Nitrogen Competition in a Tallgrass Prairie Ecosystem Exposed to Elevated Carbon Dioxide

Abstract

Because N is a limiting nutrient in tallgrass prairie and most ecosystems, changes in N availability or N cycling could control the long‐term 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 × 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 conditions. To examine plant–microbial competition and altered allocation patterns of N under elevated CO2, 15NH4–N was added to 25‐cm‐diam. polyvinyl chloride (PVC) cores (15‐cm depth) in the field, which were destructively sampled after ≈5 mo. Microbial biomass contained ≈75% of the total 15N 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%) 15N compared with ambient CO2 conditions. Though a chamber effect was apparent, greater 15N in the SOM pool and greater percentage 15N SOM/percentage 15N plant suggest greater microbial demand for N under elevated CO2