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DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

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

Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506

Corresponding author (cwrice{at}ksu.edu)

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 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 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.

Abbreviations: AC, ambient CO₂ with chamber • EC, elevated CO₂ with chamber • I, immobilization • M, mineralization • M_c NH⁺₄ consumption • i_p, NH⁺₄ production • N_p, NO^-₃ production • N_c, NO^-₃ consumption • NC, ambient CO₂ with no chamber • PVC, polyvinyl chloride • SOM, soil organic matter

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