Estimation of carbon stocks in boreal cropland soils - methodological considerations

High expectations to harness agricultural soils as sinks for atmospheric carbon (C) necessitate methods for verifying changes in soil organic C (SOC) stocks and improving the accuracy of regional SOC stock estimates specific to various land-use and management types. In this study we used a sample set included in the Finnish national soil monitoring network (n = 125) to determine the SOC stock and its vertical distribution in Finnish agricultural mineral soils using fixed depth (FD) and equivalent soil mass (ESM) methods. Further, we explored the within-field variation in SOC stock and minimum sample size in the context of SOC sequestration. Boreal agricultural mineral soils were found to be rich in SOC. The average SOC stock ranged between 8.4 and 9.8 kg m(-2)in the top 30 cm depending on cropping system and soil type. Cultivation of perennial crops tended to enrich the C close to the soil surface in comparison to dominantly annual crops, which exhibited a less steep C gradient with depth. The commonly used van Bemmelen factor of 1.724 to convert results of loss on ignition to SOC was found to be appropriate for coarse-textured Finnish agricultural soils. However, the results showed that in clay-rich soils, it is critical to take into account the structural water bound in clay particles to avoid overestimation of the SOC content. The study also demonstrated that the minimum sample size required to detect the change of 0.1 kg C m(-2)at field level is typically several hundreds. Therefore empirical soil sampling is not at all cost-effective enough for verifying the changes in SOC stocks with regard to compensation of C sequestration for farmers. Thus, alternative verification methods, such as SOC modelling in combination with field-level data collected, for example, using remote sensing should be developed further. Highlights Boreal agricultural mineral soils are rich in C Rotations where perennial crops dominated tended to enrich C in the soil surface Soil clay content should be taken into account when converting SOM to SOC The sample size required to verify SOC stock changes for C trading is extremely high