Shifting Cultivation in Northern Thailand with Special Reference to the Function of the Fallow Phase

Abstract

To clarify the various functions of the fallow phase in the shifting cultivation system in northern Thailand, the fluctuation of fertility-related properties of soils throughout land-use stages was analyzed and the soil organic matter (SOM) budget was quantitatively evaluated, with special reference to soil microbial activities. The factors that have ensured the long-term sustainability of the shifting cultivation system can be summarized as follows: (1) Some soil properties relating to soil acidity improve when SOM increases in the late stages of fallow. The litter input may be supplying bases that are obtained via tree roots from further down the soil profile to the surface soil. This simultaneous increase in SOM and bases in the surface soil through forest-litter deposition in the late stages of fallow has an increasing effect on nutritional elements. (2) The decline in soil organic C during the cropping phase may be compensated by litter input during 6–7 years of fallow. With regard to the overall budget, the organic matter input through incorporation of initial herbaceous biomass into the soil system after establishment of tree vegetation (approximately in the fourth year) was indispensable for maintaining the SOM level. (3) The succession of the soil microbial community from rapid consumers of resources to stable and slow utilizers, along with the establishment of secondary forest, retards further leaching loss of nitrogen (N) and enhances N accumulation in the forestlike ecosystems. The functions of the fallow phase listed above can be considered essential to the maintenance of this forest-fallow system. Agricultural production can therefore be maintained with a fallow period of around 10 years, which is somewhat shorter than widely believed. Traditional shifting cultivation in the study village is shown to be well adapted to its soil-ecological condition.