Does long-term warming affect C and N allocation in a Mediterranean shrubland ecosystem? Evidence from a 13C and 15N labeling field study

In the Mediterranean basin the effects of climate warming on ecosystem functioning will strongly depend on the warming intensity directly but also on its effects on evapotranspiration and nutrient cycling. Climate manipulation experiments under field conditions are a source of unique empirical evidence regarding climate-related modifications of biotic processes. A field night-time warming experiment, simulating the predicted near-future increase in ambient temperatures (+0.3 up to 1o C), was established in a Mediterranean shrub community located in Porto Conte (Italy) in 2001. After 11 years of continuous treatment, we labeled the dominant shrub Cistus monspeliensis with 13CO2 and studied the dynamics of the label allocation between aboveground and belowground pools and fluxes in warmed and ambient plots within 2 weeks of the chasing period. The interactions between C and N metabolism were assessed by parallel labeling of soil with K15NO3. Most of the assimilated 13C was respired by Cistus shoots (28-51%) within two weeks. Cistus under warming respired more 13C label and tended to allocate less 13C to leaves, branches and roots. The higher C and N content in microbial biomass in warming plots, combined with the higher N content in plant tissues and soil, evidenced a greater N mobilization in soil and a better nutrient status of the plants as compared to the ambient treatment. Acceleration of N cycling is probably responsible for higher respiratory C losses, but combined with the reduction in the number of frost days, should also positively affect plant photosynthetic performance. We conclude that, although Cistus plants are already growing in conditions close to their thermal optimum, long-term warming will positively affect the performance of this species, mainly by reducing the nutrient constraints. This positive effect will highly depend on the frequency and amount of rain events and their interactions with soil N content.