Relatively little attention has been given to the processes that control the production and subsequent availability of amino acids in temperate forest soils. We examined how differences in soil organic matter chemistry and mycorrhizal association between temperate forest tree species of the Northeastern US lead to variation in amino acid cycling. We measured amino acid cycling throughout the growing season in soils from plots dominated by either, Acer saccharum, Fraxinus americana, Tsuga canadensis, or Fagus grandifolia. Coupled with these measurements, we performed a root exclusion experiment to investigate how differences between species in belowground carbon (C) allocation dictate amino acid production rates.
Results/Conclusions
Seasonal monitoring shows significant differences in the availability and production of amino acids between tree species likely as a result of microbial demand for and access to easily assimilated substrates. A. saccharum and F. americana, tree species with low C-to-Nitrogen (N) ratio soils, exhibited smaller pool sizes and higher production rates of amino acids than species with high C-to-N ratio soils, T. canadensis and F. grandifolia. This apparent C-to-N ratio control on amino acid cycling may be explained by a simple conceptual model of enzyme investment. Along a gradient of low to high C-to-N ratios, soil microbes switch enzyme allocation from hydrolytic enzymes that attack low complexity amino acid substrates to oxidative enzymes that release amino acids from substrates of increasing complexity at both reduced rates and increased C costs. Results from the root exclusion experiment further support this model. Only in tree species with high C-to-N ratios, T. canadensis and F. grandifolia, did the exclusion of roots from soil lead to a reduction in amino acid production. This result suggests that C released by roots fuels amino acid production in these soils by providing the C capital required for microbial enzyme production.
