4.4b N: Trophic vs. Source

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Trophic amino acids

  • heavily fractionating

  • glutamic acid

  • commonly undergo transamination

Source amino acids

  • minimally fractionating - not zero fractionation

  • phenylalanine

  • commonly undergo hydroxylation

  • show the baseline

Trophic position = trophic - source = Glu - Phe

  • same source with different trophic means variation in diet

  • same trophic position/difference between trophic & source means baseline variability

TDF

  • measured TDF glu-phe values are consistently lower than expected

    • diet quality - amino acid imbalance with low diet quality leads to more de novo biosynthesis -> increased Glu transamination = higher TDF

      • In carnivores: higher protein content = higher excretion rate = higher TDF

      • In Herbivores: lower protein content = lower excretion rate = lower TDF

    • Waste type - ammonia producers have higher TDFs than urea/uric acid producers

    • This change can be included in trophic position models by using multiple TDFs as you move up the food chain

  • Basically the same things as TEF except for the rationale behind the word choice (Kelton prefers TEF)

Beta

  • β = δ15NGlu - δ15NPhe in 1° producers

  • vascularization drives beta values (not habitat or mode of photosynthesis)

  • Phe is the main precursor to many secondary compounds in plants, including lignin

  • non vascular plants have higher beta value than vascular

Microbial N

  • Microbial N metabolism is highly complex

  • mixotrophic organisms

  • Microbes have a very low & highly variable TDF

SumV

  • degree of microbial working of organic matter

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