Effects of nitrogen excess through rainfalls on growth and competition status of woody seedlings in Mt Tanzawa, near Tokyo, and mitigation of the effects using wood-chips

We examined effects of nitrogen supplied through rainfalls on growth rate of woody species distributing on the Tanzawa mountains, near large cities, Japan. We sowed three early-successional, three middle-successional and two late-successional species in a green house in April 2006, and collected those seedlings in September of the year. Two soil types and five nitrogen levels were examined with eight replications. As the nitrogen treatments, solution of NH4NO3 was added each week to Control (0kg/ha/y), Low (2kg), Ambient (10kg), High (100kg) and High plus wood-chips (20% of soils in bulk). Average of dry weights of Ambient seedlings (average of eight species) was 16 - 22% higher than ones of Low seedlings, although it was not significant. Since growth of much earlier-successional species tended to be more enhanced by additional nitrogen, the order of species changed in relation to some plant-size measurements, particularly stem weight and stem length, depending on nitrogen levels. As a result, it was suggested that nitrogen supply by rainfalls has already modified competition status of tree seedlings on the Tanzawa mountains. Growth of the seedlings treated as High plus wood-chips was substantially diminished, and it was more markedly for earlier-successional species. Wood-chips, then, promise to be available for mitigation of nitrogen excess, although more examinations are necessary to clear the adequate ratio of wood-chips to soils.