Because primary production is broadly limited by nitrogen (N) availability in cool temperate conifer forests, measurements of plant available N are necessary for purposes ranging from estimating climate change impacts to projecting timber inventory. Our understanding of soil N cycling was developed in agro-ecosystems where plant N availability is controlled by inorganic N supply. Yet in many cool temperate conifer forests, soil organic matter (SOM) accumulates and inorganic N supply measured through mineralization rates greatly underestimate forest production. Plants in these systems can instead satisfy N demand by acquiring dissolved organic N produced by microbial enzymatic breakdown of long-chain organic N. The rate of enzymatic N release is governed by stoichiometric requirements and is positively related to enzymatic release of carbon (C). The objective of this study was to measure N:C stoichiometric balance in cold temperate forests and to consider how that balance changes in response to the amount of SOM.
We characterized soil exoenzyme activity (EEA) profiles and SOM in three different forest environments in the northwest United States: 1) a dry pine/fir forest compared to a mesic cedar-hemlock forest; 2) a Douglas-fir/hemlock plantation compared to an adjacent hay field; and 3) several low (850-890m), medium (925-1440 m) and high (1615-1855 m) elevation mixed conifer forest stands. We characterized EEA profiles using fluorimetric microplate assays for protease, chitinase and cellulase activities. The log-transformed sum of protease and chitinase activities represented N release, while log-transformed cellulase activity represented C release.
Nitrogen release correlated positively with C release for all study locations and within each of the distinct ecosystems, showing a consistent slope of 0.74±0.06. However, within each location, the y-intercept for the N:C release function was greater for ecosystems with higher SOM. Specifically, N-release and SOM were higher for the mesic cedar-hemlock compared to the dry pine-fir; higher for the forest plantation compared to hay; and increased successively with elevation.
The correlation between N-releasing and C-releasing EEA supports stoichiometric theory where macronutrients are required in consistent proportions. Although the slope of the release function was constant, higher N versus C releasing EEA in soils of greater SOM was a consistent finding among forest types, ecosystems, and different elevations. Forest systems with higher SOM likely have higher microbial N demand. Exoenzyme profiles show promise as a tool for measuring N availability in cool temperate confer forests.