Seasonal changes in properties of abandoned terraced paddy field soil incubated under different water content conditions

Abstract

The chemical properties of soil samples collected in August and November from an abandoned terraced paddy field dominated by reeds were examined by in vitro incubation under normal moisture and flooded conditions. Soil pH extracted with water [pH(H2O)] was higher in soil samples collected from a depth of 0–10 cm in November than in samples collected in August; a high pH(H2O) was maintained even during nitrification under normal moisture conditions. When soil samples collected in August from a depth of 0–10 cm were incubated under flooded conditions, a significant decrease in reduction potentials (Eh) and an increase in total Fe2+ concentrations were observed. Reductive conditions during sampling were strong in soil samples collected in August from a depth of 40–50 cm. Moreover, under normal moisture conditions, soil samples collected in August showed significant decreases in pH(H2O) and significant production of water-soluble SO42− than those collected in November. Glucose addition to soil samples collected from a depth of 0–10 cm caused nitrogen immobilization under normal moisture conditions, increases in exchangeable Fe2+ and Mn2+, and decreases in exchangeable bases (Ca2+, Mg2+, K+, and Na+) under flooded conditions. Seasonal changes in soil properties were probably due to microbial activity and vegetation phenology; thus, the timing of soil sampling influenced incubation experiment results. When abandoned terraced paddy fields are created as biotopes, seasonal changes in reductive soil conditions and slope position must be considered to prevent soil acidification and base cation elution.