Evolution of the soil chemical properties in slash-and-burn agriculture along several years of fallow

Several studies have demonstrated the effects of slash-and-burn agriculture on soil chemical properties immediately or short-term after burning. This study aims to answer if: a) the slash-and-burn system affects soil chemical properties; b) management effects remain in a fallow chronosequence; and if c) these effects are restricted to the soil surface. The study was developed in the south-eastern region of the state of Paraná, located in southern Brazil. On-site, the land preparation was made through procedures comprising of fire, cropping, and fallow. The methodology was based on the chronosequence approach by different fallow periods: native forest, 30 days after burning, 2-year fallow, 5-year fallow, 7-year fallow, and 12-year fallow. The soil samples were collected at 0-5 and 5-10 cm depths for chemical analysis (pH, organic matter, ions availability (extractable phosphorus, potassium, calcium, magnesium and aluminum) and cation exchange capacity). The results indicate significant changes at 0-5 cm depth. The soil organic matter levels remained constant throughout the chronosequence. The direct effects of burning led to increased pH and nutrient availability (extractable phosphorus and calcium), which decreased within 2 to 5 years of fallow. The increase in basic cations was sufficient to guarantee the soil nutrient status up to 7 years of fallow. Successive slash-and-burn cycles in the same area caused an increase in soil sand content, and reduced cation exchange capacity.