CANCELLED - Soil nitrogen availability indirectly modifies fine root decay by structuring fungal communities

Background/Question/Methods

Less is known about the factors that control fine root decay relative to aboveground litter decay. Ectomycorrhizal (ECM) fungi can slow litter decay by competing with free-living saprotrophs, but the importance of this process for regulating fine root decay remains unclear. Experimental and theoretical studies on leaf litter suggest the competitive suppression of saprotrophic decay by ECM fungi should be strongest when litter is rich in lignin. Because fine root litter contains far greater concentrations of lignin (i.e., 35-45%) than leaf litter, we predicted that the competitive suppression of fungal saprotrophs – particularly those capable of decaying lignin – by ECM fungi would control spatial variation in fine root decay. We used a litterbag decay study across 12 northern temperate forests distributed along a gradient of soil inorganic N availability, where we previously observed a negative relationship between ECM fungi and N availability. We characterized fungal community composition in decaying fine root litter using high throughput ITS2 amplicon sequencing, and assigned fungal sequences to functional groups. We also estimated community-aggregated decay traits (CADT) from publicly available sequenced genomes to understand if these fungal dynamics controlled fine root decay by modifying the genetic decay capacity of fungal communities.

Results/Conclusions

Using generalized additive models (GAMs), we found fine root decay was positively correlated with the relative abundance of lignin-degrading saprotrophic fungi (P < 0.001). The cumulative CADT of 18 gene families associated with rapid decay was also positively correlated with lignin-degrading saprotrophic fungi (P < 0.001), suggesting these fungi accelerate root decay by increasing the community genetic potential for litter decay. We found that ECM fungi inhabiting decaying roots decreased with increasing inorganic N availability (P < 0.001) and that the relative abundance of lignin-degrading saprotrophs was negatively correlated with ECM fungi (P < 0.001), suggesting ECM fungi suppress the saprotrophs responsible for root decay. This finding indicates inorganic N availability indirectly regulates fine root decay by modifying competitive interactions between ECM fungi and lignin-degrading saprotrophs. We previously found that ECM fungi – especially those that produce peroxidase enzymes – increase with decreasing N availability, and that these compositional shifts restrict soil carbon storage by directly decaying soil organic matter. Together, this evidence and our current findings suggest ECM fungi suppress the early stages of fine root decay but enhance the decay of soil organic matter, and that the magnitude of these contrasting dynamics roles depend upon soil N availability.