Nitrogen competition in a tallgrass prairie ecosystem exposed to elevated carbon dioxide

Because N is a limiting nutrient in tallgrass prairie and most ecosystems, changes in N availability or N cycling could control the longterm response of ecosystems to elevated atmospheric CO2. If more C is sequestered into the soil, then greater microbial demand for N could decrease plant-available soil N, Alterations in N dynamics such as plant uptake, N fixation, nutrient cycling, microbial utilization, and partitioning of N into plant and soil fractions ultimately could affect the capability of ecosystems to sequester C, Our objective was to determine if competition for N between plants and microorganisms changes after 8 yr of elevated CO2 relative to ambient conditions. Treatments (three replications, randomized complete block design) were ambient CO2-no chamber (NC), ambient CO2-chamber (AC), and 2 x ambient CO2-chamber (EC), Several short laboratory incubations assessed whether turnover rates of N in soil would be altered under elevated CO2, Gross transformations of N were not altered significantly under elevated CO2 compared with ambient renditions. To examine plant-microbial competition and altered allocation patterns of N under elevated CO2, (NH4)-N-15-N was added to 25-cm-diam. polyvinyl chloride (PVC) cores (15-cm depth) in the held, which were destructively sampled after approximate to5 mo. Microbial biomass contained approximate to 75% of the total N-15 that occurred in the soil organic matter (SOM) and, thus, appeared to be a significant regulator of plant-available N, The SOM under elevated CO2 contained significantly more (> 27%) N-15 compared with ambient CO2 conditions. Though a chamber effect was apparent, greater N-15 in the SOM pool and greater percentage N-15 SOM/percentage N-15 plant suggest greater microbial demand for N under elevated CO2.