Seasonality in temperature and litterfall along with nitrogen preference of ectomycorrhizal fungi may determine the strength of the Gadgil effect

Background/Question/Methods

The Gadgil effect, in which the competition between ectomycorrhizal (ECM) fungi and free-living saprotrophs for resources like nitrogen (N) slows decomposition, has been widely cited as a mechanism to explain the differences in soil carbon storage between arbuscular mycorrhizal (AM) and ECM ecosystems. However, empirical evidence for the Gadgil effect is equivocal and confounding mechanisms have been proposed that affect its strength. Here we conduct a theoretical modeling experiment, where we explicitly incorporate mycorrhizal processes into the Carbon, Organisms, Rhizosphere, and Protection in the Soil Environment (CORPSE) model and use the model to explore the conditions under which ECM N acquisition processes can induce saprotrophic N limitation. We divided the original CORPSE model’s single microbial biomass pool into three functional pools representing saprotrophs, AM fungi, and ECM fungi, and tested the model across different scenarios of seasonal temperature variation, vegetation phenology, and mycorrhizal decomposition capabilities.

Results/Conclusions

We find that N limitation of saprotrophs only occurs during the non-growing season, when N limitation is induced by relatively higher saprotrophic carbon use efficiency (CUE) due to decreased temperatures and fresh litter inputs with relatively high C:N ratios. Saprotrophic N limitation was directly related to seasonal variations in climate and litter inputs; exclusion of seasonality in temperature and litterfall in the model led to a complete disappearance of the Gadgil effect as saprotrophs effectively recycled N from microbial necromass. Saprotrophic N limitation was particularly high when annual temperature variation was large, litterfall peaked after the hot season, and ECM fungi possessed a strong capacity to mine N from saprotrophic necromass. Our results suggest that the Gadgil effect may be strongly dependent on temporal dynamics of temperature and litterfall, and further regulated by the N preferences of ECM fungi. We would expect to see a strong Gadgil effect in temperate deciduous and boreal forests where litter peaks are observed in autumn and annual temperature variations are high, with ECM species that are specialized in mobilizing N from saprotrophic necromass. These results build a foundation for modeling how interactions between saprotrophs and mycorrhizae affect saprotrophic N limitation and soil carbon storage.