Effects of Sewage-Water Spraying on Mineral Cycling in a Forest Ecosystem

At a watershed site in Japan, diluted sewage water was sprayed in a poor pine forest to assess the impacts on mineral cycling. Spraying was conducted over the period 1976 79, and data were collected intermittently up to 1988. During each spraying period, samples of the sprayed sewage water and runoff were collected three times/week, and soil chemistry and nutrient contents of trees were assessed after completion of the spraying program. In spite of the high concentrations of COD, ammonia nitrogen, and phosphorus in the sprayed water, the concentrations of the elements in the runoff were low, and there were no differences compared to control runoff. Nitrate N concentrations increased gradually, and it took 8 9 yr after spraying for the concentration to return to its original level. The excess input of organic N caused a gradual increase in nitrate in the runoff. After 2 3 growing seasons, there was a dramatic increase in annual stem increment in the sprayed plot compared with the control watershed. Nutrient accumulation was high for N, phosphorus, potassium, magnesium, and calcium.