Supervisor: Wolfgang Wanek
PhD student: Lisa Noll
This thesis aimed to provide new insights into the environmental controls of microbial organic nitrogen cycling in soils. Proteins are the main constituent of soil organic N and the breakdown to small oligopeptides and amino acids, which can be taken up by microorganisms and plants is the rate limiting step in soil N cycling. I present a novel approach to determine gross rates of protein depolymerization and microbial amino acid uptake in soils using isotope pool dilution experiments. To analyze N isotope ratios and concentrations of amino acid-N concentrations we adopted a method for amino-group specific isotope analyses described by Zhang & Altabet (2008). The proposed method was evaluated regarding precision of concentration and isotope ratio measurements. We further applied the new method in an isotope pool dilution experiments in soils targeting the effects of C and N availability on gross protein depolymerization and microbial amino acid uptake rates. Then I discuss the integrated effects of soil properties and short term changes in soil temperature, moisture and O2 supply on gross protein depolymerization rates. The results revealed that gross depolymerization is only slightly affected by temperature or soil moisture, whereas the response to limited O2 supply limitation is pre-determined by soil mineralogy. Finally I examined large-scale controls on gross protein depolymerization rates across a European climate transect. The results highlight that gross protein depolymerization rates are restricted by substrate i.e. protein availability and not by enzyme activity. Furthermore, the chapter provides insights into the integrated effects of climate, soil parent material and land use, demonstrating that climate and bedrock effects outweigh land use effects on gross protein depolymerization rates.
Please find a list of publications here.