Plant‐available organic and mineral nitrogen shift in dominance with forest stand age

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 amino‐acid 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.