Effects of Forest Cutting in a Lower Slope on Soil Water and Stream Water Chemistry in a Watershed of Old Japanese Cedar and Cypress Stands

Abstract

The effects of forest cutting on nutrient dynamics, soil water content and chemistry, and stream water discharge rates and chemistry were examined for three years after the partial (lower slope ; 20% of total area) forest cutting of a watershed composed of artificial stands of old (99 years) Japanese cedar and cypress in Gunma prefecture, Japan. After the forest cutting, the soil water content in the surface soil layer (010 cm) decreased, although it increased in a deeper layer (3060 cm). The annual stream water discharge increased by about 100 mm after forest cutting. Compared to the period before cutting, the concentrations of NO₃^-, Ca²⁺, Mg²⁺, and K⁺ in surface soil water (5 cm) also increased during the summers of the first two years by about 37 times. However, three years after forest cutting, the concentrations decreased to the same levels as those in the period before cutting. Similarly, the stream water concentrations of NO₃^-, Ca²⁺, and Mg²⁺ increased 1.5 times. Seasonal changes in the concentrations of all analyzed ions in stream water increased in comparison with those before cutting. After forest cutting, the amount of NO₃^--N in the 3060 cm layer increased by four times the amount before cutting. Furthermore, this large amount persisted during the three consecutive years after the forest cutting. The NO₃^--N export to stream water increased up to 4.05.0 kg ha^-1 yr^-1 after cutting. However, the increase in the amount of NO₃^--N leaching to stream was smaller than that in soil water; this indicates the great potential of soil as a medium for NO₃^--N export. A higher amount of NO₃^--N in deeper soil (>30 cm) suggests retention and/or consumption of this ion in the soil even after the forest cutting.