Soil acidification and carbon storage in fertilized pastures of Northeast Thailand

Light textured soils are often characterized as acid to depth that results in low productivity levels. In an effort to address this constraint a four year study was undertaken that evaluated the productivity of Gamba grass (Andropogon gayanus) and Stylosanthes (Stylosanthes guianensis) (Stylo) to grow on these soils. In addition, Gamba grass treatments received either no nitrogen fertilizer (Gamba) or a total 287 kg N ha− 1 as either KNO3 (Gamba NO3) or (NH4)2SO4 (Gamba NH4). Average annual dry matter production levels for the Gamba, Gamba NO3 and Gamba NH4 were 11.9, 22.5, and 26.6 t ha− 1 whilst that of the Stylo treatment was 6.9 t ha− 1. However, the net annual acid addition rates associated with the export of biomass ranged from 5.1–13.3 kmol H+ ha− 1 yr− 1. Rapid acidification of the soil profile was observed to depths > 110 cm in all treatments regardless of the tempering influence of nitrate based fertilizers. Soil organic carbon levels over the study period showed a 6 fold increase at > 30 cm from the initial values, suggesting significant carbon sequestration. Whilst the study demonstrates the positive impact of a grass or legume ley in producing forage for livestock in a cut and carry system under rainfed conditions in Northeast Thailand, along with positive contributions to soil organic carbon sequestration, a precautionary approach should be adopted. Significant accelerated soil acidification has occurred to depths > 110 cm that brings into question the sustainability of these systems on these soil types.