Changes in soil phosphorus and nitrogen during slash-and-burn clearing of a dry tropical forest

Slash-and-burn clearing of forest typically results in an increase in soil nutrient availability. Throughout the tropics, ash from consumed vegetation has been accepted as the primary nutrient source for this increase. In contrast, soil heating has been viewed as a secondarily important mechanism of nutrient release. Through the use of multiple burn plots and intensive pre-burn and post-burn sampling of mineral soil, this study quantified changes in total P and N, P fractions, and KCl-extractable N in soil during the slash-and-burn conversion of a Mexican dry forest to agriculture. Slash burning resulted in large transformations of non-plant-available P and N in soil into mineral forms readily available to plants. Anion-exchange resin, NaHCO3-extractable P, and KCI-extractable N in soil increased by 37 kg P ha(-1) and 82 kg N ha(-1). Organic and occluded P (sequentially extracted with NaOH, sonication + NaOH, and NaOH fusion) and organic N (total N minus KCl-extractable N) decreased after burning by 25 kg P ha(-1) and 150 kg N ha(-1). Immediately after burning, ash from consumed aboveground biomass contained 11 kg P ha(-1) and 27 kg N ha(-1), of which 55 and 74%, respectively, were quickly transported off the site by wind. At this dry forest site, soil heating had a much larger influence on soil P and N availability than inputs of ash.