## 2 Materials and Methods
### Measurement and Analysis of Soil Gas Fluxes  
To measure soil GHG fluxes, the static chamber method was employed; gas emissions from the soil surface were trapped in a chamber and flux rates determined by taking static measurements of the change in gas concentrations over time within the headspace [41]. Soil collars were installed within each subplot on 15 May 2018 at a depth of 6 cm below ground with 5 cm of the collar above the soil surface. The sampling of soil GHG fluxes began a week after the application of fertilizer treatments (22 June 2018), and was done once a week continuously for 17 weeks until 25 October 2018. This gas sampling method was used because it was relatively inexpensive to set up compared to other methods of GHG sampling, and it also allowed for the investigation of several sites simultaneously. Sixteen 25 cm diameter polyvinyl chloride (PVC) collars were installed into the soil. The volume of each chamber was 0.0115 m\({}^{3}\) consisting of the volume of the cap (0.0095 m\({}^{3}\)) and the volume of the collar (0.01146 m\({}^{3}\)). The collars remained in the soil throughout the study. The chamber caps were made of PVC with two septa sampling ports, one for inserting a needle to serve as a pressure vent, and the other for sampling the gas from the chamber. Flexible PVC cloth was attached to the rims of sampling caps to ensure an airtight seal between the cap and the collar. A 12V, 7.5 Ah/20 h battery pack (Drenaline Long Lasting Outdoor Product Batteries, Model #12 VB) was used to power a small fan inside the chamber used to continuously circulate and mix the headspace gases during the 30 min sampling period. Samples were collected from the chamber headspace in 30 mL quantities at three timepoints (0, 15, and 30 min) after the chamber was sealed. The gas samples were each injected into a 10 mL glass serum vial fit with airtight rubber septa and placed in Ziploc bags. Gas samples were analyzed as soon as possible after sampling from the field (within 1-2 days) and stored at 2 \({}^{\circ}\)C when it was not feasible to run the samples the same day as the sampling. A gas chromatograph with a custom configuration for simultaneous CO\({}_{2}\), CH\({}_{4}\), and N\({}_{2}\)O analysis (Bruker Daltonics Inc, Billerica, MA, USA) was used to analyze the gas samples. Chamber temperatures were measured using a True RMS IndustrialMultimeter (Extech(r) Instruments, Waltham, MA, USA) at each headspace gas sampling time (0, 15, and 30 min). In addition to collecting the headspace temperature and gas samples, soil moisture and soil temperature were measured using a soil moisture meter at a distance of approximately 70 cm away from the soil chamber but within the subplot (General Tools DSMM500) and a DSMM600 Soil Condition Meter, respectively. Soil moisture and temperature were recorded during the 30-min sampling interval for each of the 16 subplots. Weekly samples were generally taken between 11 a.m. and 4 p.m. when air temperatures were near the maximum for the day. The concentration of the gases in ppm obtained from the gas chromatograph was converted from volumetric to mass using the Ideal Gas Law to calculate the amount of each gas in moles:  
\[PV=nRT, \tag{1}\]  
where \(P\) = pressure of the gas, \(V\) = volume inside sampling chamber (\(\mu\)L), \(n\) = number of moles of the gas (moles), \(R\) = universal gas constant = 0.082 L atm/k mole, and \(T\) = temperature in Kelvin. Using the molecular weights and moles of each gas, the mass (g) was calculated using the equation:  
\[m=n\ \times M, \tag{2}\]  
where \(m\) = mass (g), \(n\) = amount of gas (moles), and \(M\) = molecular mass of gas (g mol\({}^{-1}\)). Gas fluxes were obtained by performing regression analysis between the concentration of gas and the time of each gas sample. The slope (g min\({}^{-1}\)) was divided by the collar footprint (m\({}^{2}\)) and the result multiplied by 60 to estimate flux in g m\({}^{-2}\) h\({}^{-1}\) for CO\({}_{2}\). Fluxes for N\({}_{2}\)O and CH\({}_{4}\) were calculated in ug m\({}^{-2}\) h\({}^{-1}\).