Pathogen-induced tree mortality modifies key components of the C and N cycles with no changes on microbial functional diversity

Abstract

An increase in tree mortality as a consequence of biotic disturbances has been detected in forests worldwide, but little is known about their consequences on soil microbial communities and C and N cycles. We assessed the effects of Quercus suber tree mortality on functional properties of the soil microbial community and different components of the C and N pools in two forest types (open woodlands and closed forests) invaded by the exotic soil-borne pathogen Phytophthora cinnamomi. We used neighbourhood models to analyse the short-term effects of Q. suber mortality on soil variables, by comparing the impact of Q. suber trees with differenthealth status. The potential long-term indirect effects were analysed, comparing the impact of healthy Q. suber and non-declining coexistent tree species (Olea europaea var. sylvestris and Quercus canariensis). Pathogen-induced tree mortality reduced soil microbial respiration, but did not change microbial functional diversity or biomass but. The direct effects of oak decline on C and N pools depended on soil texture. We found a reduction in total soil C but an increase in the more labile forms of C in sites with low clay content. An increase in NO3- but a decrease in NH4+ was found in sites with high clay content. Coexistent species differed strongly from Q. suber in their effects on the C and N cycle. Overall, our results suggest that pathogen-induced tree mortality might translate into short- and long-term effects on C and N cycles, despite no effects on microbial functional diversity and biomass.