Interactive effects of forest die-off and drying-rewetting cycles on C and N mineralization

Mediterranean forests will experience more frequent and intense drought periods and extreme rainfall events in the coming decades. Concomitantly, drought-induced forest die-off is likely to increase. Changes in rainfall patterns and forest die-off directly influence soil microbial communities and activity and, consequently, carbon (C) and nitrogen (N) turnover, but their interactive effects have not yet been explored. We investigated the short-, and the long-term interactive effects of forest die-off and drying-rewetting cycles on soil C and N mineralization rates of a Mediterranean woodland. Soil samples collected under and out of the influence of holm oak (Quercus ilex) trees with different defoliation degrees (six healthy, six affected and six dead) were incubated under two contrasting water regimes (i.e. drying-rewetting cycles vs. constant soil moisture). Potential soil C and N mineralization responded differently to water regimes, with an overall 55% increase in C mineralization and a 22% decrease in N mineralization in the drying-rewetting cycle treatment compared to the constant moisture treatment. Holm oak decline decreased the response of C mineralization while increased the response of N mineralization to the drying-rewetting cycles at both the short- and the long-term. Moreover, N turnover showed a higher sensitivity to these environmental changes than that of C during most of the year. Our study provides solid evidence that an intensification of the drying-rewetting regimes can result in a decoupling of soil C and N cycles in Mediterranean forests and that forest die-off might enhance this decoupling at both the short- and the long-term, with important implications for the ecosystem functioning.
This study was supported by the International Laboratory of Global Change (LINCGlobal, CSIC4540), the Spanish Ministry of Economy and Competitiveness grant VERONICA (CGL2013-42271-P), the Community of Madrid grant REMEDINAL3-CM (S2013/MAE-2719) and the FCT/MEC through national funds and the co-funding by the FEDER (PT2020 Partnership Agreement and COMPETE 2020, UID/BIA/04004/2013). The authors are especially grateful to David López-Quiroga and Ana Prado Comesaña for their excellent help in the field and laboratory and to Aldo Barreiro for his assistance with data and statistical analysis. AR was supported by the Spanish National Research Council (CSIC) in the JAE-doc modality co-financed by the European Social Fund (ESF) and by a Postdoctoral Grant of the Portuguese Science and Technology Foundation (SFRH/BDP/108913/2015).