Cranberry plants and ericoid mycorrhizal fungi affect agroecosystem nitrogen recycling

Cultivated cranberry plants can be used as a model of other ericoid mycorrhizal species growing in low-nutrient, acidic environments. In the cranberry agroecosystem, it is possible to study both the effects of the plants on nitrogen (N) cycling through species-specific litter quality analyses, as well as the effects of ericoid mycorrhizal (ERM) fungi associated with cranberry roots. Cranberry plants regulate nitrogen release in this low-input (< 50 kg ha^-1 yr^-1 of ammonium fertilizer) agroecosystem, through regulation of N resorption rates and the subsequent low N levels of decomposing leaf litter.  From our leaf litter bag study, we found that newly deposited cranberry leaf litter had high C:N ratios, in part due to the N resorption efficiencies (30 – 45%) prior to leaf senescence. We also found that 52-70% of the original mass remains after three years in the field. Even though decomposition is slow, cranberry plants may not have to wait until N is re-mineralized by soil microbes to access it. We found that ERM colonization was ubiquitous throughout cultivated cranberry beds (average 37%), and these fungi can access organic forms of N, possibly mobilizing N from leaf litter prior to mineralization. The active role of the ERM symbiosis in this system is apparent in the δ¹⁵N abundance in live cranberry leaves (negative) [-4.02] – [-1.08] relative to the roots + fungi (positive) 0.25 – 3.45.  This δ¹⁵N pattern is indicative of ERM activity, in that ¹⁵N depleted amino acids are transferred to the shoots, while N remaining in the hyphae is enriched with ¹⁵N. Even in this intensively managed and fertilized agroecosystem, both the cranberry plant and the ERM symbiosis appear to play a key role in N nutrition and N recycling.