MEASURING SOIL NITROUS OXIDE EMISSIONS BY USING A NOVEL OPEN PATH SCANNING TECHNIQUE
Creators: name: Lin, Cheng-Hsien
Publisher: name: Purdue University Graduate School
Year: 2019
Subjects: Agronomy

description: A better way to improve understanding and quantification of nitrous oxide (N<sub>2</sub>O) emitted from intensive maize cropping systems is to develop an advanced emissions measurement method This study developed an open path (OP) method to measure N<sub>2</sub>O 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 NH<sub>3</sub>-N ha<sup>-1</sup>) 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 N<sub>2</sub>O concentrations, and then errors associated with the bLS model for estimating N<sub>2</sub>O emissions. The quantification of N<sub>2</sub>O 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 N<sub>2</sub>O quantification. The averaged bias of the calculated N<sub>2</sub>O 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 N<sub>2</sub>O concentrations was 5.4 part per billion<sup> </sup>(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 N<sub>2</sub>O emission rates was 1.2 µg m<sup>-2</sup> s<sup>-1</sup> (MDL; 3σ). The OP measurements showed that the NT practice potentially reduced N<sub>2</sub>O emission compared with ChP. Under the long-term NT treatments, the split-N rate application (110 kg NH<sub>3</sub>-N ha<sup>-1</sup> in the fall and spring) resulted in lower N<sub>2</sub>O emissions than the full application (220 kg NH<sub>3</sub>-N ha<sup>-1</sup> in the fall). The management of NT coupled with split-N rate application minimized N<sub>2</sub>O emissions among treatments in this study, resulting in N<sub>2</sub>O-N losses of 3.8, 13.2, and 6.6 N kg ha<sup>-1</sup> over 9-, 35-, and 20-days after the spring NH<sub>3</sub> application in 2014, 2015, and 2016, respectively. The spring pre-plant N application in 2015 also resulted in higher N<sub>2</sub>O emissions than the spring side-dress application in 2016, and the increased N<sub>2</sub>O-N loss was corresponding to lower N recovery efficiency in 2015 measurements. A comparison of chamber and OP measurements showed that soil N<sub>2</sub>O 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 N<sub>2</sub>O emissions as well as management optimization for the sustainability of the agroecosystems.