EFFECT OF BURNING ON SELECTED SOIL PARAMETERS IN A GRASS FALLOW SHIFTING CULTIVATION SYSTEM IN BHUTAN

Grass fallow shifting cultivation is an important land use practice in the highlands of Bhutan. Part of the nutrient pool contained in soil organic matter is made available for the traditional buckwheat (Fagopy rum tataricum) crop through a highly labor intensive system exposing 2 50-500 MT soil ha-1 to temperatures of 500-degrees-C and above. Dry to psoil is collected in mounds and burned using plant biomass or manure and soil organic matter as fuel. Labor input ranged from 150-401 days ha-1 with land preparation accounting for 65-85% of the total requirem ent. The burning increased soil pH from 6.0 to 6.9 and available K fro m 34 to 69 mg kg-1. Soil organic C and total N were reduced from 3.3 a nd 0. 17% to 0.8 and 0.08%, respectively. Release of P from soil organ ic matter and plant material and reduction in C/N ratio resulting in i ncreased N availability are considered the most essential effects requ ired for good crop yields. Through the burning about 16 MT of C and 47 0 kg N ha-1 are released into the atmosphere. Fallow periods of 15-20 years are required for the system to be sustainable.