Nitrogen isotopes link mycorrhizal fungi and plants to nitrogen dynamics

Contents Summary 367 I. Introduction 367 II. Background on isotopes 368 III. Patterns of soil d15N 370 IV. Patterns of fungal d15N 372 V. Biochemical basis for the influence of fungi on d15N patterns in plantsoil systems 373 VI. Patterns of d15N in plant and fungal culture studies 374 VII. Mycoheterotrophic and parasitic plants 375 VIII. Patterns of foliar d15N in autotrophic plants 376 IX. Controls over plant d15N 377 X. Conclusions and research needs 378 Acknowledgements 379 References 379 Summary In this review, we synthesize field and culture studies of the 15N/14N (expressed as d15N) 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 d15N (with no partitioning between chitin and protein), mycoheterotrophic plants are higher in d15N 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 d15N than their plant hosts. N losses via denitrification greatly influence bulk soil d15N, whereas d15N patterns within soil profiles are influenced both by vertical patterns of N losses and by N transfers within the soilplant system. Climate correlates poorly with soil d15N; climate may primarily influence d15N patterns in soils and plants by determining the primary loss mechanisms and which types of mycorrhizal fungi and associated vegetation dominate across climatic gradients.