N2O production in the organic and mineral horizons of soil had different responses to increasing temperature

Abstract

The responses of N2O emission to increasing temperature in different soil horizons are not clearly understood yet. Here, we investigated the effects of increasing temperature on sources of soil N2O emissions from organic (O) and mineral (A) horizons of a temperate forest soil. An incubation experiment using 15N as a tracer was conducted to investigate the sources of soil N2O emission and their responses to increasing temperature in the O and A horizon soils at controlled temperatures (5 °C, 15 °C, 25 °C, and 35 °C at 60% water holding capacity). The O horizon had higher total organic carbon, total nitrogen, and sand contents than the A horizon. The ratio of nitrification- to denitrification-derived N2O production decreased with increasing temperature in both soil horizons, perhaps due to the development of anaerobic volumes and the greater increase in nirS gene abundance. The nirS gene was much more abundant than the nirK gene and was more correlated to denitrification-derived N2O flux. No relationship was found between nitrification-derived N2O flux and amoA gene abundances of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) for either soil horizon. In general, nitrification dominated N2O production in the O horizon soil, while denitrification dominated N2O production in the A horizon soil. N2O emission was higher in the A horizon soil than in the O horizon soil, but the temperature sensitivity of N2O emission in the A horizon soil was lower. These differences might be explained by the higher initial anaerobic volume and higher carbon availability in the A horizon than in the O horizon. Our results suggest that the denitrification process is more stimulated by increasing temperature compared to the nitrification process in both O and A horizons in our studied soil.