Response to nitrogen loading in a P-limited larch forest ecosystem in Sapporo, Japan

We investigated nitrogen cycling in five adjacent small catchments with Andisol soils supporting mature larch plantation in Sapporo, Japan. The foliar N:P weight ratio ranged from 35 to 71, which was higher than published threshold P-limitation values of 12.5 to 26.3, suggesting that the forest ecosystem was highly P-limited. The bulk dissolved inorganic N (DIN) input was 3.0 kg N ha -1 y -1 , which was lower than in N-saturated forests in Europe (more than 10 kg N ha -1 y -1 ). The throughfall DIN inputs in the snow-free season quantified at five to six sites in each catchment (28 sites in total) was low and varied between 0.02 and 1.32 kg N ha -1 6 months -1 . The stream DIN concentration was low (between 10 and 20 μmol c L -1 ). Foliar N contents (1.83 to 2.23%) was not significantly correlated with throughfall DIN inputs. These results suggest that the forest ecosystem was not affected by N-loading. On the other hand, some biogeochemical processes may have responded to the low level of N deposition. The mean stream DIN concentration in the five catchments (13.6 to 34.2 μmol c L -1 ), the N 2 O flux (0.01 to 0.07 kg N ha -1 6 months -1 ), and the net primary production (4.83 to 15.6 m 3 ha -1 y -1 ) increased significantly with increasing throughfall DIN input, possibly due to the soil's strong P retention. The results of our study suggest that the responses of forest ecosystems on volcanic soils differ from those of European forest ecosystems on nonvolcanic soils.