Short-term effects of fire intensity on soil organic matter and nutrient release after slash-and-burn in Eastern Province, Zambia

According to the slash-and-burn technique used in Eastern Province, Zambia, cut trees are piled and burned in only a part of the cleared fields, because adequate tree biomass is not available to burn the entire field. Due to a recent decrease in emergent trees, not only emergent tree piles but also bush tree piles may exist. Therefore, our objective was to evaluate the changes in soil organic matter followed by nutrient release occurring immediately after burning in spots unburned and burned with emergent and bush trees. Fire intensity was significantly higher where emergent tree piles were present. Total carbon (C) decreased by 25.1% and 14.7% in spots burned with emergent and brush tree piles, respectively, while total nitrogen (N) decreased by 15.0% only at spots burned with emergent tree piles and did not change significantly elsewhere. Additionally, the mortality of microbes with soil heating caused an increase in C mineralization after fire. The levels of available nutrients, such as ammonium nitrogen (NH4-N), available phosphorus (P), and exchangeable potassium (K) and calcium (Ca), increased following the decomposition of soil organic matter and microbial mortality that occurred with an increase in fire intensity. Net N mineralization did not occur, especially in spots burned with emergent tree piles, because the N content of labile organic matter decreased. Maize (Zea mays L.) grain yield increased with fire intensity, because fire increased nutrient availability and limited weed biomass. Although the burned emergent and bush tree piles occupied only 6.9 and 7.5% of total cleared field, respectively, the grain yield in spots burned with emergent and bush trees accounted for 21% and 15% of the total yield, respectively. Therefore, the burning of bush trees, which is increasing because of the decreased number of emergent trees, could result in a decrease in grain yield but could also alleviate the overall severe loss of soil organic matter.