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

Abstract

Adding organic matter (OM) could minimize acidification during oxidation of sulfidic soils. Three incubation experiments were carried out under oxidizing conditions for 6weeks 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 (<2mm) 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 30gkg−1 by mixing into the soil or placing as a layer on the soil surface. After 6weeks 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 0–40gkg−1. Acidification of soil Hypo was prevented when ≥30gkg−1 Wb was added. In soil Hyper, OM addition reduced acidification. In Experiment 3 residues were mixed into the soils at 30gkg−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 6weeks, in soil Hypo the pH was highest with kikuyu and higher with Wa than Wb. The pH increased with decreasing C/N ratio.