Mediterranean soils under climate change: a drying-rewetting experiment with 14C-labelled glucose

Drying-rewetting cycles (DRC) affect litter and soil organic carbon (SOC) decomposition and mineralization, especially in Mediterranean ecosystems. Global climate change is expected to increase drought periods as well as heavy precipitation frequency, which in turn will increase soils DRC. However, the effects of DRC on the functioning of microbial communities and dynamics of dissolved organic carbon (DOC) remain elusive. Here, we investigate the effects of climate-change on organic carbon turnover rates based on a DRC approach.

Composite dehesa soil samples (0-10 cm) (Pozoblanco, Córdoba, Spain) were taken from three forced climatic treatment plots (W: warming (heat increase); D: drought (water restriction); C: Control). The plots were installed 4 yrs ago under two distinct habitats: evergreen oak canopy (designated as ‘tree’) and in the open pasture (‘open’). The soil samples were incubated for 26-days at a constant moisture (40% of water-holding capacity, WHC) and labelled ¹⁴C-glucose (150 % of C from microbial biomass). Afterwards, to simulate drought in nature, ¾ of each sample were dried and further four rewetting treatments were established: 1) constant-moisture at 40% WHC, 2) slow DRC with 5-days water addition to 40% WHC, 3) fast DRC with all water added during the first day of the experiment, and 4) dry DRC with 7-days drying and no rewetting. Following DRC period, there was an extended incubation (26 d in total), where samples were taken at three times after rewetting (4, 7 and 26 days) for further analyses. Total and ¹⁴C-glucose-derived dissolved organic carbon (DOC), microbial biomass (MBC), C, N and P related enzymatic activities, and other parameters of microbial growth were measured. During the incubation period total and ¹⁴C-CO₂ were also monitored.

The results obtained and the discussion of the DRC effects detected and main threads regarding climate change in Mediterranean dehesa agroforestal system such as increasing temperatures and drought events on microbial biomass, respiration and C turnover, will be detailed. Changes in DRC can alter organic C mineralization, in turn such effect can strongly depend on previous field-induced conditions in Mediterranean savannas. In addition, our results will help to understand the responses of soil MBC and DOC to DWC in Mediterranean ecosystems and could improve the prediction of CO₂ emission under a changing environment in the future.

Acknowledgment: EU-EJC 2^nd Call Projects MIXROOT-C and MAXROOT-C. L.M. San Emeterio thanks Ministerio de Ciencia Innovación y Universidades (MICIU) FPI research grant (BES-2017-07968) and the German Academic Exchange Service (DAAD) for funding. A.M. Carmona, M.D. Hidalgo, P. Campos and K. Schmidt are acknowledged for technical assistance.