Do shifts in ectomycorrhizal fungal communities change the sources of N and S for Bishop pine trees?

Estimates predict that more than 90% percent of land plants are hosts to mycorrhizal fungi. As these ubiquitous symbionts interact with their plant hosts along a spectrum from parasitism to mutualism, it is important to determine how different communities of these fungi might function along this continuum. In this study, we investigated the consequences of Pinus muricata association with ectomycorrhizal fungi (EcM) sampled from different environments throughout North America. Pines are rarely, if ever, found in nature without their EcM associates. As such, it is important to consider the potential consequences of shifts in EcM communities on the trees themselves and how this may also change across different geographical areas occupied by the trees. Our focus was on quantifying how mycorrhizal (M) and non-mycorhizal (NM) trees may differ in their acquisition of the nutrient resources, N and S. N is essential to life, yet it is typically the most limiting nutrient in temperate terrestrial ecosystems. Though C:N ratios did not vary between our treatments, the stable N isotope ratio between M and NM treatments did differ. This result suggests plants could be using different sources (organic versus inorganic) of N. In addition, the relative dependence on organic N pools also appears to vary with latitude of M origin. S is required for the production of many amino-acids, vitamins, chlorophyll, and important antioxidants such as glutathionine, which has been shown to mitigate reactive oxygen species (ROS). In spite of its potential nutritional and ecological importance, sulfur dynamics and the influence of EcM on sulfur uptake is poorly understood. We found that all M treatments demonstrated S isotope enrichment of approximately 6‰ compared to NM treatments. We hypothesize that this apparent enrichment is due to the EcM reduction of soil-borne sulfate to sulfite and the subsequent transport of sulfite from the fungal associate into the tree's roots.