Chemical effects on the soil–plant system in a secondary treated wastewater irrigated coffee plantation—A pilot field study in Brazil

Wastewater reuse in agriculture is recognized worldwide as an alternative water and/or nutrient source. In this study, secondary treated wastewater (STW) from an anaerobic/facultative pond system at the city of Lins (São Paulo State, Brazil) was used over 3 years and 7 months to irrigate coffee ( Coffea arabica L.). The soil type was Typic Haplustox and the crops were fertilized according to regional agronomical recommendations. Soil and leaf samples from three sampling campaigns were used to study effects on chemical quality parameters, macronutrients and Na within the soil–plant system. Due to high Na contents of the STW applied, Na concentrations showed increases throughout the soil profile compared to untreated soil conditions. Both, low C/N ratio of STW and fertilizer amendments stimulated soil microbial activity and encouraged nitrification and mineralization of wastewater organic components and soil organic matter (SOM) causing significant decreases of SOM and cation exchange capacity (CEC). Over time exchangeable sodium percentages (ESP) in the topsoil decreased due to Na exchange mainly by Ca and Mg, resulting in increasing exchangeable calcium percentage (ECP) and exchangeable magnesium percentage (EMP) associated with lower soil sodicity. Exchanged Na and available soluble Na from STW led to both elevated ESP at depth by soil migration and high plant uptake. The superficial increase of ECP and EMP favored continuous replenishment of Ca and Mg in the soil solution leading to increasing plant contents over time. The plant Ca, Mg and K contents remained high after fertilization stop and continued STW irrigation. This is expected to be rather a short-lived effect due to a reduction of the essential cation store through constantly provided Na and insufficient supply of essential cations via STW, associated with decrease of SOM and CEC and higher sodicity risk, suggesting the need of continued fertilizer use for soil maintenance. On the other hand, the plant contents of P, N and S dropped back to deficient values when irrigated solely with STW mainly due to insufficient replenishment by STW and the anion exchange complex (AEC) indicating moreover the need to continue fertilization to maintain anion levels in soil for optimum plant growth. The study revealed that STW can effectively increase water resources for irrigation, however, innovative and adapted fertilizer/STW management strategies are needed to diminish sodicity risks and to sustain adequate