Soil respiration and nutrient dynamics at a mature temperate woodland - the BIFoR FACE experiment

The atmospheric carbon dioxide (CO2) concentrations have increased approximately 50% since the beginning of the industrial era (1750), breaking the record of 415 ppm, annually. CO2 is considered on of the most important contributing gases to climate change and future predictions support a continuous increase. The rate of anthropogenic emissions increases and the ecosystemic capacity to take up and store carbon (C) govern the predicted atmospheric CO2 increase. The terrestrial biosphere is the principal aspect of the C cycle and climate change. Interestingly, the terrestrial biosphere is also associated with the highest uncertainties in its response to environmental change. Forests are of global significance in the C cycle, storing approximately 860 ± 60 Pg C, with Northern temperate forests being dominant regulators, sequestering one-third of the total C uptake. The general line of thinking was that young forests could sequester C more effectively than mature forests; however recent scientific outcomes suggest that mature forests can continue sequestering C. Nonetheless, disturbances, such as increasing atmospheric CO2, can alter the C storage capacity of the mature forests ecosystem. Although forest ecosystems serve as a CO2 sink currently, the future projections of forest ecosystems' C storage capacity are highly uncertain. This works aims to address the short-term belowground responses of mature temperate woodland, utilising Birmingham’s Institute of Forest Research Free Air CO2 Enrichment (BIFoR FACE) facility. BIFoR FACE is the only large-scale climatic experiment in the Northern Hemisphere, studying a mature forest ecosystem's responses under the atmospheric CO2 scenario of 2050. Subsequently, this works aims to provide an insight into the belowground interactions of three biogeochemical cycles (C, N, and P) and the potential shifts under elevated CO2 using high-resolution frequency measurements for the first time in the history of forest FACE experiments. During the first year of eCO2 enrichment, a 21% increase was observed in soil respiration, on an annual average, releasing approximately 283 g C m-2 y-1 more than in ambient CO2. Moreover, NO3--N availability showed a prompt and sustained decrease, up to 37% in monthly averages, in eCO2 arrays, whereas eCO2 did not affect NH4+-N and PO43--P availability in the soil. During the second year of eCO2 enrichment, we also had the opportunity to monitor a series of extreme events (Beast from the East, winter moth outbreak, and 6-week summer heatwave), alongside the belowground responses to eCO2. The drought caused by the heatwave had a severe effect on soil respiration, suppressing soil respiration by 47 and 40% in ambient and eCO2 arrays, respectively, compared with the previous year. eCO2 dampened the extreme event's impact by increasing soil respiration by ~36% in eCO2 arrays, releasing approximately 259 g C m-2 y-1 more to the atmosphere. A winter moth outbreak led to a short-lived manifold increase in soil nutrient availability during autumn for all three nutrients, with the effect being more prominent under eCO2. Moreover, we attempted to partition soil respiration to its respiratory sources (autotrophic, heterotrophic, root, and hyphal respiration), in situ, during the first two years of eCO2 enrichment. eCO2 stimulated all respiratory components, except for root respiration, during the experiment's first year. This contribution was persistent during the second year through the series of extreme events. The heterotrophic component was the main contributor to soil respiration in both years. However, the respiratory components’ sensitivities to abiotic factors (soil temperature and moisture) were altered under the combined effects of eCO2 and extreme events