Effects of fire on soil organic matter in a “cerrado sensu-stricto” from Southeast Brazil as revealed by changes in δ13C

We studied the effect of increasing fire incidence on litter and soil organic matter in a Dark Red Latosol (Typic Haplustox) under a “cerrado sensu-stricto” vegetation in Southeast Brazil. After 21 years, C4-grass population significantly increased in the plot under high fire incidence (“cerrado 3”) compared to the plots of low fire incidence (“cerrado 1” and “cerrado 2”). In general, the variability of organic carbon (OC) content, organic nitrogen (ON) content and δ13C, both in litter and in the topsoil, was higher in “cerrado 3” than in the other plots. The higher fire incidence in “cerrado 3” decreased the litter stock of carbon and nitrogen by 3703±930 and 104±26 kg ha−1, respectively, in relation to the other plots. However, no difference in carbon (195.0±8.2 Mg ha−1) and nitrogen (13.4±1.0 Mg ha−1) stock in the first meter of soil was observed between the plots. The values of δ13C increased between 2.06‰ and 3.39‰ under C3 plants, and decreased 8.31‰ under C4 plants from litter to topsoil. For all plots, the δ13C profile showed an increase in δ13C with depth. In “cerrado 1” and “cerrado 2”, the δ13C values ranged from −25.80‰ to −23.19‰, typical for a C3-dominated vegetation profile. In “cerrado 3”, the replacement of C3- by C4-derived C was in average 34.6% throughout the profile. We conclude that: (i) the studied undisturbed “cerrado sensu-stricto” is a C3-dominated vegetation; (ii) biannual fire incidence for 21 years reduced the C3 dominance and decreased the stock of C and N in litter, but not in the soil; (iii) in the same period, at least about one-third of the C stock in the soil, or 67.5 Mg ha−1 (to 1-m depth), was replaced by C4-derived below ground litter; and (iv) in choosing a δ13C reference profile for soils under cerrado, it is necessary to take fire incidence into account.