Nitrogen Saturation in Japanese Forested Watersheds

Biogeochemistry of nitrogen was evaluated in a series of small watersheds in Gunma Prefecture, 100 km northwest of Tokyo in Japan. The forest vegetation ranged in age from 7 to 86 yr and included conifer plantations and naturally regenerated hardwoods. In a watershed with 24-yr-old stands of sugi (Cryptomeria japonica) and hinoki (Chamaecyparis obtusa), stream water NO3− concentrations were lower (21 μmol/L) than watersheds composed of 7-, 84-, and 86-yr-old stands (62, 64, and 100 μmol/L, respectively). These differences among watersheds were mostly attributable to differences in the increments of N by forest vegetation. The absence of seasonal variation of stream NO3− concentration in watersheds with either coniferous or hardwood forests suggests that N availability was in excess of biotic demands during all seasons. Over 6 yr, inorganic N output by stream water (13.5 kg N·ha−1·yr−1) exceeded N input as bulk precipitation (10.5 kg N·ha−1·yr−1) in an 86-yr-old coniferous stand. The high N output was related to high N input by precipitation, high mineralization rate, and low N wood increment. The high NO3− concentrations, absence of seasonal variation, and high loss rates of N through drainage waters together suggest that some Japanese watersheds are exhibiting “nitrogen saturation.”