Combustion and Nutrient Losses During Laboratory Burns

The aim of this study was to assess the effects on combustion characteristics, and their consequences on nutrient losses, of (1) the change in load and packing ratio of the fuel bed, and (2) the change in fuel moisture content. Eighty-one experimental burns were carried out, on a test bench in the laboratory; the fuel was composed of needles and twigs of Pinus pinaster. Two levels of fuel load an dpacking ratio (8t ha-1 needles, packing ratio of 0.040; and 16t ha-1 twigs and needles, packing ratio of 0.066) were compared at constant moisture content (6%); and four levels of moisture content(6%, 12%, 24% and 30% dry weight) were compared at constant fuel load (8t ha-1 needles). At constant moisture content, an increase in the load and packing ratio of the fuel bed led to an increase in the height of flames and in the maximum temperature 25 cm above the fuel bed, in the duration of the rise in temperatures within the fuel, and in the fireline intensity. Conversely, the rate of fire spread decreased. At constant fuel load, an increase in the moisture content of the fuel led to a decrease in the rate of fire spread, in the flame height and the maximum temperature 25 cm above the fuel bed, and in the fireline intensity. In contrast, the maximum temperatures reached within the fuel, when the flaming front was continuous, did not significantly change with varying fuel loads or fuel moisture contents. The percentage fuel consumption was always high, more than 80%, but it significantly decreased with increasing fuel load and packing ratio and with increasing moisture content. Total losses of N, S, and K significantly decreased with increasing fuel load and packing ratio, with increasing moisture content and with decreasing percentage fuel consumption. Losses in P only significantly decreased with increasing fuel load and packing ratio. Losses in Mg and Ca were not significantly affected by fuel load, moisture content. or percentage consumption. An attempt was made to separate volatile from particulate losses, based on the assumption that all the losses of Ca were in particulate form. Whereas losses in particulate form remained relatively constant, losses of nutrients in volatile form seem to have been related to the percentage fuel consumption. Even if these experimental burns were of low intensity (40 to 56 kW m-1), their impact, in terms of lethal temperatures and nutrient losses, was not negligible, particularly for N and P. The increasing fireline intensity with increasing fuel load was not accompanied by an enhancement in the proportion of nutrient losses. In the same way, the strong decrease in fireline intensity with increasing fuel moisture content led only to a slight decrease in some nutrient losses. It was through their effect on the percentage fuel consumption that fuel load or moisture content modified the nutrient losses, particularly volatile losses.