title: Plant-available organic and mineral nitrogen shift in dominance with forest stand age.
abstract: Studies of soil nitrogen (N) availability over stand development have almost exclusively focused on mineral N, yet we increasingly recognize that plants can take up organic N in the form of free amino acids at biologically important rates. We investigated amino-acid and mineral N availability along a 10-site chronosequence of jack pine stands, varying in age from 4 to 60 yr following wildfire. We measured free amino-acid N and mineral N in soil extracts; native proteolytic rates; net N mineralization rates; and microbial amino-acid consumption via a 15N leucine tracer assay in 6 of the 10 sites (4, 10, 18, 22, 46, and 55-yr-old). Amino-acid N was consistently low in the youngest sites (4-10 yr), increased rapidly in mid-aged sites (15-22 yr), and was highest in stand age 46. In contrast, mineral N exhibited a parabolic shape (R2 0.499; P < 0.0001), with the youngest site and the four oldest sites containing the highest amounts of mineral N. As a result, amino-acid N as a percentage of amino-acid N + mineral N was greatest in mid-aged stands (e.g., 67% in the 22-yr-old stand). We observed no trend in proteolytic rates across the chronosequence (P = 0.632). Percentage 15N tracer recovery was lowest in the extractable organic N pool for the 4, 10, and 18-yr-old sites, though only site age 10 was significantly different from the older sites. Percentage of recovery in the organic N pool was significantly positively related (R2 = 0.798; P < 0.05) to standing pools of amino-acid N. Overall, our results suggest that heterotrophic consumption, not production via proteolysis, controls soil free amino-acid availability. Higher microbial demand for free amino acids in younger vs. older sites likely results from greater microbial C and N limitation early in stand development due to the lack of fresh litter inputs. Since aminoacid N exceeds mineral N in a time period of stand development where jack pine growth rates and N demand are highest, we speculate that amino-acid N may be important to the N economy of these forests.
authors.affiliation: Department of Forestry and the Ecology, Evolutionary Biology and Behavior Program, Michigan State University, 126 Natural Resources, East Lansing, Michigan 48824, USA. leduc.stephen@epa.gov
authors.forename: Stephen D
authors.initials: SD
authors.lastname: LeDuc
authors.forename: David E
authors.initials: DE
authors.lastname: Rothstein
mesh_terms: D000375:Aging; D000596:Amino Acids / Q000737:chemistry / Q000378:metabolism; D017753:Ecosystem; D009584:Nitrogen / Q000737:chemistry* / Q000378:metabolism*; D012987:Soil / Q000032:analysis; D012988:Soil Microbiology; D014197:Trees / Q000502:physiology*
chemical_list: D000596:Amino Acids; D012987:Soil; D009584:Nitrogen