Sulfur mineralization rates and potentials of soils

Summary Field-moist soil and glass beads mixtures were packed in glass tubes and leached with 100 ml of 5 mM CaCl₂ and incubated at 20 or 30°C. The leaching procedure was repeated every 2 weeks for 14 weeks. The leachates were analysed for SO _inf4 ^sup2– and NO₃ ^–. The S uptake by three successive croppings of corn (Zea mays L.) or soybean Glycine max (L.) Merr.] at 40- or 60-day intervals, respectively, or three cuttings of ryegrass (Lolium multiflorum L.) at 30-day intervals were studied under greenhouse conditions. Results showed that significantly greater amounts of S were mineralized at 30°C than at 20°C in each of 13 Iowa and 7 Chilean surface soils. Expressed as percentages of organic S in soils, the amounts of S mineralized in the Iowa surface soils in 14 weeks at 20 and 30°C ranged from 1.2% to 9.8% and from 2.4% to 17.5%, respectively. The corresponding values for the Chilean soils ranged from 0.9% to 7.2%6 and from 1.4% to 12.1%. The Q₁₀ values of S mineralization ranged from 1.7 to 4.4 (average 2.5) for the Iowa soils and from 1.7 to 3.1 (average 2.1) for the Chilean soils. The cumulative S mineralized at 20°C in 14 weeks was significantly correlated with the cumulative N mineralized (linear model, r=0.72^**; quadratic model, r=0.84^***). Similarly, the cumulative S mineralized at 30°C was significantly correlated with the cumulative N mineralized at this temperature (linear model, r=0.81^***; quadratic model, r = 0.82^***). The potentially mineralizable S pool (S₀), calculated by using an exponential equation for the S mineralized at 20°C, ranged from 5 to 44 mg kg^–1 for the Iowa soils and from 10 to 25 mg kg^–1 for the Chilean soils. The corresponding values obtained by using a reciprocal-plot technique ranged from 6 to 48 mg kg^–1 and from 12 to 26 mg kg^–1, respectively. The S₀ values calculated for S mineralized at 30°C, in general, were higher than those obtained at 20°C. The S mineralization rate constant (k) and the time required to mineralize 50% of S₀ (K _t), calculated by using the cumulative SO _inf4 ^sup2– released during 14 weeks of incubation, varied considerably among the soils. Up take of S by corn and soybean (tops+roots) were, in general, lower than the total SO _inf4 ^sup2– mineralized in 14 weeks at 20°C.