THE EFFECTS OF FIRE ON THE SOIL IN A DEGRADED SEMIARID WOODLAND .1. CRYPTOGAM COVER AND PHYSICAL AND MICROMORPHOLOGICAL PROPERTIES

The effects of fire on the cryptogam cover and physical and micromorphological properties of a massive red earth soil were studied in a semi-arid eucalypt woodland, heavily invadedby shrubs, near Coolabah, N.S.W. Fire reduced the cryptogam cover and concomitantly increased the depositional material produced by erosion and the area of bare surface. Annual fires for 7 years completelydestroyed the cryptogamic crusts, but they recovered slowly in the absence of fire to reachthe same cover as unburnt areas after about 4 years. A single fire also caused a majordecline in aggregate stability of the0-1cm horizon, possibly because of alteration of organiccementing materials which consist of gels secreted by algae. Micromorphological observations of surface crusts showed that, as the frequency of fire increased, there was more depositional material produced by erosion coupled with thepresence of thin laminated deposits. There was also less surface irregularity, fewer algalgels and less evidence of soil mixing by soil fauna. There was a significant negative relationship between the saturated infiltration rate and the number of fires (r2=0-63, P = 0-05). However, there was no effect of fire treatment onthe unsaturated infiltration rate measured at a supply pressure of -40 mm, at which pores>0-75 mm diameter are excluded from water flow. In our burned plots, the rate of recolonization by cryptogams was relatively fast and, with approximately 4 years recovery, cryptogam cover reached the level of unburned controls. This cryptogam cover is critical in maintaining the physical properties of the soil. It isconcluded, therefore, that irregular fires in this land system will not result in a permanentdecline in the physical properties of the soil. © 1990 CSIRO. All rights reserved.