Effect of burning of tropical deciduous forest soil in Mexico on the microbial degradation of organic matter

Slash and burn conversion of tropical deciduous forest can result in significant disruption of soil nutrient cycling, particularly in terms of the dynamics of microbial populations. This study deals with the effect of fire and ash input on microbial respiration and on distribution of C within water-stable aggregate in soils during a long-term incubation experiment (164 days). In 0-2 cm samples, the forest soil with ash had the lowest total CO2-C evolved during incubation. In the top 2 cm soil burned samples, grass amendment did not increase respiration; it did increase respiration, however, in the undisturbed forest soil. Our results suggest that the fire affected microbial activity through both soil heating and chemical changes. As indicated by the results of the grass amendment to burned sample treatment the high temperature killed some soil microorganisms, mainly those associated with the use of newly added C. In addition, ash input appears to have constrained microbial activity through changes in soil chemistry. Soil heating and ash input also affected the distribution of C across different size fractions of soil aggregates. Labile C associated with macroaggregates (>250 mu m) was destroyed during fire and did not represent an important source of available labile C for microbial activity. We concluded that the combination of organic C redistribution among size-aggregate fractions and microbial communities alteration by fire are critical for soil C dynamic under pasture condition.