Acid sulfate soils in the Mekong Delta, Vietnam, are often high in organic matter content, but net N mineralisation is low. This may be due to low soil pH or low easily decomposable organic matter content. This study aimed at investigating net N mineralisation in acid sulfate rice soil (anaerobic incubation) and acid sulfate upland soil (aerobic incubation) amended with 1% biogas sludge, 1% straw, 1% starch, 2.5‰ CaCO₃ (about 10 t CaCO₃/ha for acid sulfate soils), and 0.22‰ urea. Non-acid alluvial soils were used for comparison.

Results showed that addition of straw and starch to acid sulfate rice soil decreased net N mineralisation, but addition of biogas sludge increased cumulative N-NH₄ due to both the increase in soil pH after submergence and the supply of low C/N organic matter. Addition of biogas sludge can therefore increase N-supplying capacity in acid sulfate rice soil. During aerobic incubation of acid sulfate upland soil with biogas sludge, cumulative N (NH₄ + NO₃) was also increased compared with the control, although pH was not increased. It is concluded, therefore, that in acid sulfate soils in the Mekong Delta, the supply of easily decomposable organic matter with low C/N ratio can increase activity of microorganisms and hence increase net N mineralised compared with soils not supplied with biogas sludge. Liming can increase net N mineralisation in acid sulfate rice soil during anaerobic incubation, but not in acid sulfate upland soil during aerobic incubation. Addition of rice straw and starch to soil amended with urea increased N immobilisation; therefore, urea can be temporally immobilised in soils and hence may reduce loss of N in field conditions.