Mid-Project Analysis of Carbon and Nitrogen Transfer in Boreal Forests Using Game Theory, Optimization and Fixed Ratios

Nitrogen availability often limits photosynthesis and growth in boreal forests, where nitrogen is a key constraint for plant productivity. Trees and other plants acquire nitrogen through a complex belowground interface comprising fine roots, symbiotic, and non-symbiotic microorganisms. To sustain this interface, photosynthetically derived carbon is allocated to fine root growth, mycorrhizal symbiosis, and exudation—either into the surrounding soil or directly to associated microbial communities. These exudates serve as critical energy sources for both symbiotic and non-symbiotic microbes, which, in turn, provide nitrogen to the tree through direct transfer or organic matter decomposition. This highlights the importance of the entire belowground infrastructure in the nitrogen acquisition of trees. 

This study investigates carbon-nitrogen dynamics in boreal forest ecosystems with an emphasis on eco-evolutionary processes and ecosystem function. Specifically, three nitrogen transfer strategies—game theory, optimization, and fixed ratios—are analysed from the perspectives of Scots pine roots and their ectomycorrhizal partners. This framework aims to illuminate the underlying relationships governing carbon exchange and nitrogen acquisition, contributing to ongoing debates on carbon source-sink dynamics and contrasting models of carbon allocation, including the "Surplus Carbon Hypothesis" and "Biological Market Models". 

The approach integrates a custom soil model, an adapted ectomycorrhizal model based on, a tree growth model (CASSIA), and a modified photosynthetic assimilation model (p-hydro) that incorporates nitrogen limitations. By including both symbiotic and non-symbiotic microbes, the study aims to capture nutrient cycling feedbacks, such as the priming effect, and explore microbial community shifts driven by functional dynamics. 

Incorporating seasonal variability and rigorous modelling of tree carbon storage, allocation, and exudation provides insights into how these patterns influence next year's growth and soil ecosystem functioning. Additionally, accounting for temperature and soil moisture effects enables the disentanglement of environmental influences from sugar inputs in driving belowground processes. This comprehensive framework offers a robust tool for understanding nutrient dynamics and tree-microbe interactions in boreal forests under changing environmental conditions. 

This work is in the calibration stage so preliminary results will be presented. These include soil-side results, such as trenching simulations to capture the change in microbial composition and their contribution to the priming effect. On the tree side; simulations including determination of differing root growth by the value of nitrogen in the optimisation of photosynthesis will be presented. Additionally, a comparison of three photosynthesis input models and two sugar allocation models within the CASSIA framework is conducted to evaluate the effects of differing modelling approaches.