Nitrogen and phosphorus leaching in a tropical Brazilian soil cropped with sugarcane and irrigated with treated sewage effluent

There are concerns about groundwater contamination with N and P from fertilizers and other anthropogenic wastes. Use of treated sewage effluent (TSE) for crop irrigation can reduce the use of mineral fertilizers; however, it may add more nutrients into the soil than are necessary for crops, increasing the possibility of leaching. Thus, knowledge of nutrient dynamics in TSE irrigated soils is important for the safe use of this resource. However, the reliability of studies regarding ion leaching is limited due the high propagated variance, as these studies involve independent measurements of variables related to soil and soil solution. The objective of this research was to quantify P and N leaching in a TSE-irrigated Brazilian soil and identify the main causes of variance of this quantification. The experiment consisted of a treatment without irrigation and treatments with TSE irrigation to meet 100% and 150% of the crop water demand (CWD). Soil physical properties and soil water potential gradient were used to calculate internal drainage, and nutrient concentration was measured in soil solution samples taken with ceramic suction cups at a depth of 1 m. Variance propagation was calculated by linearization, and the contribution of each variable to the total variance was isolated and quantified. Irrigation with TSE increased N leaching; however, when applied in dosages that met 100% of the CWD, it did not threaten the groundwater quality. P leaching was as low as 100 g ha−1 and was therefore not an environmental concern. N leaching can be estimated considering the total N input and the rainfall; however, long-term data are needed to improve the accuracy of this estimation. The variance propagation of the soil water potential measurements represented up to 70% of the nitrogen leaching variance.

► Irrigation with treated sewage effluent increased nitrogen leaching.
► Wastewater irrigation to meet crop water demand did not threaten groundwater quality.
► Nitrogen leaching can be estimated from the input of nitrogen into the system and the rainfall.
► Leaching of P was as low as 100 g ha−1 and was therefore not an environmental concern.
► The soil water potential represented up to 70% of the variance of the nitrogen leaching.