An improved microelectrode method reveals significant emission of nitrous oxide from the rhizosphere of a long-term fertilized soil in the North China Plain.

Microsensors are able to accurately quantify nitrous oxide (NO) emissions in microenvironments at high spatio-temporal resolution; yet, limited studies have been conducted on agricultural soils due to the inability to obtain electrical signal under conditions of low soil moisture. This study improved the calibration of a microelectrode for measuring NO emissions from agricultural soil. The microelectrode was applied to evaluate the effect of long-term fertilization with mineral fertilizer (NPK), complemented with pig manure (MNPK), straw (SNPK), or without fertilizer (CK), all with and without urea addition, on NO emissions from the soil, with explicit separation of the rhizosphere and the bulk soil compartments. The use of soil solution instead of pure water for calibration of the microelectrode doubled the signal and significantly improved the sensor sensitivity. The optimal electrolytic concentration of the soil solution, expressed as water: soil ratio, was found at the maximum vertex of the quadratic equation fitted on the slope values of the calibration equations for different soil solutions. The application of the calibrated microelectrode revealed significantly higher NO emission from the rhizosphere compared to the bulk soil, accounting for 60% of the total emission. For the bulk soil, MNPK significantly increased NO emissions compared to SNPK and NPK, whereas the differences between these treatments for the rhizosphere soil were insignificant. The statistical modeling revealed significant relation of the NO emission with soil inorganic nitrogen contents and an additive effect of treatment (MNPK and SNPK), urea addition and rhizosphere soil. This study provides novel insights into the use of microelectrodes for measuring NO emissions from the soil microenvironment and also points on the rhizosphere compartment and the management practices of agroecosystems able to reduce the NO emission from agriculture.