Fractionation And Mobility Of Phosphorus And Heavy Metals In Sandy Forestry Soils Receiving Repeated Biosolids Application

Since the mid-1990s, a Pinus radiata (D. Don) plantation growing on a sandy, low fertility soil at Rabbit Island near Nelson, New Zealand received aerobically digested liquid biosolids. An experimental research trial was established on the site to investigate the effects of biosolids applications on tree nutrition and the quality of the environment receiving biosolids. Biosolids were applied to the trial site in 1997, 2000 and 2003, at three application rates: 0 (control), 300 (standard) and 600 kg N ha^-1 (high). Fractionation and soil testing were undertaken to investigate the amounts, forms and distribution of phosphorus and metals in biosolids-treated soils and to assess if biosolids application had raised the potential risk of phosphorus and metals leaching and groundwater pollution.Soil total P and Olsen P in the high biosolids treatment (600 kg N ha^-1 applied in every three years) had increased significantly (P<0.05) in both 0-10 cm and 10-25 cm layers. Mehlich 3 P in soil of the high treatment had increased significantly only in 0-10 cm. Olsen P appeared to be more sensitive than Mehlich 3 P as an indicator of P movement in a soil profile. Phosphorus fractionation revealed that inorganic P (Al/Fe bound P and Ca bound P) and residual P were the main P pools in soil, whereas water soluble P accounted for approximately 70% of total P in biosolids. Little organic P was found in either the soil or biosolids. Concentrations of water soluble P, bioavailable inorganic P (NaHCO₃ Pi) and potentially bioavailable inorganic P (NaOH Pi) in both 0-10 and 10-25 cm depths were significantly higher in the high biosolids treatment than in the control. Mass balance calculation indicated that most P applied with biosolids was retained by the top soil (0-25 cm). The standard biosolids treatment (300 kg N ha^-1) had no significant effect on concentrations of total P, Mehlich 3 P and Olsen P, and P fractions in soil. It implies that the risk of P losses via leaching in the sandy forest soil because of the repeated biosolids application, particularly at the standard rate, was small.Environmentally available metals analysis and sequential fractionation were carried out to study metals. The biosolids applied to the soils contained higher levels of Zn and Cu than Ni, Pb and Cr. The application of biosolids had significantly elevated the concentrations of environmentally available Cu in top soils, but had no effects on environmentally available Zn, Ni, Pb and Cr, responding to trace metal loadings in biosolids. Results of sequential fractionation showed that the application of biosolids had few effects on the distribution pattern of metals in soils. The residual fraction was the most abundant pool for all the examined metals (Cu, Zn, Ni, and Cr) in this sandy forestry soil. Exchangeable fractions contributed little to total metals. Fractionation also showed that biosolids application significantly increased concentrations of residual Cu but decreased carbonate bound Cr in topsoil. We concluded that most of biosolids-derived metals were retained by litter layer and upper soil layers (0-25cm), or taken up by trees. Therefore the mobility of metals and the possibility of leaching in this type of soil should be minimal.