Quantification of N and C cycling during aerobic composting, including automated direct measurement of N, NO, NO, NH, CO and CH emissions.

Closing the carbon (C) and nitrogen (N) balance has yet to be achieved in aerobic bioprocess due to current methodological drawbacks in the frequency of sampling and detection and the challenge in direct measurement of instantaneous N emission. To address this issue, a novel system was developed enabling simultaneous and online determination of gaseous C and N species (N, NO, NO, NH, CO and CH) from aerobic composting at a high frequency of 120 times·d. A helium‑oxygen gas mixture was used to replace the air in the system to enable direct measurement of N emission, and three different gas exchange methods were assessed in their ability to minimize atmospheric background N: 1) the N-free gas purging method; 2) one cycle of the evacuation-refilling procedure; 3) one cycle of evacuating and refilling followed by N-free gas purging. Method 3 was demonstrated as an optimum N-removal method, and background N concentrations decreased to ~66 μmol·mol within 11.6 h. During the N-free gas purging period, low temperature incubation at 15 °C reduced CO, CH, NO, NO and NH losses by 80.5 %, 41-fold, 10-fold, 11,403-fold and 61.4 %, respectively, compared with incubation at 30 °C. Therefore, a fast and low-perturbation N removal method was developed, namely the evacuating/refilling-low temperature purging method. Notably, all C and N gases exhibited large within-day variations during the peak emission period, which can be addressed by high-frequency measurement. Based on the developed method, up to 97.8 % of gaseous C and 95.6 % of gaseous N losses were quantified over a 43-day compost incubation, with N emission accounting (on average) for 5.8 % of the initial total N. This system for high frequency measurement of multiple gases (including N) provides a novel tool for obtaining a deeper understanding of C and N turnover and more accurate estimation of reactive N and greenhouse gas emissions during composting.