Impact of Elevated Atmospheric CO<sub>2</sub> in <i>Spartina maritima</i> Rhizosphere Extracellular Enzymatic Activities: Fid-Bau Portal

Impact of Elevated Atmospheric CO₂ in Spartina maritima Rhizosphere Extracellular Enzymatic Activities

Bernardo Duarte / Alexandra Baeta / João Carlos Marques / Isabel Caçador

Atmospheric CO₂ enrichment, which is caused to a large extent by anthropogenic activities, is known to interfere with sediment microbial communities via plant rhizospheres. The present work aimed to evaluate this interaction in Spartina maritima ((Curtis) Fernald.) rhizosediments, aiming to depict the impacts of atmospheric CO₂ increase in the biogeochemical processes occurring in the rhizosphere of this pioneer and highly abundant Mediterranean halophyte. For this purpose, mesocosms trials were conducted, exposing salt marsh cores with S. maritima and its sediments to 410 and 700 ppm of CO₂ while assessing rhizosediment extracellular enzymatic activities. An evident increase in dehydrogenase activity was observed and directly linked to microbial activity, indicating a priming effect in the rhizosphere community under increased CO₂. Phosphatase showed a marked increase in rhizosediments exposed to elevated CO₂, denoting a higher requirement of phosphate for maintaining higher biological activity rates. High sulphatase activity suggests a possible S-limitation (microbial or plant) due to elevated CO₂, probably due to higher sulphur needs for protein synthesis, thus increasing the need to acquire more labile forms of sulphur. With this need to acquire and synthesize amino acids, a marked decrease in protease activity was detected. Most carbon-related enzymes suffered an increase under increased CO₂. Overall, a shift in sediment extracellular enzymatic activity could be observed upon CO₂ fertilization, mostly due to priming effects and not due to changes in the quality of carbon substrates, as shown by the sediment stable isotope signatures. The altered recycling activity of organic C, N, and P compounds may lead to an unbalance of these biogeochemical cycles, shifting the rhizosphere ecosystem function, with inevitable changes in the ecosystem services level.