Dr Mark Bonner1, David Castro1, Andreas Schneider2, Görel Sundström1, Vaughan Hurry1, Nathaniel Street2, Torgny Näsholm1
1Swedish University Of Agricultural Sciences, Umeå, Sweden, 2Umeå University, Umeå, Sweden
The addition of nitrogen (N) to forest soils tends to inhibit oxidative enzyme expression by soil microbes and, likely as a consequence, reduces decomposition rates. Accordantly, soil organic matter (SOM) tends to increase following long-term N enrichment, which is typically seen chiefly as a consequence of inhibited decomposition. The mechanism for oxidative enzyme inhibition by N is unknown. It has been speculated that N-demanding microbes outcompete oxidase-synthesising microbes, or N abiotically couples with aromatic compounds to produce substrates more resistant to decomposition. The latter hypothesis is inconsistent with observed chemistry of accumulated SOM, and the former is challenged by i) the general condition of carbon limitation of soil saprotrophs, and ii) the evolutionary argument that N limited microbes would have strong selective pressure towards oxidative enzyme expression for SOM decomposition. We propose a hypothesis that includes elements kindred to both of the prior explanations, but falls more cleanly in line with empirical observation and evolutionary prediction. We suggest that high rates of N addition do indeed suppress oxidative enzyme expression through altering competitive balance, and that coupling of N with aromatics is the driver. However, the shift in competition is not between genotypes with contrasting N needs, but between phenotypes with contrasting strategies for carbon acquisition, and the products formed by coupling of N with aromatics are easily degraded and short-lived. Our hypothesis is consistent with metatranscriptomic, chemical assay, and SOM compositional results from two boreal forest fertilisation trials in northern Sweden. We discuss clear predictions that arise from the hypothesis about interactive effects of N enrichment, temperature increase, and elevated atmospheric carbon dioxide on decomposition and SOM.