Interaction with elevated CO2 and disturbances on belowground processes in a mature temperate forest

Temperate forests are a significant terrestrial carbon sink and can help mitigate rising CO₂ concentrations in the atmosphere. Under elevated CO₂ (eCO₂) conditions, a higher rate of photosynthesis can drive forest biomass productivity; however, this response can be constrained by the availability of soil nutrients, namely nitrogen (N) and phosphorus (P). To alleviate these deficiencies, increased allocation of carbon (C) belowground could drive processes which can enhance nutrient uptake, thereby supporting the growth of mature forests under eCO₂. However, the belowground processes and their interactions with above-ground disturbances (e.g. moth infestation) driving this change remain unclear. These belowground processes may involve fine root biomass growth, soil nutrient cycling, extracellular enzymatic activities, microbial biomass, and their interactions.

Here, we investigated whether eCO₂ affects belowground processes and their interaction with disturbances (moth outbreak). The experiment was conducted at the Birmingham Institute for Forest Research Free Air Carbon Dioxide Enrichment (BIFoR FACE) facility, which comprises six experimental arrays surrounding woodland patches of c. 30 m diameter. Three of these arrays are enriched with eCO2 (+150 ppm above ambient), and three are under ambient CO₂ levels. Within all six arrays, soil and root samples were collected from the top 0-30 cm covering O, A and B horizons. Fine root biomass stocks, soil N and P levels, root C, N and P content, microbial biomass C, N and P concentration, and extracellular enzymatic activities were quantified from 2017 to 2022.

We found that under eCO₂, root biomass and microbial biomass were significantly greater, especially in the O soil horizons. No change in microbial biomass C: N: P stoichiometry was observed, but root P concentrations declined, and root C: N and C:P ratios increased under eCO₂. In addition, while ammonium concentrations were significantly greater under eCO₂, there was a trend towards lower phosphate and nitrate concentrations under eCO₂. Potential C, N and P cycle enzyme activities increased under eCO₂, and the LAP: AP ratio declined under eCO_2. Overall, the changes observed under eCO₂ suggest greater belowground C allocation under eCO₂, and changes in nutrient cycling, which may have resulted in P becoming relatively less available than N. However, during the summers of 2018 and 2019, moth outbreaks caused widespread oak defoliation and reduced forest productivity substantially. The available data from this period suggests that soil nutrient availability increased substantially, likely as a result of leaf litter and frass inputs and low tree nutrient uptake following defoliation. Greater soil nutrient availability resulted in microbial biomass N pools and fine roots proliferation in the O soil horizon. These results may suggest that the insect outbreak had substantial impacts on tree responses to eCO₂ over an extended time period, potentially controlling whether eCO₂ productivity gains were allocated to long versus short-lived tissues.