Nitrogen isotopes link mycorrhizal fungi and plants to nitrogen dynamics

Abstract

Contents Summary367I.Introduction367II.Background on isotopes368III.Patterns of soil δ15N370IV.Patterns of fungal δ15N372V.Biochemical basis for the influence of fungi on δ15N patterns in plant–soil systems373VI.Patterns of δ15N in plant and fungal culture studies374VII.Mycoheterotrophic and parasitic plants375VIII.Patterns of foliar δ15N in autotrophic plants376IX.Controls over plant δ15N377X.Conclusions and research needs378Acknowledgements379References379 Summary In this review, we synthesize field and culture studies of the 15N/14N (expressed as δ15N) of autotrophic plants, mycoheterotrophic plants, parasitic plants, soil, and mycorrhizal fungi to assess the major controls of isotopic patterns. One major control for plants and fungi is the partitioning of nitrogen (N) into either 15N‐depleted chitin, ammonia, or transfer compounds or 15N‐enriched proteinaceous N. For example, parasitic plants and autotrophic hosts are similar in δ15N (with no partitioning between chitin and protein), mycoheterotrophic plants are higher in δ15N than their fungal hosts, presumably with preferential assimilation of fungal protein, and autotrophic, mycorrhizal plants are lower in 15N than their fungal symbionts, with saprotrophic fungi intermediate, because mycorrhizal fungi transfer 15N‐depleted ammonia or amino acids to plants. Similarly, nodules of N2‐fixing bacteria transferring ammonia are often higher in δ15N than their plant hosts. N losses via denitrification greatly influence bulk soil δ15N, whereas δ15N patterns within soil profiles are influenced both by vertical patterns of N losses and by N transfers within the soil–plant system. Climate correlates poorly with soil δ15N; climate may primarily influence δ15N patterns in soils and plants by determining the primary loss mechanisms and which types of mycorrhizal fungi and associated vegetation dominate across climatic gradients.

Stable nitrogen isotope patterns of trees and soils altered by long-term nitrogen and phosphorus addition to a lowland tropical rainforest

“…mid dry season) using steel ammunition. All four species associate with arbuscular mycorrhizal fungi (M. Sheldrake, unpublished data), thus accounting of ectomycorrhizal fractionation is not necessary (Hobbie and Högberg 2012 …”

Reconstructing terrestrial nutrient cycling using stable nitrogen isotopes in wood

“…Hydrologic loss of soil NO3 -through leaching and preferential uptake of NH4 + by trees amplifies the importance of fractionation between inorganic soil N pools. A third fractionation pathway involves plant associations with mycorrhizal fungi that give depleted N to plants (Hobbie and Högberg 2012). The most significant fractionation is observed in plants associated with ericoid mycorrhizal fungus, with decreasing fractionation effects in plants associated with ectomycorrhizal, arbuscular mycorrhizal, and non-mycorrhizal fungus (Craine et al 2009).…”

Attributing functions to ectomycorrhizal fungal identities in assemblages for nitrogen acquisition under stress

“…EMF community structures are strongly influenced by N deposition (Lilleskov et al, 2011;Kjøller et al, 2012). Stable isotope studies have revealed that EMF species differ in their abilities to exploit different N sources (Hobbie and Hö gberg, 2012). Furthermore, in situ ectomycorrhizal communities exhibit strong temporal differences in the capability of different EMF taxa to access litter-derived N (Pena et al, 2013a).…”

“…Ectomycorrhizal fungi (EMF) encase colonized root tips with a dense hyphal net, termed the mantle, and forage the soil for nutrients by extending extraradical hyphae or hyphal cords (Finlay, 2008). As nitrogen (N) is a major limiting nutrient in many forest ecosystems (LeBauer and Treseder, 2008), the role of EMF in the N nutrition of trees has received considerable attention (Hobbie and Hobbie, 2008;Hobbie and Hö gberg, 2012). In addition to N delivery, recent studies have suggested that EMF may also limit N transfer to host trees under N-limiting conditions (Näsholm et al, 2013).…”