Rice straw biochar affects water retention and air movement in a sand-textured tropical soil

Despite the current global attention on biochar (BC) as a soil amendment, knowledge is limited on how BC impacts the physical properties of coarse-textured soils (sand > 95%), particularly in tropical regions. A two-season field-study was conducted to investigate the effect of rice straw BC (3% w/w) on water retention, gas transport and structure of a sand-textured tropical soil. We sampled 3months and 15months after BC application and measured wet- and dry-region soil water retention, air permeability and gas diffusivity at selected matric potentials. At all measured potentials and for both sampling times, soil water retention was significantly higher (20-150%) for the BC treatment due to increased fraction of smaller pores (< 30 mu m) at the expense of macropores (< 100 mu m). Although there was no consistent effect of BC on air-filled porosity, BC significantly reduced air permeability and gas diffusivity (similar to 20%) at -30 kPa matric potential. After 15-months, air permeability decreased by similar to 15% after BC amendment, but analyses of the pore structure revealed a more tortuous and complex soil structure. Thus, application of rice straw BC to similar coarse-textured soils will improve soil-water relations and over time provide better structure for agricultural purposes.