Abstract
Net nitrification properties of forest soils were evaluated in watersheds of three natural forests (T6, T7, T8) and a conifer plantation (T9) at Ichmomata in Kochi, southern Japan. Soil nitrate pool was less than 2 mg N kg^<-1>. Nitrate production during the 90-day laboratory incubation was 152〜314 mg N kg^<-1>. In the soils from T6 and T8, nitrate was produced at a constant rate. In contrast, in the soils from T7 and T9, nitrate was not produced in 30 days, but was produced rapidly after 30 days. In these soils, low nitrate production and active nitrate consumption by soil heterotrophic microbes may keep soil nitrate concentrations very low at the initial period of incubation. In the later period, microbial consumption of nitrate may decrease due to the depletion of available carbon sources. Soil production of nitrate after 90-day incubation was positively correlated with stream nitrate concentration. This indicates that consumption of nitrate by soil microbes is an important factor for retaining nitrogen within forest ecosystems.
