505I.506II.506III.508IV.512V.513VI.514515References515 Summary Fine roots acquire essential soil resources and mediate biogeochemical cycling in terrestrial ecosystems. Estimates of carbon and nutrient allocation to build and maintain these structures remain uncertain because of the challenges of consistently measuring and interpreting fine‐root systems. Traditionally, fine roots have been defined as all roots ≤ 2 mm in diameter, yet it is now recognized that this approach fails to capture the diversity of form and function observed among fine‐root orders. Here, we demonstrate how order‐based and functional classification frameworks improve our understanding of dynamic root processes in ecosystems dominated by perennial plants. In these frameworks, fine roots are either separated into individual root orders or functionally defined into a shorter‐lived absorptive pool and a longer‐lived transport fine‐root pool. Using these frameworks, we estimate that fine‐root production and turnover represent 22% of terrestrial net primary production globally – a c. 30% reduction from previous estimates assuming a single fine‐root pool. Future work developing tools to rapidly differentiate functional fine‐root classes, explicit incorporation of mycorrhizal fungi into fine‐root studies, and wider adoption of a two‐pool approach to model fine roots provide opportunities to better understand below‐ground processes in the terrestrial biosphere.

SummaryRatios of nitrogen (N) isotopes in leaves could elucidate underlying patterns of N cycling across ecological gradients. To better understand global-scale patterns of N cycling, we compiled data on foliar N isotope ratios (δ 15 N), foliar N concentrations, mycorrhizal type and climate for over 11 000 plants worldwide. Arbuscular mycorrhizal, ectomycorrhizal, and ericoid mycorrhizal plants were depleted in foliar δ 15 N by 2‰, 3.2‰, 5.9‰, respectively, relative to nonmycorrhizal plants. Foliar δ 15 N increased with decreasing mean annual precipitation and with increasing mean annual temperature (MAT) across sites with MAT ≥ −0.5°C, but was invariant with MAT across sites with MAT < −0.5°C. In independent landscape-level to regionallevel studies, foliar δ 15 N increased with increasing N availability; at the global scale, foliar δ 15 N increased with increasing foliar N concentrations and decreasing foliar phosphorus (P) concentrations. Together, these results suggest that warm, dry ecosystems have the highest N availability, while plants with high N concentrations, on average, occupy sites with higher N availability than plants with low N concentrations. Global-scale comparisons of other components of the N cycle are still required for better mechanistic understanding of the determinants of variation in foliar δ 15 N and ultimately global patterns in N cycling.

Review SummaryStudies using carbon isotope differences between C 3 and C 4 photosynthesis to calculate terrestrial productivity or soil carbon turnover assume that intramolecular isotopic patterns and isotopic shifts between specific plant components are similar in C 3 and C 4 plants. To test these assumptions, we calculated isotopic differences in studies measuring components from C 3 or C 4 photosynthesis. Relative to source sugars in fermentation, C 3 -derived ethanol had less 13 C and C 3 -derived CO 2 had more 13 C than C 4 -derived ethanol and CO 2 . Both results agreed with intramolecular isotopic signatures in C 3 and C 4 glucose. Isotopic shifts between plant compounds (e.g. lignin and cellulose) or tissues (e.g. leaves and roots) also differed in C 3 and C 4 plants. Woody C 3 plants allocated more carbon to 13 C-depleted compounds such as lignin or lipids than herbaceous C 3 or C 4 plants. This allocation influenced 13 C patterns among compounds and tissues. Photorespiration and isotopic fractionation at metabolic branch points, coupled to different allocation patterns during metabolism for C 3 vs C 4 plants, probably influence position-specific and compound-specific isotopic differences. Differing 13 C content of mobile and immobile compounds (e.g. sugars vs lignin) may then create isotopic differences among plant pools and along transport pathways. We conclude that a few basic mechanisms can explain intramolecular, compound-specific and bulk isotopic differences between C 3 and C 4 plants. Understanding these mechanisms will improve our ability to link bulk and compound-specific isotopic patterns to metabolic pathways in C 3 and C 4 plants. © New Phytologist (2004) 161 : 371-385

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.

The successful use of natural abundances of carbon (C) and nitrogen (N) isotopes in the study of ecosystem dynamics suggests that isotopic measurements could yield new insights into the role of fungi in nitrogen and carbon cycling. Sporocarps of mycorrhizal and saprotrophic fungi, vegetation, and soils were collected in young, deciduous-dominated sites and older, coniferous-dominated sites along a successional sequence at Glacier Bay National Park, Alaska. Mycorrhizal fungi had consistently higher δN and lower δC values than saprotrophic fungi. Foliar δC values were always isotopically depleted relative to both fungal types. Foliar δN values were usually, but not always, more depleted than those in saprotrophic fungi, and were consistently more depleted than in mycorrhizal fungi. We hypothesize that an apparent isotopic fractionation by mycorrhizal fungi during the transfer of nitrogen to plants may be attributed to enzymatic reactions within the fungi producing isotopically depleted amino acids, which are subsequently passed on to plant symbionts. An increasing difference between soil mineral nitrogen δN and foliar δN in later succession might therefore be a consequence of greater reliance on mycorrhizal symbionts for nitrogen supply under nitrogen-limited conditions. Carbon signatures of mycorrhizal fungi may be more enriched than those of foliage because the fungi use isotopically enriched photosynthate such as simple sugars, in contrast to the mixture of compounds present in leaves. In addition, some C fractionation may occur during transport processes from leaves to roots, and during fungal chitin biosynthesis. Stable isotopes have the potential to help clarify the role of fungi in ecosystem processes.

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When soil nitrogen is in short supply, most terrestrial plants form symbioses with fungi (mycorrhizae): hyphae take up soil nitrogen, transport it into plant roots, and receive plant sugars in return. In ecosystems, the transfers within the pathway fractionate nitrogen isotopes so that the natural abundance of 15N in fungi differs from that in their host plants by as much as 12% per hundred. Here we present a new method to quantify carbon and nitrogen fluxes in the symbiosis based on the fractionation against 15N during transfer of nitrogen from fungi to plant roots. We tested this method, which is based on the mass balance of 15N, with data from arctic Alaska where the nitrogen cycle is well studied. Mycorrhizal fungi provided 61-86% of the nitrogen in plants; plants provided 8-17% of their photosynthetic carbon to the fungi for growth and respiration. This method of analysis avoids the disturbance of the soil-microbe-root relationship caused by collecting samples, mixing the soil, or changing substrate concentrations. This analytical technique also can be applied to other nitrogen-limited ecosystems, such as many temperate and boreal forests, to quantify the importance for terrestrial carbon and nitrogen cycling of nutrient transfers mediated by mycorrhizae at the plant-soil interface.

To determine the dominant processes controlling nitrogen (N) dynamics in soils and increase insights into soil N cycling from nitrogen isotope (d 15 N) data, patterns of 15 N enrichment in soil profiles were compiled from studies on tropical, temperate, and boreal systems. The maximum 15 N enrichment between litter and deeper soil layers varied strongly with mycorrhizal fungal association, averaging 9.6 ± 0.4% in ectomycorrhizal systems and 4.6 ± 0.5% in arbuscular mycorrhizal systems. The 15 N enrichment varied little with mean annual temperature, precipitation, or nitrification rates. One main factor controlling 15 N in soil profiles, fractionation against 15 N during N transfer by mycorrhizal fungi to host plants, leads to 15 N-depleted plant litter at the soil surface and 15 N-enriched nitrogen of fungal origin at depth. The preferential preservation of 15 N-enriched compounds during decomposition and stabilization is a second important factor. A third mechanism, N loss during nitrification and denitrification, may account for large 15 N enrichments with depth in less N-limited forests and may account for soil profiles where maximum d 15 N is at intermediate depths. Mixing among soil horizons should also decrease differences among soil horizons. We suggest that dynamic models of isotope distributions within soil profiles that can incorporate multiple processes could provide additional information about the history of nitrogen movements and transformations at a site.