MASS BALANCE STUDY OF APPLICATION OF MUNICIPAL WASTE-WATER TO FORESTS IN MICHIGAN

A mass balance study of the effects of secondary municipal wastewater on P and N cycling was conducted in 1976 and 1977 on three 1.2-ha plots in a late successional, sugar maple (Acer saccharum Marsh.)-beech (Fagus grandifolia Ehrh.) forest in southern Michigan [USA]. The treatments included an unirrigated control plot and 5 and 10 cm/wk wastewater application plots. The 10 cm/wk site was deliberately overloaded hydraulically. Cl concentrations at the 150 cm depth on the wastewater application plots did not approach input concentrations until the end of the 1976 irrigation season. Data [from 1977] are emphasized in mass balance calculations. About 96% of the added inorganic N from 5 cm/wk of wastewater application leached past the root zone. Application of 10 cm/wk wastewater increased inorganic N retention on a mass balance basis to 69% of input, apparently as a result of denitrification under anaerobic conditions in the water logged soils. The 10 cm/wk application resulted in nitrate concentrations approaching those of wastewater input concentrations during peak runoff periods. The 10 cm/wk application also resulted in death of trees in the lower areas of the forest where water tended to pool. P retention by the system was 96.5% at an application rate of 5 cm/wk but decreased to 66% when the irrigation rate was increased to 10 cm/wk as a result of runoff losses. Changes in soils included increased available P and exchangeable Mg and K in the top 15 cm of soil and increased Na and Cl concentrations throughout the 150 cm depth sampled.