Transfer of elements to the atmosphere during low-intensity prescribed fires in three Australian subalpine eucalypt forests

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Abstract
Measurements were made of the transfer of N,P,K, Ca, Mg, Mn, and B to the atmosphere during low-intensity (350-600 kW m-1) prescribed burns in three Australian subalpine eucalypt forests dominated by overstoreys of either Eucalyptus pauciflora (Sieb. ex Spreng), E. dives (Schau.), or E. delegatensis (R. T. Baker). Elemental transfer was calculated as the difference between the quantity of an element in the fuel (litter plus understorey) before burning and that present in the postfire residues which were recovered quantitatively using small aluminium trays. Complete recovery of fine ash is essential for accurate budgeting for elements other than N. The mass ranges of elements transferred to the atmosphere (kilograms per hectare) were as follows: N, 74-109; P, 1.96-3.04; K, 12.1-21.0; Ca, 18.7-29.7; Mg, 4.5-9.7; Mn, 1.6-4.3; B, 0.08-0.12. These transfers represented, as a percentage of the element initially present in the fuel, the following; N, 54-75; P, 37-50, K, 43-66; Ca, 31-34; Mg, 25-49; Mn, 25-43; B, 35-54. The percentage loss of elements was positively linearly correlated with the percentage loss in fuel weight. High concentrations of P and cations occur in fine ash, especially grey (mineral) ash. In comparison with unburnt litter, concentrations of Ca, Mg, and P were increased by 10- to 50-fold, 10- to 35-fold, and 10-fold in fine (< 1 mm) ash produced at the E. pauciflora site, respectively. Hence, transport of a relatively small mass of fine ash either during or after a fire may result in the removal of a significant quantity of nutrients. Prescribed fire rotations of about 10-12 years are required in the forest studied to permit natural inputs of N to approximately replace the amounts transferred to the atmosphere in a single fire. For P replacement, rotation length would need to exceed 20 years. The major impact of regular burning would appear to be on the N cycle because of the rapid reaccumulation of N in shrub biomass and surface litter, thus rendering it highly susceptible to volatilization in a subsequent burn.