Effects of burning on nutrient balance in an area of gorse (Ulex europaeus L.) scrub

Wildfires affect nutrient balance as a result of combustion of biomass, increased surface and subsurface runoff and increased soil erosion. In the present study, nutrient inputs and outputs to burnt and unburnt Ulex scrub plots were monitored over a 2-year period. During burning, between 50 and 75% of the nutrients contained in above-ground plant tissues were directly lost due to volatilization and upward movement of particulates to the atmosphere. Only small amounts (less than 3% for all elements) were deposited at the soil surface as ash. During the first rains after burning, N, P and K losses were largely due to sediment transport in surface runoff, while Ca and Mg losses were roughly equally distributed between sediment losses and soluble-form losses (in surface runoff and subsurface flow) and Na losses were largely in soluble form. Post-burning nutrient inputs to the soil in throughfall were lower than in the control plots for N and K; in the case of the remaining elements (P, Ca, Mg and Na), inputs to the burnt plots and control plots differed little. In general, burning led to clear net losses of nutrients; annual losses were approximately 2.5–3.5 g m−2 in the case of N and approximately 6.5–9.0 g m−2 in the case of K. In the unburnt plots, by contrast, outputs were approximately equal to inputs.