Nitrate retention and leaching in variable charge soils of a watershed in São Paulo state, Brazil

Abstract

Two Oxisols [Dusky Latosol (LR) and Dark‐Red Latosol (LE)] and one Entisol [Quartzous Sand (AQ)] of a watershed situated at the boundaries of the most important Brazilian aquifer, and intensively cropped to sugarcane, were studied in relation to their nitrate retention and leaching potential, and related electrochemical properties. Nitrate leaching tests carried out with repacked soil columns showed that deep layers of the subsoils, in which electrochemical conditions favor nitrate retention (e.g., net charge near zero, and ZPC larger than pH in water) required a larger number of pore volumes (npv) of leaching solutions for nitrate peak elution and nitrate depletion than surface layers, in which nitrate retention is not favored. Well defined regions of nitrate accumulation were detected in the 220–460 cm layer in the LE and 40–600 cm layer in the AQ. In these regions nitrate concentrations varied from 0.06 to 0.22 and 0.08 to 0.20 cmol_c kg^‐1, against 0.03 to 0.05 cmol_c kg^‐1 in the upper and underlayers. Nitrate distribution curves in the LE profile in the years 1995 and 1997 presented similar shapes and positions, indicating that the accumulation region remained almost unaltered, and stationary, in this period. Nitrate accumulation was related to soil electrochemical conditions favorable to nitrate retention.