The erodibility of soils beneath wildfire prone grasslands in the humid tropics, Hong Kong

The paper analyses factors influencing soil erodibility in a grassland area of the humid tropics subject to frequent wildfires. The interrelationships between aggregate stability, soil physicochemical properties and pore size characteristics are examined. At the multifire scale aggregate stability was significantly (P>0.01) correlated with cation exchange capacity (CEC) (R=0.76) and, in order of explained variance, exchangeable magnesium, aluminium, potassium, calcium and hydrogen concentration. At the individual fire scale only CEC and exchangeable magnesium had a significant ameliorating influence in the more recently burnt areas, whilst both exchangeable magnesium and calcium were significant in the area of longest post-fire recovery. At the multifire scale aggregate, stability was also significantly correlated with soil texture and organic matter partly through their influence on CEC. These interactions between aggregate stability and soil physicochemical properties influence soil drainage, and low but significant correlations were recorded between aggregate stability and total porosity (R=0.55 P>0.05) and the relative proportion of transmission pores to storage pores (R=0.44 P≥0.05). Water release curves showed that areas with the longest post-fire recovery times held higher amounts of water at most suctions and hence contained a greater volume of pores of different sizes, than more recently burnt areas. Soil erodibility in wildfire prone areas therefore reflects complex interactions and feedbacks between soil physicochemical properties, aggregate stability and pore size characteristics which vary spatially and with time as soil and vegetation recovery progresses.