The role connectivity in soil for stability of soil organic matter degradation: a biophysical model

Soils contain the largest terrestrial reservoir of organic carbon, and dynamics in soil organic carbon turnover will drive carbon-climate feedbacks over the coming century. To date, most SOC dynamics have been simulated with pool-based models, which assume homogeneous physical properties in soil. However, there is increasing evidence which suggests that soil carbon turnover is not just determined by carbon inputs, but by restrictions on microbial access to organic matter in a spatially heterogeneous soil environment. Pore geometry is a critical factor in affecting the accessibility of organic matter for microorganisms, but this accessibility has not been explicitly considered in models. Therefore, we have a challenge to mechanistically predict how environmental change will impact on the balance between soil stored in soil and in the atmosphere.

We will exemplify the impact of pore-connectivity on organic matter turn-over by modelling fungal mediated processes in soil. The fungal model includes important fungal processes such as growth, death and spread, secretion of enzymes to degrade soil organic matter, and translocation of dissolved organic matter through its hyphal structure. For such a complex system the question of what constitutes a connected habitat and how this affects soil processes may not be as easy to address and needs to consider more than the pore connectivity often highlighted. For example, for these fungal mediated processes, habitat connectivity may be determined by various characteristics, namely: (i) the total volume of the connected pore space; (ii) the connected air-filled pore volume, through which fungal spread predominantly occurs and gasses diffuse, (iii) the connected water phase volume, through which dissolved C diffuses, (iv) the distribution of particulate organic matter that fuels fungal growth, and (v) biological traits such as those enabling translocation through for example fungal hyphal networks. We demonstrate how various aspects of habitat connectivity differentially impact on two contrasting fungal species, representing e.g. R and K strategists. The results suggest that: i) connectivity of the water phase is critical as it regulates diffusion of dissolved organic matter; this is even so for fungi that preferentially spread through air-filled pores, and ii) whether fungi behave as R or K strategists is not just determined by fungal traits but to a large extend depends on the physical environment. Consequently, selective pressures can be exerted by physical conditions. It was however not possible to identify a key physical driver as the dynamics were mostly determined by interactions between the various types of connectivity. These different pathways tend to compensate each other, enhancing stability of the function. We argue that the fact that multiple connected pathways underpin a soil function leads to resilience of soils to perturbations and underpins soil health.