Quantification of N and C cycling during aerobic composting, including automated direct measurement of N2, N2O, NO, NH3, CO2 and CH4 emissions

Abstract

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 N₂ emission. To address this issue, a novel system was developed enabling simultaneous and online determination of gaseous C and N species (N₂, N₂O, NO, NH₃, CO₂ and CH₄) from aerobic composting at a high frequency of 120 times·d^-1. A helium‑oxygen gas mixture was used to replace the air in the system to enable direct measurement of N₂ emission, and three different gas exchange methods were assessed in their ability to minimize atmospheric background N₂: 1) the N₂-free gas purging method; 2) one cycle of the evacuation-refilling procedure; 3) one cycle of evacuating and refilling followed by N₂-free gas purging. Method 3 was demonstrated as an optimum N₂-removal method, and background N₂ concentrations decreased to ~66 μmol·mol^-1 within 11.6 h. During the N₂-free gas purging period, low temperature incubation at 15 °C reduced CO₂, CH₄, NO, N₂O and NH₃ 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 N₂ 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 N₂ emission accounting (on average) for 5.8 % of the initial total N. This system for high frequency measurement of multiple gases (including N₂) 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.

Keywords: Aerobic composting; Gas pollution; High-frequency measurement; N(2) measurement; Online measurement.