Simulation models indicate that the nitrogen (N) cycle plays a key role in how other ecosystem processes such as plant productivity and carbon (C) sequestration respond to elevated CO and warming. However, combined effects of elevated CO and warming on N cycling have rarely been tested in the field. Here, we studied N cycling under ambient and elevated CO concentrations (600 omol molp#), and ambient and elevated temperature (1.5 : 3.0pC warmer day:night) in a full factorial semiarid grassland field experiment in Wyoming, USA. We measured soil inorganic N, plant and microbial N pool sizes and NO uptake (using a p#eN tracer). Soil inorganic N significantly decreased under elevated CO, probably because of increased microbial N immobilization, while soil inorganic N and plant N pool sizes significantly increased with warming, probably because of increased N supply. We observed no CO x warming interaction effects on soil inorganic N, N pool sizes or NO uptake in plants and microbes. Our results indicate a more closed N cycle under elevated CO and a more open N cycle with warming, which could affect long-term N retention, plant productivity, and C sequestration in this semiarid grassland.
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