New Zealand and Australia generate large quantities of agricultural, industrial, and municipal wastes. As authorities move to protect the environment by regulating waste disposal practices, environmentally sound methods of waste disposal are being sought. In particular, land application of wastes as a means of disposal, nutrient re-cycling, and water conservation is becoming increasingly popular. This paper provides an overview of the types, quantities, and characteristics of wastes generated in New Zealand and Australia, and highlights the problems with current waste disposal practices, including landfilling, incineration, and discharging into waters. This is followed by a detailed review of the beneficial effects and adverse impacts of land application of wastes on plant production and soil and environmental quality, and possible hazards to human health. The management of waste application on land is a challenging task and requires rigorous scientific input. Sludges and euents contain significant concentrations of plant nutrients, particularly nitrogen, phosphorus, and organic matter. Their application on land has been shown, in many cases, to result in significant increases in plant yields and improvements in soil physical conditions and chemical fertility. The constraints with some wastes, particularly those of industrial and municipal origin, are that they contain undesirable constituents, e.g. heavy metals, toxic organics, pathogens, and salts, or have extremely high or low pH. High concentrations of nitrate and phosphate derived from wastes are also of concern for ground and surface water contamination. The processes that control the fate of wastes in the soil are complex and many of them are poorly understood, e.g. rate of release of nutrients and other chemicals; leaching of nutrients, metals, and organics through macropores and as suspended solids; emission of greenhouse gases; impact of solvents, surfactants, and sludge organic matter on the sorption, degradation, and leaching of hydrophobic organics; and the long-term bioavailability and fate of metals and organics fixed by soil organic matter. More research is urgently required to develop a sound understanding of waste characteristics and the processes affecting their fate in the soil in order to ensure that land application of wastes is safe.

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The effect of cow urine, dairy shed effluent (DE), and urea fertiliser on nitrate leaching was determined using undisturbed soil lysimeters (500 mm diameter by 700 mm deep) with ryegrass (Lolium perenne)–white clover (Trifolium repens) pasture. Cow urine was applied to the lysimeters, at rates of 0 and 1000 kg N/ha in May 1996. Urea and DE were applied to urine-applied and non-urine-applied lysimeters at rates of 0, 200, and 400 kg N/ha in 4 split equal applications in May, August, and November 1996 and February 1997. Natural rainfall was supplemented with simulated rainfall in winter and spring (May–October) to achieve the 75th percentile of winter–spring rainfall records in the region. Flood irrigation was applied 6 times during summer–autumn (November–April) at 100 mm per application, which is a typical practice used by dairy farmers in the region. Drainage water was collected and analysed for nitrate, nitrite, bromide (added tracer), and ammonium. Over the first year of the experiment (May 1996–April 1997), 12% of the urine-N applied was lost through leaching, mainly in nitrate form. When urine (1000 kg N/ha) was applied in combination with DE (200 kg N/ha) and urea (200 kg N/ha), the mineral N leaching loss increased to 14% of the total N applied. The annual average nitrate concentrations in the drainage from the lysimeters that received urine alone, or urine+DE and/or urea, were 33–57 mg N/L, with a mean peak concentration of 110 mg N/L. These nitrate concentrations were significantly higher than in those treatments that did not receive urine (1–5 mg N/L). Because, on average, about 25% of the area of a grazed dairy paddock receives urine per year, the field-scale leaching losses were calculated by taking into account the dilution effect of drainage water from non-urine patch areas of the paddock. The calculated annual paddock losses were 33–60 kg N/ha, and on average the annual paddock nitrate concentrations were 10–17 mg N/L. This demonstrates the importance of accounting for the dilution of nitrate in the leachate from non-urine patch areas of the paddock. The annual average concentration from the treatment DE at 400 kg N/ha was significantly lower than that from the urea treatment at the same rate. This was probably because of the different chemical forms of N in each material, and needs to be taken into account when developing regional rules for land application of urea and effluents.

Intact lysimeters (50 cm diameter, 70 cm deep) of silt loam soil under permanent grassland were used to investigate preferential transport of phosphorus (P) by leaching immediately after application of dairy effluent. Four treatments that received mineral P fertilizer alone (superphosphate at 45 kg P ha À1 year À1 ) or in combination with effluent (at $ 40-80 kg P ha À1 year À1 ) over 2 years were monitored. Losses of total P from the combined P fertilizer and effluent treatments were 1.6-2.3 kg ha À1 (60% of overall loss) during eight drainage events following effluent application. The rest of the P lost (40% of overall loss) occurred during 43 drainage events following a significant rainfall or irrigation compared with 0.30 kg ha À1 from mineral P fertilizer alone. Reactive forms of P (mainly dissolved reactive P: 38-76%) were the dominant fractions in effluent compared with unreactive P forms (mainly particulate unreactive P: 15-56%). In contrast, in leachate following effluent application, particulate unreactive P was the major fraction (71-79%) compared with dissolved reactive P (1-7%). The results were corroborated by 31 P nuclear magnetic resonance analysis, which showed that inorganic orthophosphate was the predominant P fraction present in the effluent (86%), while orthophosphate monoesters and diesters together comprised up to 88% of P in leachate. This shows that unreactive P forms were selectively transported through soil because of their greater mobility as monoesters (labile monoester P and inositol hexakisphosphate) and diesters. The short-term strategies for reducing loss of P after application of dairy effluent application should involve increasing the residence time of applied effluent in the soil profile. This can be achieved by applying effluent frequently in small amounts.

The rates at which nutrients are released to, and removed from, the mineral nutrient pool are important in regulating the nutrient supply to plants. These nutrient transformation rates need to be taken into account when developing nutrient management strategies for economical and sustainable production. A method that is gaining popularity for determining the gross transformation rates of nutrients in the soil is the isotopic dilution technique. The technique involves labelling a soil mineral nutrient pool, e.g. NH4+, NO3−, PO43−, or SO42−, and monitoring the changes with time of the size of the labelled nutrient pool and the excess tracer abundance (atom%, if stable isotope tracer is used) or specific activity (if radioisotope is used) in the nutrient pool. Because of the complexity of the concepts and procedures involved, the method has sometimes been used incorrectly, and results misinterpreted. This paper discusses the isotopic dilution technique, including the theoretical background, the methodologies to determine the gross flux rates of nitrogen, phosphorus, and sulfur, and the limitations of the technique. The assumptions, conceptual models, experimental procedures, and compounding factors are discussed. Possible effects on the results by factors such as the uniformity of tracer distribution in the soil, changes in soil moisture content, substrate concentration, and aeration status, and duration of the experiment are also discussed. The influx and out-flux transformation rates derived from this technique are often contributed by several processes simultaneously, and thus cannot always be attributed to a particular nutrient transformation process. Despite the various constraints or possible compounding factors, the technique is a valuable tool that can provide important quantitative information on nutrient dynamics in the soil–plant system.