Addressing Key Questions in the Bioatmospheric Nitrogen Cycle Using Stable Isotope Techniques

The application of stable isotope techniques enables determination of the fate of applied or deposited reactive N, quantification of biogeochemical process rates, differentiation between microbial sources of N emissions, and determination of impacts on microbial biodiversity. Here we will introduce the 15N, 18O and 13C techniques we have developed and present results from studies where they have been applied to address the following key questions in the bioatmospheric N cycle: What is the fate of atmospheric deposited N in a plant-soil system? What impact does this N addition have on rates of key biogeochemical processes in the rhizosphere? What are the effects on the soil microbial community structure? How much of this N is returned to the atmosphere as N2O or N2? Using 15N and 18O enrichment techniques we are able to quantify gross mineralization rates and microbial immobilization, plant uptake and leaching of deposited or applied N, gross nitrification rates, quantify N2O and N2 production from ammonia oxidation, nitrifier denitrification and denitrification and distinguish between these sources, and derive a system N balance. These stable isotope techniques are being combined with molecular techniques (analysis of gene expression, 13C stable isotope probing (SIP)) to relate the measured processes to the microbial populations responsible and to assess effects of reactive N on community structure. Here we will give a synthesis of results from N deposition studies in semi-natural ecosystems and from N addition studies in agricultural systems, the information from which can be used to drive policy through the formulation of appropriate management and mitigation strategies for reactive N.