Modelling nitrogen-limited litter decomposition with fungal dynamics

Nitrogen (N) availability influences aboveground productivity, yet the mechanisms governing the retention and release of soil N remain poorly understood. In high latitude regions, N availability often limits decomposition, though this critical factor is rarely integrated into existing decomposition models, which predominantly focus on carbon quality and accessibility. To address this gap,  we developed a process-based model of litter decomposition to investigate the effect of low N availability on decomposition. Distinct from most decomposition models, our model explicitly features mechanisms of resource reallocation within the fungal mycelium. Fungal biomass is divided into three fractions: 1) cytoplasmic cells active in decomposition, 2) vacuolised cells with a lower N content and without decomposition capacity, and 3) dead cells (necromass). The model can predict mass loss trajectories of a variety of litter types with different N content based on a single parameter set. The fungal mycelium responds to N limitation by increasing the proportion of vacuolised, inactive cells with a low N content, reducing decomposition rates. Under N limitation, N accumulates in the necromass pool. To predict the observed patterns of N immobilization and release, the rate of fungal necromass decomposition has to be slow and close to that of lignin. Moreover, we found that slow mycelial growth facilitates exploitation of low N resources, whereas fast growth intensifies N-limitation. Our model disentangles the interplay between N availability, mycelial dynamics, and decomposition, pointing towards the potentials of more explicit incorporation of fungal traits in models of N-limited ecosystems.