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 signif icant disruption of soil nutrient cycling, particularly in terms of the dyn amics of microbial populations. This study deals with the effect of fire an d ash input on microbial respiration and on distribution of C within water- stable aggregate in soils during a long-term incubation experiment (164 day s). 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 amend ment did not increase respiration; it did increase respiration, however, in the undisturbed forest soil. Our results suggest that the fire affected mi crobial activity through both soil heating and chemical changes. As indicat ed by the results of the grass amendment to burned sample treatment the hig h temperature killed some soil microorganisms, mainly those associated with the use of newly added C. In addition, ash input appears to have constrain ed microbial activity through changes in soil chemistry. Soil heating and ash input also affected the distribution of C across diffe rent size fractions of soil aggregates. Labile C associated with macroaggre gates (>250 mu m) was destroyed during fire and did not represent an import ant 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 dynami c under pasture condition.