Soil organic matter (SOM) contains more carbon (C) than the atmosphere and terrestrial vegetation combined, but the microbial processes that regulate the amount and biochemical composition of SOM are not well-understood. Microbial communities release inorganic nitrogen (N) from SOM (i.e., net N mineralization) as a function of SOM biochemistry, with higher rates of net N mineralization occurring when SOM is rich in organic N and depauperate in energy-rich organic compounds. A separate body of laboratory studies and N deposition field experiments has demonstrated that experimentally-elevated inorganic N can reduce the abundance and activity of “white-rot” fungi that degrade lignin and polyphenolic compounds, which increases SOM storage and alters its composition by enhancing the relative abundance of lignin-derived compounds. Although somewhat counterintuitive, these observations suggest that naturally high rates of net N mineralization by soil microbial communities may feed back to suppress white-rot fungi and, in turn, modify the amount and biochemical composition of SOM. To test if this microbial feedback regulates the amount and biochemistry of SOM in forest ecosystems, we characterized SOM using pyrolysis gas-chromatography-mass spectrometry across a net N mineralization gradient in 72 plots spanning several forest ecosystems in northern Lower Michigan.
Results/Conclusions
Net N mineralization was negatively related to soil C:N and the relative abundance of aromatic compounds, lipids, and polysaccharides in SOM, whereas it was positively related to proteins and other N-bearing compounds (linear regression; P < 0.05). These results confirm that net N mineralization is more rapid when SOM contains more N and fewer energy-rich compounds. We detected significant threshold values of net N mineralization (Davies test; P < 0.05) in the relationships between net N mineralization and both soil total C and the relative abundance of lignin-derived compounds in SOM. Neither soil C nor lignin-derived compounds were related to net N mineralization below these thresholds, whereas both soil properties were positively related to net N mineralization above the threshold values. Furthermore, soil C (+29%; ANOVA, P = 0.069) and lignin-derived compounds (+22%; P = 0.046) were greater above the threshold values of net N mineralization than below. Together, these findings suggest high rates of N mineralization feed back to increase soil C and lignin-derived compounds by suppressing white-rot fungi, which is not considered in current conceptualizations of SOM formation. We are currently characterizing fungal community composition across the net N mineralization gradient to confirm this mechanism.