Nitrogen and heavy metal dynamics following the application of municipal sewage sludge to an acid forest (Pinus radiata) soil

Abstract

The application of sewage sludge to forests holds much potential, both as a waste disposal option, and as a source of cheap fertilizer. The possible risks include nutrient (particularly nitrogen) overloading beyond the forests assimilative capacity, resulting in nutrient leaching, and increased concentrations of heavy metals causing reductions in the size and efficiency of the soil microbial biomass.

Two forests close to Christchurch city (New Zealand) were studied. Eyrewell Forest, on which the majority of the trials were conducted, is located on Lismore stony silt loam, and Chaneys Forest is located on Waikuku loamy sand. Laboratory and field studies were undertaken to determine the changes in, and potential fate of, sludge-applied N, sludge-borne heavy metals, and the response of the microbial population to sludge amendment.

Laboratory leaching and non-leaching (batch) incubation studies were used to determine the maximum potential mineralization of sludge organic N which, when combined with the inorganic N present in the sludge, gives the maximum N supplying capacity of the sludge. Maximum values of 41 % mineralization from the leaching incubation and 51 % mineralization from the batch incubation were obtained. Values fell to as little as 0.6% at higher sludge loading rates. This was attributed to poor aeration, and illustrates the vulnerability of sludge N mineralization to environmental conditions.

A field trial using in situ containment of soil was conducted at Eyrewell Forest to determine the actual rates of mineralization likely to occur in the field. A single application of sludge, aimed at supplying 400 kg N ha⁻¹ , was applied in June 1992. The production of NH₄⁺-N and N0₃⁻-N in the (L+F+H) horizon and the mineral soil (0-5 cm and 5-10 cm) was followed for 18 months after application.

Sludge application significantly increased the quantity of inorganic and total N in the (L+F+H) horizon and mineral soil, 0-5 cm. Inorganic N concentrations in sludge amended soils declined to original concentrations, 24-30 weeks after sludge application, but total N remained high, indicating little mineralization of sludge organic N occurred after application.

The high pH and high concentration of ammoniacal N (NH₄⁺-N + NH₃⁻-N) in sewage sludge offers a potential for volatilization of NH₃₍g₎, despite the acid soil conditions. Using fresh sewage sludge applied to undisturbed (L+F+H) horizon collected from Eyrewell Forest, NH₃ volatilization was measured for one week. Negligible quantities volatilized from unamended (L+F+H) horizon (0.2 kg N ha⁻¹ ), but significant quantities volatilized from sludge-amended (L+F+H) horizon (29.1 kg N ha⁻¹). This was equivalent to 7.5% of total sludge N, or 30% of sludge ammoniacal N.

The mobility and thus bioavailability of heavy metals depends on the chemical species of the metal. These are affected by a combination of sludge and soil factors. Heavy metals examined were cadmium, chromium, copper, lead, nickel and zinc. A sequential extraction scheme separated these into "exchangeable", "organically bound", "oxide" and "residual" fractions. Utilizing the site created for in situ N mineralization, sewage sludge combined with (L+F+H) horizon was analyzed 0, 3, 6, and 12 months after application, and the concentration and ratios of each species compared with those present in sewage sludge and unamended (L+F+H) horizon. The relative proportions of each species varied with each metal, and the metal source (sludge or soil).

Overall, the species composition was dominated by sludge factors, and showed no significant changes with time. Sludge was the main, and for some metals the only source of bioavailable metals. There was negligible movement of metals from the (sludge + (L+F+H) horizon) layer into underlying mineral soil.

Due to their ability to bind heavy metals, aluminium, iron and manganese oxide fractions were determined in sewage sludge, amended and unamended (L+F+H) horizon, and in the mineral soil. The quantities of all species were much greater in the mineral soil than in any other sample type. This suggests the mineral soil offers a large capacity for immobilizing heavy metals through binding with Al, Fe and Mn oxides.

Soil microbial biomass size (biomass C) and activity (respiration, urease, phosphatase and arylsulphatase activity) were determined in the (L +F +H) horizon of sludge amended and unamended sites. The (L+F+H) horizon was collected from the Christchurch City Council drainage board trial sites at Eyrewell and Chaneys forest, 18 months after cessation of sludge application. A large decrease in biomass size and activity per gram of soil was generally noted in sludge-amended (L +F +H) horizon, and was attributed to the elevated heavy metal concentrations in these sites. On an area basis, sewage sludge appeared to have a positive effect on the microbial parameters. The increase in (L+F+H) horizon weight due to residual sludge organic matter offset the lower values per gram of soil to give this result. It appears that the short term effect of sludge application is to enhance the soil microbial biomass.

The microbial response observed indicated that the biomass was sensitive to lower concentrations of metals than those present in the Christchurch City Council trial. A laboratory incubation was run, incorporating a range of heavy metal concentrations up to and including those present in the City Council trial. The incubation also incorporated heavy metals from different sources (sewage sludge and/or metal chloride salts) to represent different concentrations of bioavailability. Microbial biomass quantity (biomass C) and activity (respiration), and heavy metal species were determined. Heavy metal concentrations of up to 1360 µg g⁻¹ did not affect biomass C, but concentrations above 1660 µg g⁻¹ caused decreases in biomass C. Respiration was not reduced at heavy metal concentrations of up to 3400 µg g⁻¹. The source of heavy metal had a significant effect at a given concentration, with metal salts eliciting the greatest response. The extractants used for heavy metals did not provide a good indication of metal bioavailability.