Effects of nitrogen fertilizer, soil temperature and moisture on the soil-surface CO2 efflux and production in an oasis cotton field in arid northwestern China

- Soil-surface CO2 efflux increased with increasing nitrogen (N) fertilizer rate.
- Soil CO2 production (SP) decreased with increasing soil depth.
- N fertilization enhanced SP in the 0-20 cm layer.
- Increasing temperature increased SP in the 0-40 cm layer.
- Increasing soil moisture increased SP in 0-10 cm but decreased it in 10-20 cm.

In several studies of agricultural ecosystems, researchers have focused on the soil-surface carbon dioxide (CO2) effluxes, but the nature of CO2 production in the soil profile and its influencing factors remain unclear. In this study, the soil-surface CO2 effluxes in an oasis cotton field were measured using the chamber method, and the CO2 concentrations were used to estimate the CO2 production in different layers of the soil profile using the gradient method. The soil CO2 concentrations increased with increasing soil depth, whereas CO2 production decreased with increasing soil depth. Both soil-surface CO2 effluxes and CO2 production in the 0-40 cm layers exponentially increased with increasing temperature. Irrigation temporarily reduced the soil-surface CO2 effluxes by 19-63% through inhibiting CO2 production in the 10-40 cm layer but did not affect the CO2 production in the 0-10 cm layer. CO2 production mainly occurred in the 0-10 cm layer, and this cumulative production accounted for 63-67% of the total production throughout the soil profile (0-40 cm). The application of nitrogen (N) fertilizer enhanced the rate of CO2 production in the 0-20 cm layer by increasing the root biomass and soil mineral N content. A positive correlation was detected between the soil-surface CO2 efflux and soil NO3− content in 2015, but no significant correlations were found between the soil-surface CO2 efflux and soil NH4+ contents in any treatment. A higher soil-surface CO2 efflux was observed under high soil temperature and a certain soil moisture range (0.21-0.23 cm3 cm− 3). An analysis of the soil profile revealed higher CO2 production rates detected in the 0-10 cm layer under high soil temperature and moisture conditions, but higher rates were observed under high soil temperature and low soil moisture conditions in the 10-20 cm layer. Therefore, our results suggest that the effects of fertilization, soil temperature and moisture on CO2 production vary depending on the soil depth. These findings might improve our understanding of the mechanisms underlying soil respiration in soil profiles.