Loss of mass and nitrogen from biosolids applied to a pine plantation

Abstract

Application of biosolids (sewage sludge suitable for beneficial use) to tree plantations has the potential to increase productivity, but also to leach nitrogen (N) if application rates are excessive. This project aimed to determine rates and mechanisms of loss in mass and N from 3 dewatered biosolids (1 from aerobic and 2 from anaerobic digestion of waste water) applied to the surface of a radiata pine plantation. Biosolids were applied at operational rates (nominally 30 dry t/ha), supplying between 965 and 1425 kg N/ha, of which 210–550 kg/ha was NH₄⁺-N.

Results showed that, for mass, biosolids were characterised by 2 discrete pools, an active pool comprising 52–67% of total mass, and a pool that can be considered inert within the time frame of 5 years. Most of the active organic matter and carbon was lost within 2 years; much of the loss of total mass was derived from the inorganic pool.

There was a large and rapid loss of total N from biosolids during the first 3 weeks after application, most of which was accounted for by depletion of NH₄⁺-N, presumably by volatilisation. Much of the subsequent release of N was apparently through mineralisation, with 300 kg/ha (anaerobically digested) to 500 kg/ha (aerobically digested) being released in the first year.

Applications at these rates have the potential to contaminate waterways with N in the first year after application, the risk being greatest from aerobically digested biosolids. In subsequent years there appears to be little risk. However, applications at lower rates appear well suited to forests, with a small proportion of the inorganic N providing an initial input to soil and mineralisation of organic N providing a steady supply to the plantation over a number of years.