Soil nitrogen and carbon changes in a biosolid amended sodic vertisol with a history of treated sewage effluent-enriched water irrigation

The application of biosolids to cropping soils may improve soil health, supply nitrogen and other nutrients for crops. There are specific guidelines on the quantity of biosolids that can be applied to a soil to grow selected crops. However, some recommendations overlook environmental factors such as soil organic N and N supplied by irrigation water. We conducted a three-year field monitoring programme to study the effect of biosolid application on sodicity, soil nitrogen and organic carbon status to a depth of 1.8 m in a vertisol with a history of sewage effluent water irrigation. Biosolid application had no effect on soil sodicity. Improvements in soil organic carbon (SOC) were short-lived and occurred only during the first year. N accumulation and leaching below the root zone (1.2 m) occurred after two years and suggest a possible off-farm environmental risk. The higher residual mineral N in biosolid treatment 2 years after application implies an opportunity to reduce N inputs in vertisols treated with biosolids and effluent water. The biosolid application rate of 35 Mg ha⁻¹ was well below the nitrogen-limited biosolid application rate (NLBAR) (100 Mg ha⁻¹) under the current guidelines. This warrants further refinement of NLBAR to suit specific circumstances such as effluent-enriched water irrigation and additional in-crop fertilizer application.