Treated Wastewater Affects the Fertility and Geochemistry of Degraded Soil in the Brazilian Semi-Arid Region

Projections for the Brazilian semi-arid (BSA) region estimate a reduction in water bodies and an increase in degraded areas. Recovering degraded soils using treated wastewater (TWW) is a strategy to increase the resilience of the local population to these climatic adversities. This study aimed to evaluate the impact of deficit irrigation with treated effluent on the (geo)chemistry of degraded soil in the BSA. An experiment with the application of TWW was conducted on soil degraded within an agroforestry system. The treatments arranged in randomized block design were WS0.5 (water supply at 0.5 L/plant/week), TE0.5 (treated effluent at 0.5 L/plant/week), and TE1 (treated effluent at 1 L/plant/week). Soil samples were collected (0–15 and 15–30 cm) at the initial condition, after two years of irrigation, and two years after the end of irrigation. Analyses of chemicals and geochemicals were carried out. All treatments increased soil fertility after two years in both layers, with TE1 resulting in higher Ca2+ (0–15 cm: 2.88; 15–30; 3.14; cmolc kg−1), Mg2+ (0–15 cm: 2.13; 15–30; 2.00; cmolc kg−1), and K+ (0–15 cm: 0.11; 15–30; 0.12; cmolc kg−1), generating a residual effect two years post-irrigation suspension and no risk of salinization. However, TE1 and mainly TE0.5 showed an increase in sodium content, making the soil solodic (6–11%). The application of TWW changed the CaO, MgO, and K2O contents of silt fraction, contributing to the availability of Ca, Mg, and K in soils. Future studies should monitor sodium levels and confirm K-bearing phyllosilicate (illitization) after irrigation with TWW. The application of TWW for a short period (two years) and in small volumes (0.5 L/plant/week) affects (geo)chemistry of degraded soil from the BSA.