Soil Carbon Content and Stable Isotopic Composition of Abandoned Agricultural Fields in Western Japan.

The aim of this study was to investigate how the cessation of cultivation influenced the belowground carbon pools in former agricultural fields. We measured soil stable carbon isotopic ratios (δ¹³C) and carbon contents in three abandoned agricultural fields in Higashi-Hiroshima, Japan. The abandoned fields were located within 0.20 km of one another and had been abandoned for 2, 5, or 10 years (2Y, 5Y, 10Y fields). The return of carbon in plant litters and roots to the soil caused the soil δ¹³C to increase after abandoned; thus, the soil δ¹³C value was lowest in the 2Y field (-24.2‰) and highest in the 10Y field (-22.1‰). While the large root biomass in fields after the long-term cessation of cultivation is a potential source of carbon input into the soil, the negative linear relationship between the soil δ¹³C value and the soil carbon concentration (δ¹³C = -5.0(soil carbon concentration) - 16.6, r² = 0.44, P < 0.001) indicates that succession (from C₃ crops to C₄ grasses) led to a decrease in the soil carbon concentration in the fields after abandonment. Before these abandoned fields were abandoned, long-term paddy rice cultivation might have supplied more organic matter or have caused the carbon loss to be less. Soil carbon dynamics in paddy soils is very different from that in upland soils because paddy soils are water-logged during the rice growing period. This anaerobic condition caused by water-logging leads to a delay in soil carbon decomposition and results in a higher soil carbon content level in paddy soils than in upland soils.