Abstract
Changes in nutrient budgets and hydrological processes due to the natural disturbance of pine wilt disease (PWD) were monitored in a small, forested watershed in Japan. The disturbance caused changes in soil nitrogen transformations. Pre‐disturbance, mineralized nitrogen remained in the form of NH~4~^+^, whereas in disturbed areas most mineralized nitrogen was nitrified. Stream NO~3~^−^ concentrations increased following PWD. There was a delay between time of disturbance and the increase of NO~3~^−^ in ground and stream waters. Stream concentrations of NO~3~^−^ and cations (Ca^2+^ + Mg^2+^) were significantly correlated from 1994 to 1996, whereas the correlation among NO~3~^−^, H^+^, and SO~4~^2−^ was significant only in 1995. Although both cation exchange and SO~4~^2−^ adsorption buffered protons, cation exchange was the dominant and continuous mechanism for acid buffering. SO~4~^2−^ adsorption was variable and highly pH dependent. The disturbance also resulted in slight delayed changes of input–output nutrient balances. The nitrogen contribution to PWD litter inputs was 7·39 kmol ha^−1^, and nitrogen loss from streamwater was less than 0·5 kmol ha^−1^ year^−1^ throughout the observation period. This large discrepancy suggested substantial nitrogen immobilization. Copyright © 2004 John Wiley & Sons, Ltd.