Seasonal changes in nitrogen availability, and root and microbial uptake of 15N13C9-phenylalanine and 15N-ammonium in situ at a temperate heath

In the plant biosynthesis of secondary compounds, phenylalanine is a precursor of condensed tannins. Tannins are deposited into the soil in plant root exudates and dead plant material and have been suggested to precipitate some soil nutrients and hence reduce nutrient availability for plants. Free amino acid, inorganic and microbial N concentration during the growing season was investigated in an ecosystem with a natural tannin chemosphere. The influence of tannins on the uptake of nitrogen in plants and microbes was followed by injecting tannic acid (TA), ammonium-15N and phenylalanine-15N/13C9.Plants preferred ammonium over phenylalanine, while microbes had no preference. Soil microbes had a 77% uptake of intact phenylalanine. Phenylalanine was acquired intact by both grasses and Calluna, with 63% and 38% uptake of intact phenylalanine in grass fine roots and Calluna roots, respectively. Inorganic N and amino acid concentrations were lowest in the period with highest plant activity and grass root biomass but were unaffected by TA addition.

► Plants preferred ammonium over phenylalanine, while microbes had no preference.
► Soil microbe uptake of phenylalanine was 77% intact.
► Phenylalanine uptake by grass fine roots was 63% intact, by Calluna roots 38% intact.
► Addition of TA to the heath soil had no precipitating effect on nutrients.
► Grass fine roots, NH4 and amino acids had large seasonal variation.