in situ glycine mineralization is closely correlated to SOM turnover across six North American forest ecosystems that vary in substrate quantity and quality

Free amino acids (FAA) represent a significant fraction of dissolved organic nitrogen (N) in forest soils, and play an important role in the N cycle of these ecosystems. While a number of experiments have elucidated factors controlling the production and/or turnover of FAA, the primary motivation behind previous efforts has centered on plant nutrition or the overall N economy of soils. Relatively little attention has been given to establishing linkages between the turnover of these labile substrates and the metabolic status of resident microbial populations. Heterotrophic growth in soil environments is often energy limited as most soil carbon (C) consists of complex polymers that are resistant to biodegradation. Still, in many soils the microbial biomass maintains a high level of endogenous energy which stems from the need to rapidly utilize exogenous inputs of labile substrate. We hypothesized that the residence time of simple C substrates such as FAA, are mechanistically linked to the turnover of indigenous C pools. We tested this hypothesis across a latitudinal gradient of forested ecosystems that sharply differ in climate and edaphic properties. Using a combined laboratory and field approach, we compared the turnover of isotopically labeled glycine in situ to the turnover of mineralizable soil C (C_min) at each site. Our results indicate that the turnover of glycine is rapid regardless of soil type. However, we noticed that across all ecosystems, glycine mineralization rates were correlated with some indices of soil organic matter quality. For example C:N ratios for the upper soil horizons explained ~ 80% of the variability observed in glycine turnover, and there was a strong positive correlation between the turnover constants for glycine and C_min. This suggests the overall decomposability of native C and hence the starvation-survival strategies of resident microbial populations influence the turnover dynamics of low-molecular-weight organic substrates such as glycine.