Since the industrial revolution, human activities have more than doubled inputs of nitrogen to global ecosystems. Increased nitrogen (N) inputs can affect a variety of ecosystem characteristics and processes including soil chemistry and plant-soil interactions involving ectomycorrhizal fungi (EMF). These fungi (EMF) form symbiotic relationships with trees, transferring nutrients such as nitrogen, phosphorus, and calcium to the trees in return for photosynthate. EMF are categorized by the length and organization of their extramatrical mycelia (EMM) into different exploration types (ET), each denoting a specific foraging strategy. This study investigated how chronic N additions to a deciduous forest affected nitrogen mineralization and nitrification, EMF colonization of tree roots, EMF community composition, and N, phosphorus (P) and calcium (Ca) in forest soils. The research was conducted at the two-plus decade chronic nitrogen addition experiment (3 treatments -- control, 50kgN/ha/yr, 150kgN/ha/yr) at the Harvard Forest in central Massachusetts. Measurements included nitrogen mineralization and nitrification rates, phosphatase activity, and calcium levels in leaf litter and surface soil. EMF colonization of root tips was quantified and DNA from soil cores and sequenced amplicons were used to obtain species-level data. Species and colonization data were used to estimate EMM biomass.
Results/Conclusions
Chronic N-additions had far-ranging effects on soil nutrient dynamics. Concentrations of NO3- and NH4+ were notably high in the high N-addition plot, where soil N:P ratios increased more than 3-fold. Phosphatase enzyme activity increased with available N. Ca levels in the soil and leaf litter were low in the high N-addition plots. Ectomycorrhizal colonization decreased with both soil nitrate concentration and N-mineralization. Dominant ET shifted from medium to short and contact ET in the high N-addition plot, dominated by nitrophilic taxa. EMM biomass decreased with available N. EMF facilitate tree uptake of nutrients, including N. If chronic N-addition suppresses ectomycorrhizal colonization, trees could become limited by other nutrients such as Ca. We suggest that chronic N-addition alters the symbiosis between trees and EMF, decreasing colonization and favoring nitrophilic taxa with shorter ET. Shorter ET and lower percent colonization imply that less carbon (C) is cycled through EMF before entering the soil. Presence of shorter ET, likely lacking proteolytic capacity, suggests that EMF-mediated decomposition decreases with N-additions. This conjecture is supported by observed increases in soil C with added N. Our data suggest that chronic N-additions may limit other nutrients like Ca and P, potentially altering tree stoichiometry and forest productivity.