Availability of nitrogen from municipal biosolids for dryland forage grass

A 3-yr study, initiated in 1996, evaluated the availability of N from appli ed biosolids for dryland forage grass production under the cool, continenta l climatic conditions in central British Columbia. Treatments included 600 (LB), 1200 (MB) and 1800 (HB) kg total N ha(-1) applied as municipal biosol ids, a single application of 150 kg N ha(-1) as urea in the first year of t he experiment (SF), a multiple application of 150, 60 and 30 kg N ha(-1) as urea in the first, second, and third years of the experiment (MF), and a c ontrol that received no biosolids or urea. All treatments were roto-tilled to 15-cm depth and seeded to a mixture of four grasses. The LB treatment wa s predicted to supply a similar quantity of plant-available N as the MF tre atment, assuming 25, 10, and 5% of biosolids N is available in the first, s econd, and third year, respectively. Soil N fertility was poor as indicated by the very low forage yield and N uptake in the control, and minimal appa rent net soil N mineralisation. Recovery of urea N in the crop over 3 yr av eraged only 27%, likely reflecting net immobilisation in this recently brok en site and accumulation of N in non-harvested portions of the crop. Cumula tive recovery of N from biosolids in the harvested forage averaged only 11% . However, the fertiliser N equivalency of the biosolids N (ratio of recove ry of biosolids N to urea N) was estimated at 41%, close to the predicted v alue of 40%. Forage yield and N uptake were similar for the LB and MF treat ments, suggesting that actual biosolids N availability was similar to that predicted. Limited forage yield increase for the HE compared with the MB tr eatment early in the experiment, and high forage nitrate content for the HE treatment in the first year, suggest that the HE treatment initially suppl ied an excessive quantity of N. Both urea and biosolids applications increa sed cumulative uptake of other macro- and micro-nutrients, with forage Cu c oncentrations reaching values in the establishment year that may be of conc ern for some animal species. Monitoring of forage NO3 and Cu concentrations is advisable where biosolids are applied.