Long-term warming affects 13C and 15N allocation in a field-grown Mediterranean shrub Cistus Monspeliensis.

The greenhouse gas emissions continue unabated, projecting rapid changes in the global climate. During the 20th century the global temperatures have raised by 0.6°C. A further increase of 0.3 to 0.7°C is projected for the period 2016-2035 (IPCC 2013). Long term field manipulation experiments, which mimic expected changes in future air temperatures and water regimes, could give realistic picture on the response of different aspects of ecosystem C cycle to changing climate. Temperature was manipulated for 10 years in a Mediterranean shrubland of Porto Conte (Italy). Reflective curtains spanned over the vegetation at night decreased the loss of IR radiation from plots, resulting in the increase of average air and soil temperature by approximately 1oC. Passive night time warming reflects global heating patterns where major increases are registered in night minimum temperatures rather than in day maximums. We took an advantage of this experimental set-up to study changes in allocation patterns of C between pools and fluxes in Cistus monspieliensis shrub - a dominant species within the experimental site. For this purposes we labeled control and warmed plots in 13C enriched atmosphere and studied the distribution of labeled assimilates between numerous pools and fluxes. To better explain C allocation patterns a particular attention was given to interactions with plant N availability which was evaluated by labeling of soil with N 15N-enriched KNO3. Higher enrichment was found in non-soluble leaf sugar fraction under Warming. This additional C was however just temporary stored in reserves rather than stabilized in the cell walls and was further utilized to fuel metabolic processes. By the end of the chasing period more C was allocated to leaves in Control plots. Shoot respiration resulted to be a considerable sink for newly assimilated C especially in warming plots where 40% of recovered 13C was allocated to this flux immediately after the labeling. Tissues N content explained patterns of root-derived respiration, which was more enriched in Control plots. Lower N availability here strengthen the sink capacity of roots for C in confront to leaves. Higher root-derived respiration rate in Control was therefore a result of higher substrate availability. Changes in soil water content determined patterns of tissue's N. Highly mobile components of soil N pool were lost with rain water runoff. However, our data indicate that N losses should be promptly counterbalanced by increased N mineralization under Warming resulting in higher tissues N content when meteorological conditions of the site are stable. Higher allocation of C to shoot respiration coupled to lower amount of C remaining in biomass constrains C sequestration capacity of plants under Warming. For a positive C balance, which is observed by a long-term monitoring survey in this site, gain of C should exceed its losses, therefore we predict higher assimilation rates in Warming plots over the season. Longer growing season and reduction of the frost days in Warming plots should add to the Cistus productivity success.