Nutrients contained in piggery pond sludge are a potential source of nutrients for crop production. In order to efficiently utilise nitrogen (N) contained in piggery pond sludge for crop production knowledge of the availability of N is required. Understanding of N availability following the application of piggery pond sludge to soil was obtained from two field experiments on the Darling Downs in southern Queensland on contrasting soil types, a cracking clay (Vertosol) and a hardsetting sandy loam (Sodosol), and from laboratory incubations conducted under controlled conditions. Piggery pond sludge was applied as-collected (Wet PPS) and following stockpiling to dry (Dried PPS).
Measurement of chemical and physical constituents of Wet PPS and Dried PPS showed the two materials differed in their composition. A large proportion of total N in the Wet PPS was in the form of NH4+
(31 % of total N). Whereas, Dried PPS contained <l% of total N as NH4+ and 14% of total N was present as NO3-. Organically bound N was the main form of N present, 69% of total N in Wet PPS and 85% in Dried PPS. Carbon:N ratios were similar, 5.4 Wet PPS and 7.8 Dried PPS. However, analysis of lignin composition using the Van Soest method indicated there was a greater presence of lignin in the Dried PPS compared to Wet PPS, and the lignin:organic N ratio was 19:2 for Dried PPS and 5.9 for Wet PPS. In addition a larger proportion (54%) of the total solids in Wet PPS (8.5% total solids) were present as organic matter (OM) compared to Dried PPS, 14% (86% total solids). Analysis of composition suggested the Dried PPS would be a stable material less susceptible to mineralisation than Wet PPS.
Laboratory incubation treatments were unamended soil (Control),
Dried PPS applied at rates of 6 t ha-1 (D1) and 18 t ha-1 (D2), and Wet PPS applied at rates of 17.5 t ha-1 (W1) and 52.5 t ha-1 (W2) to both soils and an additional treatment of 120 t ha-1 (W3) on the clay soil. Cumulative net N mineralised data, obtained from the long-term (30 weeks) leached aerobic incubation, was described by a first-order single exponential model modified for an initial flush. The mineralisation rate constant (0.0569 week-1) was not significantly different between Control and PPS treatments and across soil types, when the effects of inorganic N applied in PPS treatments were excluded from the cumulative net N mineralisation data. Thus indicating different treatments of PPS did not affect the stability of organic N and its susceptibility to mineralise. Potentially mineralisable N (No) was significantly
increased by the application of Wet PPS increased with increasing rate of application (W1 treatment 256 mg kg-1 and the W2 treatment 335 mg kg-1 on the sandy soil). Whereas, mineralisation of organic N applied in Dried PPS was negligible (total inorganic N leached during 30 weeks of incubation in the Control treatment on the sandy soil 286 mg kg-1 and D1 treatment 294 mg kg-1). This was attributed to Wet PPS having a greater amount of OM with a lower presence oflignin resulting in increased substrate available for mineralisation in a more easily degradable form. Measurement of carbon (C) mineralisation by determination of CO2 evolved during 30 weeks of aerobic leached incubation supported the N mineralisation data with the Dried PPS being more stabilised (19 to 28% of organic C applied mineralised, excluding the Dl treatment on the sandy soil)
than the Wet PPS (35 to 58% of organic C applied mineralised). Mineral N applied in Wet PPS contributed as much to the total mineral N status of soil as did that which mineralised over time from organic N. The clay soil impacted on the initial availability of NH4+ applied in Wet PPS and delayed nitrification due to the retention of NH4+ in an exchangeable form. Clay soil had a negative effect on the overall availability of N as well as net N mineralised.
The non-leached aerobic incubation (12 weeks) demonstrated NH4+ applied in Wet PPS was rapidly nitrified to nitrate (NO3-), and there was no apparent immobilisation of inorganic N applied in Wet PPS and Dried PPS. Application of Wet and Dried PPS significantly increased total inorganic N extracted following 12 weeks of aerobic
non-leached incubation. However, only the W2 and W3 treatments (17 to 22% of organic N applied mineralised) significantly increased net N mineralised compared to the Control treatments. Similar to the aerobic leached incubation, the aerobic non-leached incubation demonstrated the main effect of applying Wet PPS or Dried PPS on N availability was related to the quantity of mineral N applied in PPS, except where Wet PPS was applied at high rates and net mineralisation of organic N occurred.
In the field experiments, release of soil mineral N, yield response and N uptake of cereal crops following application of PPS was studied. Wet PPS and Dried PPS was applied to the field experiments in December 1997. Dried PPS was applied at rates of 6 t ha-1 (D1) and 18 t ha-1 (D2) to both trial sites and Wet PPS at 20 t ha-1 (W1) and 65 t ha-1 (W2) to the sandy loam soil, and 40 t
ha-1(W1) and 120 t ha-1 (W2) to the clay soil. Urea was also applied at rates of 50, 100 and 150 kg N ha-1 for calculation of fertiliser equivalence of Wet and Dried PPS treatments. Mineral N accumulations during a subsequent fallow period were determined by core sampling to various soil depths in January, February, March, May and September 1998. The clay soil affected the initial availability of NH4+ applied in Wet PPS and differences between treatments were not apparent until the February soil sampling. This effect was also observed in the laboratory, aerobic leached incubation. Ammonium applied in Wet PPS to the sandy soil was rapidly nitrified to NO3- (for example, 74 kg N ha-1 in the surface soil (0-10 cm) of the W2 treatment in January 1998). Rapid nitrification of
NH4+ was also observed in the non-leached aerobic incubation. Anaerobically mineralisable N was well correlated (R2=0.93 clay soil; R2=0.9 sandy soil) with net N mineralised during 30 weeks of aerobic leached incubation. However, anaerobically mineralisable N was not useful for prediction of N availability in the field as it was unable to detect differences between Control and PPS treatments following the January 1998 sampling.
With one exception, wheat yield on the clay soil, grain yields were not significantly increased by application of PPS. However, the application of PPS resulted in increased crop N uptake and grain protein concentrations on both soil types. For example, the barley crop grown on the sandy soil on the W2 treatments achieved 14.1% grain protein concentration (82 kg N ha-1 taken up) compared to 10.8% on the Control treatment
(57 kg N ha-1 taken up). Increased crop N uptake and grain protein concentrations were a result of increased soil NO3- levels with increased application rates of Wet and Dried PPS. The field experiments verified the observations of the laboratory incubations, that the main impact of Wet and Dried PPS on N availability is the mineral N applied in PPS. Low yields (1.6 to 2.61 ha-1), grain protein concentrations (7.3 to 8.5%) and crop N uptake (32 to 56 kg N ha-1) of the sorghum crop grown on the clay soil following the barley crop demonstrated an insignificant residual value of N applied in PPS resulting from the low rate of mineralisation of organic N applied in PPS.
In the field experiment, there was evidence of deep leaching of NO 3- from the W2 treatment on the sandy soil following the harvest of the barley crop with 18 kg N
ha-1 being present in the 120 to 150 cm depth. High concentrations of residual NO 3- in the W2 treatment on the sandy soil (131 kg NO 3- ha-1) following the barley crop grown in 1998, combined with the sandy texture of the soil, suggest that leaching of NO 3- could be an issue for concern when high rates of Wet PPS are applied in times of high precipitation.