Seven decades of Atlantic rainforest conversion to slash-and-burn agriculture: Effects on soil's physical properties

Slash-and-burn agriculture is a millennia-old no-till farming technique that is still widely practiced in developing tropical countries. This practice is currently employed in Brazil by subsistence family farms, Indigenous groups, and maroons, in a primitive manner, using only cutting, fire, and fallow as soil preparation techniques for food production. In recent years, this practice has been drastically reduced. However, the fallow periods have become even shorter. Therefore, there is a need to understand how the recurrence of fire with the reduction of fallow time affects the soil. This study aims to evaluate the cumulative effects of fire recurrence in slash-and-burn agriculture for seven decades on the physical properties of the soil and the minimum fallow times for post-burn resilience. Using a chronosequence approach, different fallow periods: a) native forest (control); b) recently burned (30 days after fire); c) 2-year fallow; d) 5-year fallow; e) 7-year fallow; f) 12-year fallow were considered. In each area, six disturbed and six undisturbed samples were randomly collected at 0-5 cm and 5-10 cm depths each to estimate: soil bulk density, soil total porosity, penetration resistance, aggregate stability, mean weight diameter, and soil water retention capacity. The results show that the seven decades of slash-and-burn agriculture affected the topsoil (0-5 cm deep) more prominently. Curiously, soil bulk density and total porosity were not affected at any time between the areas. The fire increased penetration resistance immediately after burning in the 0-5 cm layer by 162.5%, showing resilience after five years of fallow. Soil aggregate stability showed a delayed effect, with a significant increase in two years after the fire by 64.2%, remaining high after 12 years of fallow, indicating the influence of other mechanisms and reactions in the soil after the fire. The mean weight diameter did not change at any time while soil water retention increased immediately after burning by 25.2% with resilience after a two-year fallow. Most physical properties were not directly affected by fire heating but by post-fire environmental conditions and the interaction of various mechanisms during the fallow period. In general, most physical parameters showed good resilience over an average of five years of fallow, with cumulative effects only for aggregate stability.