Small scale spatial variability of organic carbon stocks in litter and solum of a forested Luvisol

Soil organic carbon (OC) stocks are highly variable at the forest stand level. In order to analyse spatial variability at the stand scale, a study was conducted on a Luvisol developed from loess in Thuringia, Germany. At a 10,000 m² beech (Fagus sylvatica L.) forest site, soil samples were taken at 54 points from the litter layer and the complete solum whose thickness varied between 0.24 m and 0.96 m. With a sampling scheme that incorporated a minimum horizontal sampling distance of 0.2 m, OC stocks in the litter layer and the solum were characterised by ordinary statistics and geostatistics. With respect to the vertical distribution of OC stocks, results showed that the OC stocks were 0.4 kg OC m^{- 2} in the litter layer, 3.0 kg OC m^{- 2} in the solum 0-0.12 m depth, and 4.5 kg OC m^{- 2} in the solum > 0.12 m depth. This highlighted the importance of subsoils in terrestrial carbon balances. No spatial pattern was observed in the litter layer, whereas semivariograms of OC stocks in the solum 0-0.12 m depth and the solum > 0.12 m depth exhibited a range of less than 5.4 m. The nugget variance accounted for around 20% of the sill in the solum 0-0.12 m depth and for over 40% of the sill in the solum > 0.12 m depth. These results emphasised the high small scale variability at the stand scale. Ordinary and spatial correlation indicated that the OC stocks are strongly affected by the thickness of the solum, whereas a relation between the distance to the nearest tree and the magnitude of OC stocks was not found. Considering the variability of the study area, the minimum detectable difference (MDD) of OC stocks for n = 9 was 0.3 kg OC m^{- 2} in the litter layer, 2.0 kg OC m^{- 2} in the solum 0-0.12 m depth, and 3.6 kg OC m^{- 2} in the solum > 0.12 m depth. At this site more than 300 samples would be required to detect a relative OC stock change of 10% in the solum.