Nitrogen saturation in Japanese forests: an evaluation

Nitrogen biogeochemistry of 24 forested sites in Japan was used in evaluating the status of ‘nitrogen saturation’ for this region. Bulk deposition of inorganic N ranged from 3.5 to 10.5 kg N ha⁻¹ yr⁻¹ and losses in drainage waters ranged from 0.6 to 28 kg N ha⁻¹ yr⁻¹. Concentrations of NO₃⁻ in drainage waters during the growing season either remained fairly constant or increased during periods of high precipitation inputs. This pattern is markedly different than that exhibited for most forested watersheds in Europe and North America where during the growing season nitrate concentrations often reach their lowest values. These Japanese sites had high rates of N mineralization as a function of abundant moisture and warm temperatures. Nevertheless, most sites, except those with elevated atmospheric inputs of N, had high levels of nitrogen retention. The general absence of seasonal patterns of NO₃⁻ concentrations also suggests that this condition cannot be used to evaluate nitrogen saturation in Japanese forests as has been done for some watersheds in North America and Europe.