We investigated stream water chemistry at 81 forested watersheds in the Oki-Dogo Island in relation to topography and dominant vegetation in each watershed. Concentrations of Cl^-, Na⁺, K⁺, and Mg²⁺ in Oki-Dogo forest streams were much higher than those in Japanese average values, suggesting strong influence of sea salt. Positive correlations among HCO₃^-, Mg²⁺ and Ca²⁺ indicates that HCO₃^- is important counter anion of Mg²⁺ and Ca²⁺. Relatively low pH and concentrations of HCO₃^-, Mg²⁺ and Ca²⁺ were observed in the stream waters in higher elevation. Therefore, acid neutralizing capacity in the forested watersheds at higher elevation may be small compared to watersheds at the lower elevation forests. Mean NO₃^- concentration was 23.4 μmol L^-1, whereas one-fourth of the sampling sites exhibited higher NO₃^- concentration than expected value at the stream in nitrogen-saturated forest under N deposition level in Oki Islands. NO₃^- concentration was positively correlated to the highest elevation and range of slope steepness within the watersheds, suggesting that nitrogen loss tends to occur frequently in watersheds with hilly topography.

Non-crystalline minerals are formed through weathering processes from volcanic materials and strongly bind phosphorus (P), which may reduce the P availability for forest trees on volcanic soils. On the other hand, volcanic materials are a direct source of P and enhance soil total P. It has not been clarified how P availability and P cycling in forest ecosystems relate to non-crystalline minerals in volcanic soils. The objective of this study is to clarify the differences of soil nutrient availability and foliar nutrients of trees by comparing volcanic vs. non-volcanic soils. We selected eight secondary forests where Quercus serrata was dominant in Japan as study sites. We collected surface soils from three layers (0-5, 5-15, 15-30 cm depth) from each study site and canopy fresh leaves of Q. serrata. We measured the concentration of oxalate-extractable Al and Fe (Al_o and Fe_o), P fractions, and exchangeable-cations in soils and N and P in leaves. Both Al_o and Fe_o and soil nutrients (P fraction and exchangeable cations) were significantly higher in volcanic soils than in non-volcanic soils. Al_o + 1/2Fe_o was significantly correlated to NaOH-P (indicator of P bound to Al and Fe). Both plant available-P and NaOH-P were significantly positively correlated with foliar N and P concentrations. Estimated maximum heights of trees tended to be higher in volcanic soils. Our results indicated that the addition of volcanic ash directly increased soil P availability via increasing the pool of P and flux of P from recalcitrant and organic forms of P to available inorganic forms of P in soils.