Carbon and nitrogen gas and leachate fluxes, soil and vegetation properties in oceanic-alpine ecosystems during a drought-rewetting mesocosm experiment

Plant-soil mesocosms (10 cm diameter, 15 cm depth) were collected from Nardus snowbed grassland and Empetrum shrub heath in the Allt a'Mharcaidh catchment, western Cairngorm mountains, Scotland, and transferred to a controlled-environment facility the following day. Mesocosms were maintained under simulated diurnal light, temperature and humidity conditions and randomly assigned to combinations of drought (control or 16-day drought) and rewetting intensity (low or high) treatments within a structured experimental timeline (acclimation, baseline, drought, rewetting and recovery phases). Artificial rain, formulated to reflect northwest Scotland precipitation chemistry, was applied according to treatment design. Throughout the experiment, CO₂ fluxes (net ecosystem exchange and ecosystem respiration) were measured using an infrared gas analyser and chamber system, with fluxes calculated from concentration changes over time. Leachate was collected at defined intervals, filtered and analysed for ammonium, nitrate, total dissolved nitrogen and dissolved organic carbon, enabling calculation of nutrient fluxes. Vegetation cover was assessed visually and analysed using principal coordinate analysis, while soil temperature and mesocosm mass were monitored regularly to determine moisture dynamics. At harvest, soils and vegetation were destructively sampled to quantify soil carbon and nitrogen content, pools, microbial biomass carbon and nitrogen (via fumigation-extraction), and aboveground biomass using combustion analysis and standard laboratory procedures. Together, these measurements quantify ecosystem carbon and nitrogen fluxes and soil-plant responses to drought and rewetting intensity treatments under controlled conditions.