An innovative soil mesocosm system for studying the effect of soil moisture and background NO on soil surface C and N trace gas fluxes

Nitric oxide (NO) is a key substance in atmospheric chemistry, influencing the formation and destruction of tropospheric ozone and the atmosphere's oxidizing capacity. It also affects the physiological functions of organisms. NO is produced, consumed, and emitted by soils, the effects of soil NO concentrations on microbial C and N cycling and associated trace gas fluxes remain largely unclear. This study describes a new automated 12-chamber soil mesocosm system that dynamically changes incoming airflow composition. It was used to investigate how varying NO concentrations affect soil microbial C and N cycling and associated trace gas fluxes under different moisture conditions (30% and 50% WFPS). Based on detection limits for NO, NO₂, N₂O, and CH₄ fluxes of < 0.5 µg N or C m⁻² h⁻¹ and for CO₂ fluxes of < 1.2 mg C m⁻² h⁻¹, we found that soil CO₂, CH₄, NO, NO₂, and N₂O were significantly affected by different soil moisture levels. After 17 days cumulative fluxes at 50% WFPS increased by 40, 400, and 500% for CO₂, N₂O, and CH₄, respectively, when compared to 30% WFPS. However, cumulative fluxes for NO, and NO₂, decreased by 70, and 40%, respectively, at 50% WFPS when compared to 30% WFPS. Different NO concentrations tended to decrease soil C and N fluxes by about 10–20%. However, with the observed variability among individual soil mesocosms and minor fluxes change. In conclusion, the developed system effectively investigates how and to what extent soil NO concentrations affect soil processes and potential plant–microbe interactions in the rhizosphere.