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