Long-term response of forest plantation productivity and soils to a single application of municipal biosolids

After 16 to 19 yr, we revisited four experimental trials set up in the early 1990s to evaluate the long-term impact of municipal biosolids applied in forest plantations. Tree growth and the soil were sampled to determine the effects of a single application of biosolids applied at (liquid equivalent) rates of 0, 130, 200, and 400 m(3) ha(-1). Tree radial growth responded markedly to biosolids in the young plantations, increasing from 18% for Pinus resinosa to 62% for Picea glauca, and up to 700% for Quercus sp. Increases in phosphorus (P) concentrations in the tree foliage in response to biosolids could still be detected in the conifer trials. In the top 0-5 cm soil layer, organic carbon (C), total nitrogen (N), P, and copper (Cu) concentrations and pools increased, while soil compaction and bulk density decreased. In the deepest soil layer sampled (20-40 cm depth), the total N and calcium (Ca) pools were reduced by the biosolids treatments, while the pool of exchangeable acidity increased. Our observations indicate that a single application of liquid biosolids up to 400 m(3) ha(-1) (30 t ha(-1) DM) in young forest plantations is a sustainable practice without undue risk to such podzolic soils.