Continuous Monitoring of Nitrous Oxide from Agricultural Field: New Data on Temperature and Soil Moisture Effect

Agricultural soils are the most significant terrestrial source of nitrous oxide (N₂O), one of the three major biogenic gases contributing to greenhouse effect. Emissions can increase after manure fertilization, which is commonly applied in the Northeastern US, where dairy farms are ubiquitous.

Measurements of N₂O emission were conducted on manure-amended alfalfa in 2006 and corn in 2007 fields in central New York State. Fluxes were measured with the micrometeorological eddy covariance technique using tunable diode laser absorption spectroscopy (EC-TDLAS), which allowed high-frequency (10Hz), high-resolution, continuous monitoring of N₂O flux from soils. The experimental setup also included simultaneous measurements of meteorological and soil parameters, such as air and soil temperature, soil moisture, precipitation, humidity, water vapor and CO₂ flux, and net radiation. The results were averaged over 30 min periods and daily values were calculated.

The preliminary analysis of data obtained during two years of observations showed emission increases after manure application, as well as the high variability and strong instantaneous ‘spike-like' nature of N₂O flux. The background level of emissions stayed below 0.05ug N₂O-N m^-2 s^-1, whereas under favorable conditions the peaks could reach up to 0.7ug N₂O-N m^-2 s^-1 within a short time. The main variables effecting N₂O production were the soil moisture and temperature. However, most of the time soil moisture variations were not substantial enough (<2%) to cause immediate flux response. The explicit N₂O increase due to moisture changes only occurred during/after strong rainfall or freezing/thawing events. When soil moisture was not a limiting factor (WFPS 50-70%), soil temperature and its rapid fluctuations had more direct and immediate impact on flux formation. The N₂O ‘spikes' mostly followed sharp temperature drop/rise cycles (>10^oC). The highest observed N₂O peaks were generated as a result of combination of recent manure application, significant soil moisture increases and strong temperature drop/rise events.