Soil organic carbon accumulation following afforestation in a Japanese coniferous plantation based on particle-size fractionation and stable isotope analysis

Afforestation of grasslands often reduces the soil organic carbon (SOC) stock. However, in an earlier study, we observed a larger SOC stock in a 55-year-old Cryptomeria japonica D. Don. plantation stand converted from Miscanthus sinensis Andress grassland than in an adjacent natural forest stand (Arai and Tokuchi, 2010). To elucidate the factors resulting in the larger SOC stock in the plantation stand, we conducted particle-size fractionation (63–2000 μm as the coarse fraction and < 63 μm as the fine fraction) and 13C isotope analysis of the soils. It was expected that the larger SOC stock in the plantation stand resulted from the lack of difference in SOC stocks between stands in the fine fraction and greater accumulation of SOC in the coarse fraction.The results showed that the fine fraction contained more SOC than the coarse fraction in both stands, suggesting that physically protected SOC was predominant in the total SOC at this study site. On the other hand, no significant differences were found between stands in SOC stocks in the fine fraction, whereas marginally larger SOC accumulation was found in the coarse fraction in the plantation stand than in the natural forest. It was particularly obvious that the stock of water-extractable organic carbon (WEOC; C lost by wet sieving during particle-size fractionation) was larger in the plantation stand than in the natural forest. These results suggest a small initial SOC reduction caused by afforestation and a larger C accumulation in a relatively fresh, undecomposed, and uncomplexed form with mineral particles. Our findings indicate that afforestation on grasslands can increase SOC stocks when the physically protected SOC dominates and the decomposability of newly input C after afforestation is degraded.

Research Highlights
► No significant difference is observed between the fine fractions.
► Plantation stand contains marginally larger SOC stock in the coarse fraction than natural forest.
► Larger WEOC stock in the plantation stand is estimated than natural forest.
► Maintenance of old SOC and restriction of newly input C decomposition is important.