Assessing Soil Sulfur (S) Fractions as Indicators of Long-Term Fertilization Residual Effects

Sulfur (S) fertilization can alter the distribution of soil S fractions with varying degrees of bioavailability. However, long-term studies on the accumulation of these fractions and their relationship with plant availability are limited. This study aimed to: (i) quantify changes in soil S fractions using both physical and chemical fractionation methods, and (ii) assess their relationship with S bioavailability, as indicated by uptake in a test crop, after 10 years of continuous phosphorus (P) and S fertilization. The experiment consisted of a factorial combination of three P rates (0, 20, and 40 kg P ha⁻¹) and four S rates (0, 12, 24, and 36 kg S ha⁻¹) applied to cereals from 2000 to 2010, within a crop sequence of maize—full season soybean—double-cropped wheat/soybean. In 2010, a maize crop was sown as a test crop, and S uptake was considered indicative of bioavailable S. Soil samples were collected before sowing the test crop in 2010, and S fractions were separated physically as S in particulate organic matter (S-POM) via wet sieving, and chemically into inorganic S (Sin), ester sulfate (SOC; organic S not directly bound to C), and C-bonded organic S (SC). After 10 years of fertilization, S-POM, Sin, and SOC increased by 60%, 300%, and 83%, respectively, corresponding to increases of 4.6, 2.8, and 24.8 mg kg⁻¹ per 100 kg of cumulative applied S. S uptake by the test crop was positively associated with Sin and SOC, with uptake increases of 1.7 and 0.18 kg S ha⁻¹ for each 1 mg kg⁻¹ increase in these fractions, respectively. These results suggest that the Sin and SOC fractions explained the residual effects of 10 years of S fertilization in a Typic Argiudoll of the Pampas region, and may serve as reliable soil indicators for assessing long-term S fertility.