The effect of organic matter application on carbon sequestration and soil fertility in upland fields of different types of Andosols

We evaluated the effect of long-term application of organic matter on soil organic carbon (SOC), and physico-chemical properties of different types of Andosols in the cool temperate zone. In 2012, a soil survey was conducted in 34 fields in Toya, Hokkaido, Japan, using reference data collected from 2001 to 2003 in the same fields. The region contains three groups of soil series (K1, K2 and HO), with soils of K1 (Vitric Andosols) being strongly affected by newly (1769–2000) erupted volcanic ash compared with K2 and HO (Silandic Andosols). The mean application rate of organic matter was 1.45 ± 1.27 Mg carbon (C) ha ⁻¹ yr ⁻¹. Mean SOC contents in 2001–2003 and 2012 were 26.5 ± 8.1 and 25.7 ± 5.7 g C kg ⁻¹, respectively, and mean SOC stocks were 39.6 ± 10.2 and 37.9 ± 7.4 Mg C ha ⁻¹ 0.15 m ⁻¹, respectively. There were no significant differences in either SOC content or stock. However, the rate of change in SOC between 2001–2003 and 2012 (ΔSOC) was significantly higher in K1 than in K2, and SOC decreased significantly in K2. There was a significant negative correlation between initial SOC and rate of ΔSOC: rate of ΔSOC content (g C kg ⁻¹ yr ⁻¹) = −0.046 × Initial SOC content (g C kg ⁻¹) + 1.12 and rate of ΔSOC stock (Mg C ha ⁻¹ 0.15 m ⁻¹ yr ⁻¹) = −0.052 × Initial SOC stock (Mg C ha ⁻¹ 0.15 m ⁻¹) + 2.09. The thresholds of initial SOC content and stock (intercept at the x-axis) were calculated as 24.3 g C kg ⁻¹ and 36.1 Mg C ha ⁻¹ 0.15 m ⁻¹, respectively. These values were higher than those obtained in previous studies conducted in croplands probably because of the specific features of Andosols. ΔSOC content was significantly negatively correlated with Δbulk density, Δavailable phosporus and base saturation, but positively correlated with Δtotal porosity, Δphosphate absorption coefficient and cation exchange capacity. These findings indicate that application of organic matter increases SOC and improves soil fertility when the initial SOC is lower than the threshold, and highlight the difficulties in maintaining or accumulating SOC in Andosols because of the high SOC.