To understand the origin of the ultrafine pedogenic components responsible for the magnetic susceptibility (MS) enhancement remains a major challenging problem, in linking the magnetic signal with paleoclimatic conditions. Here we examine the influence of natural fires on the MS signal of both plants and modern soils and in particular the MS difference between C3 and C4 plant ashes and their influence possibly on soil magnetic susceptibility. We demonstrate that burning of C3 and C4 plants can enhance MS signal of modern soils. We show that C4 plants have greater potential to enhance the MS signal. The average MS value of C4 plant ashes is (532¿61)¿10-8 m3 kg-1, much higher than the average MS value of (120¿65)¿10-8 m3 kg-1 of C3 plant ashes. The Fe2O3 concentration in C4 plant is two to four times higher than that in C3 plant. One burning of the grassland, mainly consisting of C4 plants, can enhance MS value of the surface soil up to about 30--40%. ¿ 2000 American Geophysical Union