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 water quality and on the output fluxes of dissolved and particulate material 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. Additonal data from another 22.5 ha catchment burnt in 1982, without preburn information, were used. Burning increased mean annual streamflow by at least 36% during the first 2 years after fire. Overland flow on burnt slopes had higher NO-3 and K+ concentrations than that on unburnt slopes. Fire had little impact on streamwater chemistry; only slight but statistically significant increases in NO-3 concentrations and in pH and decreases in K+ and in SO2-4 were found. As a result of the increased water yield, dissolved nutrient losses via streamflow increased by at least 36% in the first 2 years after burning. Despite this increase, nutrient budgets calculated at one burnt catchment showed that, except for Na2+ and Mg2+, it received more dissolved nutrients from bulk deposition than it lost through streamflow. Bed load transport, the dominant process of particulate matter export, remained low during the first 4 years after burning. Both burnt and unburnt catchments received much more particulate matter from bulk deposition than they lost through bed load transport. It is concluded that the studied prescribed burn of these dry heathlands had little effect on catchment outputs of dissolved and particulate material. Lack of change of streamwater chemistry after a complete burn of the heathland testifies to the efficiency of the retention by the soil of nutrients leached from the ash, and nutrient uptake by regrowing vegetation. © 1993.