Spatial variability of soil solution chemistry under Hinoki cypress (Chamaecyparis obtusa) in Tama Hills

A monitoring study on precipitation and soil solution was conducted to analyze soil acidification processes at the Rolling Land Laboratory (RLL), Hachioji, Tokyo based on the spatial variability of the soil solution chemistry around the Hinoki cypress (Chamaecyparis obtusa) trunk. Soil solution samples were taken st various distances from the tree trunks and at various depths. Soil solution pH at the depth of 10 cm decreased to 4.1-4.2 on the downslope side of large tree trunks, presumably due to the heterogeneity of throughfall input and extensive infiltration of acidic stemflow. Ammonium ions brought by throughfall and stemflow were nitrified and provided large amounts of H(+). Protons were replaced with exchangeable cations, When base cations were depleted, aluminum ion became the dominant cation species. On the average, Ca(2+) concentration in the soil solutions at the depth of 10 cm decreased from 0.28 mmol(c) L(-1) at the reference site to 0.18 mmol(c) L(-1) on the downslope side and Mg(2+) concentration decreased from 0.30 mmol(c) L(-1) to 0.15 mmol(c) L(-1). Arithmetic mean aluminum concentration at the depth of 10 cm on the downslope side was 0.35 mmol(c) L(-1). Here aluminum dissolution was the main acid sink. Based on the spatial variability of the soil solution chemistry, soil solution acidification processes were divided into four stages.