Organic matter addition can prevent acidification during oxidation of sandy hypersulfidic and hyposulfidic material: Effect of application form, rate and C/N ratio

• Organic matter (OM) addition can reduce acidification of oxidizing sulfidic material.
• OM mixed into soil is more effective than mulching.
• OM effectiveness can be optimized by mixing OM with C/N ratio ≤ 50 at ≥ 30 g kg− 1.
• Ameliorative effect of OM addition is greater in soil with low pyrite concentration.

Adding organic matter (OM) could minimize acidification during oxidation of sulfidic soils. Three incubation experiments were carried out under oxidizing conditions for 6 weeks using two acid sulfate soils: one with hyposulfidic material (soil Hypo) and one with hypersulfidic material (soil Hyper). The organic materials used were dried and finely ground (< 2 mm) plant shoots: mature wheat straws a (Wa, C/N ratio 54) and b (Wb, C/N ratio 137) and young kikuyu shoots (C/N ratio 15). In all experiments, acidification in the un-amended treatments was smaller in soil Hypo than soil Hyper. In Experiment 1, Wa was added at 30 g kg− 1 by mixing into the soil or placing as a layer on the soil surface. After 6 weeks in both soils the pH was lowest in the un-amended control; soil pH was higher when OM was added by mixing than as a layer. In Experiment 2, Wb was mixed into soils at 0–40 g kg− 1. Acidification of soil Hypo was prevented when ≥ 30 g kg− 1 Wb was added. In soil Hyper, OM addition reduced acidification. In Experiment 3 residues were mixed into the soils at 30 g kg− 1: Wa, Wb and kikuyu alone and different mixtures of Wb and kikuyu (with C/N ratio 107, 76 and 46, respectively) for soil Hypo, or only Wa and Wb for soil Hyper. After 6 weeks, in soil Hypo the pH was highest with kikuyu and higher with Wa than Wb. The pH increased with decreasing C/N ratio.