Pathogen-Induced Tree Mortality Modifies Key Components of the C and N Cycles with No Changes on Microbial Functional Diversity

An increase in tree mortality rates as a consequence of biotic disturbances has been detected worldwide. Biotic disturbances in forests might cause significant impacts at ecosystem-level, with strong effects on soil biological activity and organic matter transformation. However, very few empirical studies have explored to date the consequences of biotic disturbances for soil microbial communities and C and N cycles. Here, we assessed the effects ofQuercus suberdieback, caused by the exotic root pathogenPhytophthora cinnamomi,on the soil microbial community and key components of the C and N cycle. We used a spatially explicit neighborhood approach to analyze the effects ofQ. subertrees with different health status and non-declining coexistent species on soil variables. The study was replicated in the two mainQ. suberforest types of the region (closed forests and open woodlands) with contrasting soil texture characteristics. Pathogen-induced tree dieback did not affect microbial functional diversity or biomass, but translated into lower soil microbial respiration. Tree mortality induced changes in several variables of the C and N cycle, but the sign and magnitude of these effects varied depending on the local characteristics of soil texture. Coexistent species differed strongly fromQ. suberin their effects on the C and N cycle. Overall, our results show that tree dieback due to invasive pathogens translates into complex short- and long-term effects on different components of the C and N cycles, despite no effects on microbial functional diversity and biomass.