BURNINGMICROBES · Fire-induced changes on surface soil microbial communities in Mediterranean forests. Rehabilitation by addition of exogenous organic matter

Fire-induced changes on surface soil microbial communities in Mediterranean forests. Rehabilitation by addition of exogenous organic matter

INTRODUCTION Fire is an ecological factor intrinsic of Mediterranean ecosystems. However, during the last century the extension of burned areas in the Mediterranean basin has increased partly due to the temperature rise. Increasing wildfire frequency has been acknowledged as a major consequence of climate change that threatens the European public health and environment. Direct effects of forest fires are human injuries and death, as well as emission of greenhouse gases, destruction of habitats and wildlife and, importantly as regards this project, depletion of the soil’s ability as a reservoir of organic carbon and genetic resources. Fire subjects soil microbes to extreme temperatures and modifies their physical and chemical habitat. This might immediately alter the soil microbial community structure, and thus ultimately the functioning of the ecosystems. The overall aim of the project BurningMicrobes was to investigate the effect that fire has on the community structure of the soil microbiota and its consequences on the biogeochemical cycles in Mediterranean forest soils. Linking microbial community structure with the performance of ecosystems is essential for predicting the consequences of the increasing wildfire frequency and designing restoration strategies of burned areas. Therefore, the results of this project are relevant both to land-use planners and policy makers. EXPERIMENTAL SET-UP An experimental fire was performed in a 1000 square meter Rosmarinus officinalis dominated area (Fig. 1A, attached). Experimental plots were located in Ayora (Valencia, eastern Spain; UTM: 676565.50, 4332416.06 m) and burning was organised within the project GRACCIE (www.graccie.eu). Fire completely burned the plant cover (Fig. 1B, attached). During burning temperature reached 611 ± 94 ºC (average ± standard error) at 50 cm over the soil surface, 338 ± 83 ºC on the soil surface and 106 ± 35 ºC within the upper two cm below the surface. Surface (0-2 cm) soil samples (n = 10) were randomly taken before (pre-fire) and 1 day, 1 week, 1, 4.5, 9 and 12 months after fire. Pre-fire samples were taken as the unburned control. This considerably reduces the spatial heterogeneity in all microbial parameters that results from sampling an adjacent unburned area, which is the only option when wildfires are investigated. RESEARCH RESULTS We specifically investigated whether: A) Fire induces changes on the soil microbial biomass, total activity, and enzymatic activities linked to the carbon (C), nitrogen (N) and phosphorous (P) cycles. Fire caused a significant shift in the microbial biomass carbon, respiration and soil hydrolases. Soil microbes became significantly more abundant and metabolically more active immediately after fire, as illustrates the two-fold increase in carbon dioxide production within the first month. This enhanced activity responded to the release of easily degradable substances and nutrients to the soil solution. Such a pulse was detected for all total organic carbon, water-soluble carbon, carbohydrates, and most notably, for ammonium and nitrate (see Fig. 2A as an example; attached). Hydrolases involved in the C, P and N cycles responded variously to burning. We detected an enhanced hydrolysis of C and P organics that was probably related to the increased abundance of oxidizable compounds and the higher nutrient demand of an activated microbial community. However, the degradation of N organic compounds decreased coinciding with the pulse of mineral N forms. One year after burning the control levels of chemical and biochemical soil properties had been generally restored. B) Fire alters the soil microbial community structure. Fire significantly altered the bacterial, fungal and archaeal community structure analysed as banding patterns of 16S or 18S rRNA gene amplicons separated through denaturing gradient gel electrophoresis. Immediate changes in bacterial and fungal community structure correlated to the rise in total organic carbon and nitrates caused by the combustion of plant residues. Bacterial communities shifted further in time forced by desiccation and increasing concentrations of macronutrients. Shifts in archaeal community structure were unrelated to any of the eighteen environmental variables measured. Fire-induced changes in the community structure of bacteria, rather than archaea or fungi, were correlated to the enhanced microbial biomass, carbon dioxide production and hydrolysis of glycosides and phosphorated compounds. We concluded that fire interrupts the conservative cycling typical of mature ecosystems through shifts in microbial biomass, activity and bacterial community structure. In order to identify the major changes in the bacterial communities, we pyrosequenced the 16S rRNA gene in pre- and post-fire soils using 454 Roche GS FLX. We observed in the post-fire community an overrepresentation of endosporulating bacteria owing to their superiority under extreme temperature and desiccation. C) Fire modifies the abundance and diversity of nitrogen cyclers The abundance and diversity of nitrifiers and denitrifiers was investigated in pre- and post-fire communties by quantitative PCR and terminal fragment length polymorphism of several functional markers, namely, the bacterial and archaeal amoA genes, and the narG, napA, nirS, nirK and nosZ genes. The pulse detected in mineral N after burning was mirrored in the abundance of all functional markers involved in the N cycle as illustrated in Fig. 2 (attached). We also detected shifts in the community structure of all functional groups studied. Our results indicate that fire accelerates the nitrogen cycle by promoting the communities of nitrifiers and denitrifiers. This might increase the emissions of nitrous oxide to the atmosphere and reduce the soil’s ability to store assimilable N. One year after burning the numbers of nitrifiers and denitrifiers tended to be similar to those under undisturbed conditions. Longer-term experiments are needed in order to determine the system’s resilience.

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