MEASURING SOIL NITROUS OXIDE EMISSIONS BY USING A NOVEL OPEN PATH SCANNING TECHNIQUE

Abstract

A better way to improve understanding and quantification of nitrous oxide (N2O) emitted from intensive maize cropping systems is to develop an advanced emissions measurement method This study developed an open path (OP) method to measure N2O emissions from four adjacent maize plots managed by tillage practices of no-till (NT) and chisel plow (ChP), and different nitrogen (N) treatments from 2014 to 2016. Anhydrous ammonia (220 kg NH3-N ha-1) was applied in once or equally split (full vs. split rate) and applied in different timing (Fall vs. Spring). The spring N application occurred either before planting (pre-plant) or in season (side-dress). Emissions measurements were conducted by using the OP method (the scanning OP Fourier transform infrared spectrometry (OP-FTIR) + the gas point-sampling system + a backward Lagrangian stochastic (bLS) dispersion model) and static closed chamber methods. The performance and feasibility of the OP measurements were assessed by a sensitivity analysis, starting with errors associated with the OP-FTIR for calculating N2O concentrations, and then errors associated with the bLS model for estimating N2O emissions. The quantification of N2O concentrations using the OP-FTIR spectrum was influenced by ambient humidity, temperature, and the path length between a spectrometer and a retro-reflector. The optimal quantitative method mitigated these ambient interference effects on N2O quantification. The averaged bias of the calculated N2O concentrations from the spectra acquired from wide ranges of humidity (0.5 – 2.0 % water vapor content), temperature (10 – 35 °C), and path length (100 – 135 meters) was 1.4 %. The precision of the OP-FTIR N2O concentrations was 5.4 part per billion (3σ) in a stationary flow condition for a 30-minute averaging period. The emissions measurement from multiple sources showed that the field of interest was likely interfered by adjacent fields. Fields with low emission rates were more sensitive to the adjacent fields with high emissions, resulting in substantial biases and uncertainties. The minimum detection limit of the N2O emission rates was 1.2 µg m-2 s-1 (MDL; 3σ). The OP measurements showed that the NT practice potentially reduced N2O emission compared with ChP. Under the long-term NT treatments, the split-N rate application (110 kg NH3-N ha-1 in the fall and spring) resulted in lower N2O emissions than the full application (220 kg NH3-N ha-1 in the fall). The management of NT coupled with split-N rate application minimized N2O emissions among treatments in this study, resulting in N2O-N losses of 3.8, 13.2, and 6.6 N kg ha-1 over 9-, 35-, and 20-days after the spring NH3 application in 2014, 2015, and 2016, respectively. The spring pre-plant N application in 2015 also resulted in higher N2O emissions than the spring side-dress application in 2016, and the increased N2O-N loss was corresponding to lower N recovery efficiency in 2015 measurements. A comparison of chamber and OP measurements showed that soil N2O emissions were likely underestimated by 10x without considering the wind-induced effect on gas transport at the ground-atmospheric interface. This study showed that the OP method provides a great opportunity to study agricultural N2O emissions as well as management optimization for the sustainability of the agroecosystems.