Nitrogen conservation strategies of cranberry plants and ericoid mycorrhizal fungi in an agroecosystem

Ericoid mycorrhizal fungi (ERM) are ubiquitous in cultivated cranberry beds, but no prior studies have used 15N natural abundance measurements to track ERM influence on nitrogen (N) cycling in these agroecosystems. Cranberries (Vaccinium macrocarpon) evolved in low-nutrient peat bogs, and ERM symbioses often occur in nutrient-poor environments where N is limiting. We investigated ERM symbiosis, resorption rates, as well as leaf litter quality and decomposition to infer N conservation strategies in cultivated cranberry plants. We expected moderate to high resorption rates and low leaf litter decomposition rates. We also expected to find evidence of ERM fungi mediating N uptake. Resorption rates (35.7 ± 3.5% [s.e.]) were moderate, and the resulting cranberry leaf litter had high C:N ratios (56.9 ± 0.02). Decomposition was slow with 57 ± 2% of initial leaf litter mass remaining at the end of 2.5 years. The negative δ15N values of cranberry leaves (−2.61 ± 0.27‰) and positive δ15N values of cranberry roots (1.04 ± 0.35‰) suggest that N uptake was mediated by ERM fungi as 14N is preferred over 15N during transfer from mycorrhizal fungi to host plant. We demonstrated that even in this intensively managed and fertilized agroecosystem, cranberry plants retain N conservation strategies observed in wildland cranberry ecosystems, namely high resorption efficiencies, low-quality leaf litter, and active ERM symbiosis. This study has management implications, since current nutrient plans mainly focus on inorganic N fertilizer regulation and do not recognize the role of ERM fungi in mediating N uptake.