Fire alters soil organic matter inducing quantitative and qualitative changes that presumably will affect post-fire soil microbial recolonisation. Several studies have evidenced marked soil organic carbon reduction after moderate and high intensity fire, which limit the total recovery of microbial biomass during years. In order to evaluate the role of soil organic matter alteration in short-term microbial colonization process, we perform a preliminary experiment where unaltered soil from Sierra Nevada Natural Park was heated at 300 ºC during 20 minutes in a muffle furnace (H300) to simulate a medium-high intensity fire. After heating, soil samples were inoculated with unaltered fresh soil, rewetted at 55-65% of water holding capacity and incubated during 3 weeks. At the same time, unheated soil samples were incubated under the same conditions as control (UH). In addition, trying to partially alleviate soil organic matter fire-induced alterations effects on microbial colonization, we include an organic amendment treatment (M+). So, part of heated and unheated samples were amended with a mix of ground alfalfa:straw (1:1) and soil microbial abundance and activity were monitored together with soil organic matter changes. Heating process reduces total organic carbon content. After one week of incubation carbon content in heated samples was lower than the control one, in both, amended and un-amended samples. Microbial biomass and respiration were negatively affected by heating. Ground mulching addition increase microbial biomass and respiration but was not enough to reach control values during the whole study. Nevertheless, viable and cultivable fungi and bacteria showed different pattern. After two weeks of incubation both, fungi and bacteria were higher in heated samples. Ground mulching addition appears to stimulate fungal response in both, heated and unheated samples. Preliminary results of this experiment evidence the transcendence of soil organic matter fire-induced changes on microbial colonization process and the importance to determine several microbial parameters to obtain a more faithful conclusion about microbial response. The organic amendment appears to alleviate partially heated-induced damage, highlighting the positive stimulation on fungal abundance in both, heated and unheated samples.Ministerio de Economía y Competitividad CGL2013-47862-C2-1- RMinisterio de Economía y Competitividad CGL2012-38655-C04-0
