Effects of burning on heathland soil chemical properties: an experimental study on the effect of heating and ash deposits

Published source details Forgeard F. & Frenot Y. (1996) Effects of burning on heathland soil chemical properties: an experimental study on the effect of heating and ash deposits. Journal of Applied Ecology, 33, 803-811.
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Summary

Once heavily grazed and vegetation also subject to regular cutting and removal, heathlands in Brittany are now largely neglected. In consequence natural succession has lead to tall gorse Ulex europaeus shrub-dominated communities. Such succession may be prevented by regular fires. The low soil fertility maintained by traditional management and also fires, are considered responsible for the development and maintenance of the distinctive small ericaceous shrub communities characteristic of West European heathlands. Few studies have looked at properties of burnt heathland soil to assess the effect of the role of heat and nutrient supply from ash after light and hot fires. In this study Britanny heathland soils were subject to fire treatments and analysed in the laboratory.

Soil collection: From a heath at Paimpont (48º00’N, 2º15’W) in Bretagne (Brittany), north-west France, 60 blocks of soil (20 x 20 x 10 cm depth) were collected randomly arfter three days of dry weather. Vegatation was dominated by gorse around 2 m in height. The brown acidic soil was 55 cm deep with a thick (10 cm) litter horizon, the C/N ratio suggesting a low humification rate.

Fire treatments: Now in Brittany fires usually occur accidentally and are not prescribed burns. Two kinds of fire intensity were implemented to simulate the types of fires now observed. In a 1985 experimental fire maximum temperatures varied from 50 to 430 ºC at the ground surface with ash deposits estimated at 3.6 t/ha, providing guidance for this current experiment.

Of the 60 soil blocks, 18 were stored in a cool place to act as controls. Other blocks were placed in large boxes of moist sand and the litter removed. The soil surface was heated either to 150 ºC for 15 min or 300 ºC for 30 min (using an epiradiator) and the effects of the percolation of two ash solutions were studied.

The results showed that soil heating as a result of heathland burns, has a significant effect on soil properties only at higher temperatures (300 ºC). The organic matter content decreased during this treatment inducing a decrease in cation exchange capacity and in exchangeable bivalent cation concentrations. The nutrient inputs from ash did not compensate for these losses, except for potassium. Nearly all sodium was leached and its exchangeable concentration always remained lower than the control samples. The major part of nutrient input by ash was immobilized as insoluble forms, thus futher contributing to lowering soil fertility (as mostly unavailable to plants in such forms).

Conclusions: These results suggest that a hot burn has transient effects on the soil fertility of the heathlands. Repetitive fires (on a sensible rotation) will induce nutrient loss, thus contributing to the maintenance of both low soil fertility and the characteristic ericaeous-dominated vegetation communities.

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