Nitrogen dynamics in a eucalypt plantation irrigated with sewage effluent or bore water

Abstract

Irrigation of treated effluent onto tree plantations is a popular method of land treatment but can lead to unacceptable levels of groundwater degradation. Knowledge of nitrogen transformations and balances is essential to design and operate plantations so as to keep groundwater degradation within acceptable limits. APSIM for Effluent, a model of water, salt, and nitrogen in effluent-irrigated plantations was tested against data from a plantation of Eucalyptus grandis (flooded gum) irrigated with either secondary-treated sewage effluent or bore water. APSIM was then used for quantifying nitrogen transformations, leaching, and balance, within the plantation.

Summed over 5 years, the predicted nitrogen balance of the effluent-irrigated treatment showed that the accumulation of nitrogen in the aboveground biomass and litter (335 and 19 kg/ha) was significantly less than the amount of nitrogen added in effluent (508 kg/ha). Denitrification at this site was low, about 52 kg/ha over 5 years, because the soil was permeable and unlikely to become anaerobic for substantial periods of time. After 5 years, organic nitrogen decreased by 167 kg N/ha, and 269 kg N/ha was leached.

In the trees irrigated with bore water, accumulation of nitrogen in the biomass and litter (301 and 34 kg/ha) was not much less than for the effluent-irrigated treatment and was considerably greater than the nitrogen added in the bore water irrigation (14 kg/ha). After 5 years, the predicted fluxes were 10 kg/ha denitrified, 389 kg/ha depleted from soil organic matter, and about 58 kg/ha leached. About 75% of the nitrogen leaching occurred in the first year of the experiment.