Contrasting effects of elevated CO2 and warming on nitrogen cycling in a semiarid grassland

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 2 and warming. However, combined effects of elevated CO 2 and warming on N cycling have rarely been tested in the field. Here, we studied N cycling under ambient and elevated CO 2 concentrations (600 μmol mol -1), and ambient and elevated temperature (1.5: 3.0°C 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 3 - uptake (using a 15N tracer). Soil inorganic N significantly decreased under elevated CO 2, 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 2 x warming interaction effects on soil inorganic N, N pool sizes or NO 3 - uptake in plants and microbes. Our results indicate a more closed N cycle under elevated CO 2 and a more open N cycle with warming, which could affect long-term N retention, plant productivity, and C sequestration in this semiarid grassland.