THE EFFECTS OF FIRE ON WATER-QUALITY, DISSOLVED NUTRIENT LOSSES AND THE EXPORT OF PARTICULATE MATTER FROM DRY HEATHLAND CATCHMENTS

Dry heathlands at La Calma (Montseny, NE Spain) are periodically burnt by shepherds to improve the pasture. The effects of such fires on wat er quality and on the output fluxes of dissolved and particulate mater ial were studied by comparing an unburnt 6.3 ha catchment with a 10.1 ha catchment burnt in 1983 for which some pre-burn data exist. Additon al data from another 22.5 ha catchment bumt in 1982, without pre-burn information; were used. Burning increased mean annual streamflow by at least 36% during the first 2 years after fire. Overland flow on bumt slopes had higher NO3- and K+ concentrations than that on unburnt slop es. Fire had little impact on streamwater chemistry; only slight but s tatistically significant increases in NO3- concentrations and in pH an d decreases in K+ and in SO42- were found. As a result of the increase d water yield, dissolved nutrient losses via streamflow increased by a t least 36% in the first 2 years after burning. Despite this increase, nutrient budgets calculated at one burnt catchment showed that, excep t for Na2+ and Mg2+, it received more dissolved nutrients from bulk de position than it lost through stream-flow. Bed load transport, the dom inant process of particulate matter export, remained low during the fi rst 4 years after burning. Both bumt and unburnt catchments received m uch more particulate matter from bulk deposition than they lost throug h bed load transport. It is concluded that the studied prescribed burn of these dry heathlands had little effect on catchment outputs of dis solved and particulate material. Lack of change of streamwater chemist ry after a complete burn of the heathland testifies to the efficiency of the retention by the soil of nutrients leached from the ash, and nu trient uptake by regrowing vegetation.