OOS 34-10
Ectomycorrhizal fungal impacts on plant nitrogen nutrition: emerging isotopic patterns, latitudinal variation, and possible mechanisms
Ectomycorrhizal (EcM) mediated nitrogen (N) acquisition is one main strategy used by terrestrial plants to overcome growth limitation. Natural abundance nitrogen isotope ratios (expressed as δ15N) increasingly serve as proxies for N loss patterns and the dependency of plants on ectomycorrhizal (EcM) derived N due to fractionating processes that alter source 15N:14N ratios. However, current understanding is latitudinally biased. Much less is known about relative δ15N patterns in low-latitude tropical and subtropical forests where N is typically in excess of plant demand.
Using δ15N values and other site characteristics from 40 high- and 9 low-latitude ecosystems, we assessed relationships among co-occurring soil, EcM and saprotrophic fungi, and EcM and arbuscular mycorrhizal (AM) plants using structural equation modeling, AIC model selection, and isotope mass balance. The SEM approach allowed us to examine hypothetical causal pathways in a complex network of direct and indirect relationships. These datasets also permitted a more geographically balanced examination of how co-occurring ecosystem δ15N values are structured across broad latitudinal gradients.
Results/Conclusions
Overall, our SEM and linear models supported the presence of distinctive pathways by which the δ15N values of EcM plants and fungi deviate from co-occurring AM plants and decomposer fungi. The δ15N values of AM plants (R2=0.55) were directly influenced by latitude-dependent processes and soil δ15N values. The δ15N values of EcM plants (R2=0.63), in contrast, were directly influenced by latitude-dependent processes and the δ15N values of EcM fungi. The δ15N values of EcM fungi were directly, yet differentially, influenced by soil N concentrations and δ15N values.
In agreement with biogeochemical theory, soils, saprotrophic fungi, and EcM plants were 15N-enriched in tropical latitudes suggesting greater fractionating N losses in warmer and wetter ecosystems. Yet, EcM fungi were consistently 15N-enriched relative to saprotrophic fungi. In contrast to current theory, EcM plants were not 15N-depleted relative to co-occurring AM plants in the tropics and established isotope mixing models could not explain observed patterns. Combined, our results suggest that low-latitude EcM trees are less reliant on the delivery of 15N-depleted N compounds from associated EcM fungi and there may be distinct pathways of N processing in low-latitude ecosystems not limited by N availability. Understanding the role of mycorrhizae in global N cycles will require reevaluation of high latitude perspectives on fractionation sources that structure ecosystem δ15N patterns.