Natural Occurrence of Stable Isotopes Stable Isotopes Stable Isotopes as a probe for Carbon, Mineral and Water Cycles Several elements are found in more than one form E.g., Hydrogen can be found in its normal form called Hydrogen, but has a variant with an elemental weight of 2, called Deuterium E.g., Nitrogen is usually found at elemental weight of 14 N, but can also be found in other states, most common of these rarer states is 15 N Stable Isotopes are those that don t decay and remain in their state Stable isotopes of normal forms are fairly rare in nature, but do consistently occur. Ecologists have mostly used H, C, O, N, S and Strontium. Isotope variation is expressed in Delta Notation, which is the ratio of the sample (rare/common isotope) divided by the ratio of a standard (rare/common isotope) : Samples are considered heavier or enriched with the heavier isotopes with a larger delta number relative to a standard or another sample. Plants vary in their amount of a parfcular isotope depending on circumstances Enzymes discriminate against heavier isotopes (e.g. 13 C vs. 12 C) called FRACTIONATION circumstances which restrict isotope choice will result in recognizable paperns. Carbon example: Circumstances which promote full or parfal stomatal closure Plants fracfonate against 13 C in CO 2 But water stress increases its abundance because of stomatal closure (no choice) Shaded plants are less drought stressed, and can fracfonate to a greater degree This can be expressed also as WUE, or as a seasonal value 1
Leaves may show a C isotope signal based on the kind of environment in which they develop. Leaf Carbon Isotope values reflect patterns of stomatal opening and closure. The more they are closed, or relatively closed, the more enzymes have less choice to discriminate against heavy Carbon. Thus, leaves exposed to greater light (& wind & drought) end up with higher C delta values (enriched in 13 C). Seasonal frequencies of drought stress results in a similar pattern, heavier C during dry seasons. (Note scale opposite from previous scale). Carbon & Nitrogen: Mycorrhizal Symbiosis SymbioFc relafonship between host plant and fungus associated with roots Fungus assists plant with uptake of soil nutrients and moisture Fungus receives C (energy) from plant Mycorrhizal Fungi SubstanFal porfon of fungi involved in decomposifon Occur on over 90% of plant species of the world High diversity of EM fungi with suggesfons of funcfonal specializafon Dependent on host species in some cases Mycorrhizal Types: Arbuscular Mycorrhizae (AM, formerly endomycorrhizae or VAM- type) AM all from one lineage of Zygomycetes (may be separated out) Ectomycorrhizae (EM, various subtypes) EM from various Ascomycetes and Basidiomycetes 2
AM mycorrhizae Internal mycorrhizae, common in herbaceous plants and some woody plants. Proliferate within root cells, hyphal filaments extend into soil. Not believed to be very host specific. EM Fungi Often the common mushrooms we see in forests. EM Mycorrhizae Large numbers of fungal species are involved with EM mycorrhizae. Believed to be relatively host(genus)-species specific. In cross-section, EM mycorrhizae appear to be a thick sheath of hyphal filaments with numerous hyphae entering root between cells of epidermis and cortex. EM fungal hyphae look like a net in a root cross-section, called a Hartig Net. X-section Mantle Hartig Net Cenococcum Mycorrhizal Fungi Mycorrhizal fungi receive their carbon from plants And they deliver N in organic compounds back to their plant host 13 C amounts in mycorrhizal fungi depend on whether their host is overstory or understory, water stress of host, and degree that host fracfonates sugars 3
Within the same habitat, plants may vary in their particular Delta C values depending on their particular circumstances (birches in more shade in this example) and their fungal mutualists will show correlated values.. Plant hosts fracfonate carbon delivered to mycorrhizal fungi Plant carbon pools differ in their Delta C The lightest pools are in the leaves The heaviest pools are in wood and roots Mycorrhizal fungi are enriched compared with their host with respect to 13 C, but lighter than saprophytic fungi that break down lignins. Mycorrhizal fungi are the principal source of N for many woody plants The fungi fracfonate N before passing N to the host The degree to which plants differ from the environment depends on the abundance of N in the soil and the extent to which they depend on mycorrhizae enriched depleted N Mycorrhizae absorb N compounds from the soil, but pass them on to their plant mutualists in compounds associated with carbohydrates. Because these compounds pass through fungal membranes, the fungi fractionate against the heavier N compounds, resulting in the fungi becoming heavy with N and the plant host light. The DEGREE to which plants are DEPLETED in 15 N depends upon how limiting N is in the environment. 4
Downward-facing triangles represent non-mycorrhizal pine seedlings, all other symbols are pine seedlings with an EM fungus. Mycorrhizal seedlings are always lighter than nonmycorrhizal seedlings. Variation in values depend on what proportion the pine takes up, and species of fungus. Water has two stable isotopes, regular H, and deuterium (D). RaFos will vary by source (rain, fog, stream, water table) due to differences in evaporafon rates. Source rafos can vary seasonally, depth in soil, etc Water transpiring from plants will also differenfally lose H vs. D. Evaporation and condensation modify D % in H2O. lighter heavier even lighter heavier even lighter Soil water, streamwater, and the water table all differ & this was used to determine that plants in riparian zones actually use mostly water table, not stream water. Small trees use soil water as their rooting depth is limited. Larger trees, whether on streams or not, reach the water table and use that water. Italian Macchia (similar to chaparral) on the coast 5
Macchian plants varied in water isotope content Values above rainwater interpreted as saltwater uptake as well Fog is isotopically enriched compared to rainwater in the same region (differential condensation processes) Interpreted as mixing of rain and groundwater When fog is the principal source of water, plants will be enriched' compared to others in the same habitat. Essentially groundwater HYDRAULIC LIFT In dry soil, some plants will bring water up from depth and it will move in reverse flow (acropetally) in lateral roots to the tips of roots, and out roots into the soil. % xylem water coming from fog. Stable isotopes can demonstrate this because D is enriched near the surface compared to soil water at depths (differential evaporation of lighter H). Hydraulic lift (reverse movement of water to lateral root branches) was also conclusively detected by using stable isotopes which determined source of water by depth. DAYTIME DYE ADDED NIGHTTIME Dye was later used to convince the skeptics who can t understand what a stable isotope is. 6
Summary Stable Isotopes occur naturally Because physical and biofc processes discriminate, stable isotopes can be used to answer a number of otherwise difficult quesfons Summary - 2 Leaves under stress get enriched in heavy carbon Mycorrhizal fungi are lighter in carbon than saprophyfc fungi because their carbon pools differ in enrichment Mycorrhizal plants are lighter than non- mycorrhizal plants because the fungi discriminate which nitrogen to pass on Summary - 3 Because of the differenfal dynamics of H 2 O vs HOD vs D 2 O dynamics of soil water can be invesfgated For example roofng depth of plants in riparian areas Extent of importance of fog drip BioFc influences like hydraulic lig 7