Changes in Soil Chemistry and Soil Nutrient Stocks after 30 Years of Treated Municipal Wastewater Land Disposal: A Natural Experiment

The benefits and risks of irrigation with treated municipal wastewater (TMW) on soil quality and crop production have been largely investigated. However, there is a lack of knowledge on the effect of plant species on the interaction between soil quality and TMW. We leveraged a natural experiment investigating the effect of 30 years of TMW irrigation at a rate of 4 m y⁻¹ (eq. 1860 kg N ha⁻¹ y⁻¹, and 264 kg P ha⁻¹ y⁻¹) on a sandy soil under pine plantation and pasture, compared with soil under New Zealand native Kunzea robusta. There was a consistent increase in soil P with irrigation under both pasture (Olsen P in topsoil 40 mg kg⁻¹ vs. 74 mg kg⁻¹) and pine (18 mg kg⁻¹ vs. 87 mg kg⁻¹), which was significant down to 2 m deep. The pH, electrical conductivity, total organic C and N, inorganic N and Na were affected by both irrigation and vegetation type. Beyond P soil accumulation, there was no evidence of soil degradation by Na or trace element accumulation. Estimations of nutrient mass balance indicated that 80% and 60% of the total applied P was lost under pine and pasture, respectively. This percentage increased to 96% and 83% for N, respectively. Although plant species had a significant effect on soil quality and N and P losses from TMW-irrigated areas, adjusting irrigation rates to levels that can be managed by plants is the only way to design sustainable TMW irrigation schemes.