Inhibition of Elevated Atmospheric Carbon Dioxide to Soil Gross Nitrogen Mineralization Aggravated by Warming in an Agroecosystem

The response of soil gross nitrogen (N) cycling to elevated carbon dioxide (CO2) concentration and temperature has been extensively studied in natural and semi-natural ecosystems. However, how these factors and their interaction affect soil gross N dynamics in agroecosystems, strongly disturbed by human activity, remains largely unknown. Here, a N-15 tracer study under aerobic incubation was conducted to quantify soil gross N transformation rates in a paddy field exposed to elevated CO2 and/or temperature for 9 years in a warming and free air CO2 enrichment experiment. Results show that long-term exposure to elevated CO2 significantly inhibited or tended to inhibit gross N mineralization at elevated and ambient temperatures, respectively. The inhibition of soil gross N mineralization by elevating CO2 was aggravated by warming in this paddy field. The inhibition of gross N mineralization under elevated CO2 could be due to decreased soil pH. Long-term exposure to elevated CO2 also significantly reduced gross autotrophic nitrification at ambient temperature, probably due to decreased soil pH and gross N mineralization. In contrast, none of the gross N transformation rates were affected by long-term exposure to warming alone. Our study provides strong evidence that long-term dual exposure to elevated CO2 and temperature has a greater negative effect on gross N mineralization rate than the single exposure, potentially resulting in progressive N limitation in this agroecosystem and ultimately increasing demand for N fertilizer.