Thursday, August 10, 8:00-11:30 am
COS 78 - Nitrogen cycling I
L-4, Lobby Level, Cook Convention Center
Presiders: K Buckeridge

Amino acid and mineral nitrogen availability along a chronosequence of Michigan jack pine stands.

LeDuc, Stephen*,1, Rothstein, David1, 1 Michigan State University, East Lansing, MI

ABSTRACT- Studies of successional changes in soil nitrogen (N) availability have almost exclusively focused on mineral N; yet, we now realize that plants can take up organic N in situ at biologically important rates. We investigated changes in amino acid- and mineral N availability along a ten-site chronosequence of jack pine stands, varying in age from 4 to 60 y post-wildfire. We measured total soluble N, free amino acid N and mineral N in soil extracts. We also measured native proteolysis in laboratory incubations and net N mineralization in situ. Both total N and amino acid N increased with stand age, reaching asymptotes at ca. 45 and 27 y, respectively. We observed no difference in proteolytic rates. Mineral N and net N mineralization followed a parabolic-shaped curve with stand age: higher in young and old stands, lower in mid-aged stands. Amino acid N as a percentage of bioavailable N (amino acid N + mineral N) was high in mid-aged stands, intermediate in old stands and low in young stands (e.g. 65%, 35% and 16% in the 27, 60 and 4 y stands, respectively). Overall, we observed differences in standing pools of amino acid N but no difference in production, suggesting that amino acid N availability is driven by heterotrophic consumption. Since above-ground jack pine growth rates are relatively high in mid-aged sites, amino acid N may be important to the N economy of these forests.

Key words: organic nitrogen cycling, biogeochemistry, jack pine forests

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