Organic Nitrogen Cycling at the Duke Free Air CO2 Enrichment Forest
Abstract
A recent shift in the paradigm of nitrogen cycling suggests that amino acids provide an important source of plant-available nitrogen. Field experiments have documented intact amino uptake in boreal, alpine and wetland ecosystems. Our work shows that intact amino acid assimilation may contribute substantially to plant N demand in temperate pine stands with shallow organic horizons. By using universally labeled alanine and ammonium in field trials, we were able to demonstrate that pine trees assimilated amino acid-N at approximately half the rate of ammonium. Mean fine root 15N assimilation of alanine was 0.7 % compared to 1.8 % recovery of ammonium 15N (p= 0.008). On average the 13C: 15N ratio of fine roots in alanine tracer plots was 3:1, suggesting intact uptake occurred. Cycling of amino acids compared to inorganic N was calculated by tracer recovery in plant, microbial and soil pools. Microbes out competed trees for N, immobilizing up to 10% of the tracer N, while retention in SOM dominates the interim (weeks) distribution. Tracer recovery was greatest in the SOM, followed by microbes, extractable NH4 and roots. These data confirm that nitrogen mineralization assays do not accurately measure the pool of available N in temperate forests. These results advance the growing body of evidence for amino acid assimilation by quantifying its importance and bioavailability relative to inorganic N sources in the field.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFM.B13C0247H
- Keywords:
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- 4870 Stable isotopes;
- 1600 GLOBAL CHANGE (New category);
- 1615 Biogeochemical processes (4805);
- 0400 Biogeosciences