Slash burning and the effects on nutrient dynamics and soil faunal compostition in an Eucalyptus grandis plantation in South Africa

Abstract

Sustainability of commercial plantations is dependent on the conservation of soil nutrients, especially on the ancient, leached soils of the tropics. This is particularly important when limited fertilization is practiced. To assess the sustainability of commercial plantations, various sustainability indicators have been suggested. One such indicator is that of biodiversity. The measurement of soil faunal diversity is important as soil fauna play an active role in the cycling of nutrients through the decomposition of plant residues and organic matter. The diversity and different feeding activities of soil fauna influence the rate at which nutrient cycling occurs either through microbial grazing, faecal deposition, the mixing of litter with the mineral soil as well as through the spreading of microbial inoculum. Management practices may, however, negatively influence nutrient availability through losses associated with the burning and removal of slash residues, thus changing the soil faunal community composition. In Eucalyptus grandis plantations in South Africa, the site is usually burned following the harvest and prior to replanting, which may markedly disrupt soil faunal function and nutrient availability. Soil biological processes, nutrient loss and soil faunal composition were the focus of this study. Nutrient losses associated with fire and slash management practices were determined by comparing the quantity of nutrients lost from the slash that was either spread or piled following burning. The in situ nitrogen and phosphorus mineralization rates and soil faunal composition were measured in six randomly located plots, three burned and three unburned, prior to and after a low intensity fire. Results indicate that 78 – 99% of phosphorus, nitrogen and carbon were lost from the slash as a result of burning, with a greater percentage loss occurring from the slash that was spread. It was further shown that within the burned plots the nitrogen availability was enhanced four fold within the first month following the fire. Phosphorus availability was also enhanced ten fold as a result of the burning, however, these effects on nitrogen and phosphorus availability were short lived. Soil faunal diversity was low both prior to and after burning. Ants are the dominant soil fauna. Soil faunal composition was more markedly influenced by season than by fire, with millipedes occurring in all plots in spring, prior to the fire and ants dominating in summer, post fire.