Comparison of nutrient budgets between three forested mountain watersheds on granite bedrock

The budgets of nutrients and major ions between the input flux from atmospheric deposition and the output flux to streams were measured on three forested small mountain watersheds, Aburahi-N (23.8 ha), Aburahi-S (3.8 ha, 2 km south from Aburahi-N) and Myokoji (1.77 ha, 28 km northwest from Aburahi-N), located at 34-35 N and 136 E around the central part of Japan. The bedrock of the watersheds is granite. The main vegetation of Aburahi-N and Aburahi-S was planted with Japanese cypress (Chamaecyparis obtusa Sieb. et Zucc), and Myokoji was a secondary forest growing deciduous broadleaf trees and Japanese red pine. The chemical analysis of the stream waters was carried out once a week at Aburahi-N from 1987 to 1991, at Aburahi-S from 1995 to 1998, and at Myokoji from 1991 to 1993. Wet and dry depositions were collected every month at Aburahi-N. The annual mean concentrations of total nitrogen (TN) of the stream water of Aburahi-N, Aburahi-S and Myokoji were 0.408, 0.614 and 0.349 mg/l, respectively, total phosphorus (TP) contents were 0.0074, 0.0046 and 0.0096 mg/l, and the potassium ion (K+) contents were 0.83, 0.49 and 0.77 mg/l. The average annual output fluxes of TN from the watersheds of Aburahi-N, Aburahi-S and Myokoji were calculated to be 5.85, 4.10 and 12.7 kg/ha/y, TP 0.131, 0.045 and 0.280 kg/ha/y, K+ 9.63, 3.23 and 9.70 kg/ha/y, and discharge 1062, 508 and 1265 mm/y, respectively. The annual outputs tended to increase as the annual precipitation increased. The average annual input fluxes of TN, TP and K+ for six years from 1990-1998 were 12.5, 1.31 and 7.64 kg/ha/y, respectively, and the precipitation averaged 1449 (1019-1947) mm/y.