign improves with the estimates based on ESM as compared to FD (R2: 0.95 vs R2: 0.90). Thus, our results show that ESM is more accurate than FD for studying SOC changes over time.