Sulfur speciation in a tropical soil amended with lime and phosphogypsum under long-term no-tillage system

Sulfur (S) is present mainly in organic forms in the most superficial layers of well-drained soils, despite sulfate (SO₄²⁻) being the main S-species absorbed by plants. Coupled broadcast application of lime and phosphogypsum (PG) is widely deployed as an agricultural practice to improve soil chemical properties and increase crop yields in highly weathered tropical soil areas under no-tillage system (NTS). However, the effects of large-scale application of these soil amendments to S-species distribution in soil in NTS are largely unknown. We combined S wet-chemical analysis with Synchrotron-based X-ray Absorption Near-Edge Structure Spectroscopy (XANES) to evaluate the effects of lime and PG application to S-species distribution along the soil profile (up to 150 cm). Soil samples were collected from a field-scale experiment under NTS for over 15 years. Soil was amended with lime and PG, either individually or in combination, four times during the experiment. Soil sampling was conducted 16 months after the last amendments’ application, and treatments (lime, PG and lime + PG) were compared to a control (under NTS without amendments’ application) and with soil samples from a native forest (reference for an undisturbed site). Broadcast PG application increased the contents of Ca-phosphate extractable S and pseudo-total S (microwave assisted acid digestion) contents in the deeper layers. Two peaks representing reduced and oxidized S species were detected in S K-edge XANES spectra of all treatments and NF soil. The oxidized S peak gets sharper with increase in depth, as reduced S peak intensity decreases, and fingerprint analysis suggested a greater contribution of reduced organic S species in the most superficial soil layer followed by an increase in highly oxidized S species as depth increased. Linear combination fitting analysis confirmed a greater contribution of highly oxidized S species to the total S-species as depth increased, especially in soil samples that received PG application.