Effects of rain shortage on carbon allocation, pools and fluxes in a Mediterranean shrub ecosystem - a C labelling field study.

Hydrological cycle is expected to become the primary cause of ecosystem's degradation in near future under changing climate. Rain manipulation experiments under field conditions provide accurate picture on the responses of biotic processes to changed water availability for plants. A field experiment, mimicking expected changes in rain patterns, was established in a Mediterranean shrub community at Porto Conte, Italy, in 2001. In November 2011 Cistus monspeliensis, one of the dominating shrub species in the Mediterranean basin, was C labelled on plots subjected to extended rain shortage period and on control non manipulated plots. Carbon (C) allocation was traced by C dynamics in shoots, shoot-respired CO, roots, microbial biomass, KSO-extractable C and CO respired from soil. Most of the recovered C (60%) was respired by shoots within 2weeks in control plots. In rain shortage treatment, C remained incorporated in aboveground plant parts. Residence time of C in leaves was longer under the rain shortage because less C was lost by shoot respiration and because C was re-allocated to leaves from woody tissues. The belowground C sink was weak (3-4% of recovered C) and independent on rain manipulation. Extended rain shortage promoted C exudation into rhizosphere soil in expense of roots. Together with lowered photosynthesis, this "save" economy of new C metabolites reduces the growing season under rain shortage resulting in decrease of shrub cover and C losses from the system on the long-term.