Nitrogen isotopes link mycorrhizal fungi and plants to nitrogen dynamics
Summary In this review, we synthesize field and culture studies of the 15 N/ 14 N (expressed as δ 15 N ) 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 15 N ‐depleted chitin, ammonia, or transfer compounds or 15 N ‐enriched proteinaceous N . For example, parasitic plants and autotrophic hosts are similar in δ 15 N (with no partitioning between chitin and protein), mycoheterotrophic plants are higher in δ 15 N than their fungal hosts, presumably with preferential assimilation of fungal protein, and autotrophic, mycorrhizal plants are lower in 15 N than their fungal symbionts, with saprotrophic fungi intermediate, because mycorrhizal fungi transfer 15 N ‐depleted ammonia or amino acids to plants. Similarly, nodules of N 2 ‐fixing bacteria transferring ammonia are often higher in δ 15 N than their plant hosts. N losses via denitrification greatly influence bulk soil δ 15 N , whereas δ 15 N 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 δ 15 N ; climate may primarily influence δ 15 N patterns in soils and plants by determining the primary loss mechanisms and which types of mycorrhizal fungi and associated vegetation dominate across climatic gradients. Contents Summary 367 I. Introduction 367 II. Background on isotopes 368 III. Patterns of soil δ 15 N 370 IV. Patterns of fungal δ 15 N 372 V. Biochemical basis for the influence of fungi on δ 15 N patterns in plant–soil systems 373 VI. Patterns of δ 15 N in plant and fungal culture studies 374 VII. Mycoheterotrophic and parasitic plants 375 VIII. Patterns of foliar δ 15 N in autotrophic plants 376 IX. Controls over plant δ 15 N 377 X. Conclusions and research needs 378 Acknowledgements 379 References 379