DOI: 10.5683/sp3/xosfs4
title: Greenhouse Gas Data ( collected in lab)

description: Read me: 
Greenhouse gas data collection and measurement in 2018. A detailed description of methods used to derive the data is below. Further questions can be directed to shresthabm@gmail.com

Methods: Soil preparation, incubation, gas sampling, and analysis

Water holding capacity of sieved soils at different matric potentials was determined using the pressure-plate method [41]. Sub-samples of air-dried soils were first placed in O-rings on porous ceramic plates and saturated for 24 h. Saturated soils were then pressurized at 0.1, 0.5, 1.0, 5.0, and 15.0 bars for 72 h, after which the moisture content at each pressure level was quantified by drying at 105 ºC for 27 h to a constant mass and reweighing. Water content at 15 bar was considered the permanent wilting point (PWP), while 0.1 bar was the field capacity (FC) of sandy soils [42] and 0.33 bar the FC of clayey soils [41]. Water content at 0.33 bar was estimated by linear extrapolation of water contents at 0.1, 0.5, and 1.0 bar. The moisture content of air-dried sieved soil was also determined following the oven-dry method (described above) to help maintain the desired soil moisture level throughout the subsequent incubation experiment. 
For each grassland investigated, 100 g of oven-dry equivalent air-dried soil was placed in each of six 500 mL Mason jars for the incubation experiment. Sufficient water was added (with a dilute 0.005 M CaSO4 to protect micro-aggregates from disruption) to bring these soils to a moisture level of either FC, 40% FC, or PWP [43]. One set of Mason jars with soil from each moisture treatment was placed in an incubator at 5 ºC, while the other set was placed in another incubator at 25 ºC. The tops of all jars were covered with perforated aluminum foil for five consecutive days to stabilize microbial activity. On the fifth day (collection day 0), initial GHG samples were collected from the headspace air of the jars immediately after closing them using a lid equipped with a rubber septum. Soils were further incubated for 24 h with the lids closed, and then headspace samples were collected again to determine the change in GHG concentrations. Subsequent sampling of GHGs occurred on days 1, 2, 4, 7, 10, 13, 18, 23, 28, 35, 42, 52, 62, 72, 82, 92 and 102. The change in gas concentration between the 0 and 24 h headspace samples on each sampling day was used to calculate daily GHG flux per unit dry mass of soil. Soil moisture levels were maintained throughout the incubation period by tracking water loss by weighing the jars and replenishing the water at least 3 days prior to each gas sampling event. Headspace air samples were collected with an air-tight 20 mL syringe (Norm-Ject, Henke Sass Wolf, Tuttlingen, Germany) and injected into 12 mL pre-vacuumed soda glass Isomass Exetainers (Labco Limited, Lampeter, Wales, UK). 

Greenhouse gas samples were analyzed with a Varian CP 3800 gas chromatograph (Varian Canada, Mississauga, Canada) containing three detectors. A thermal conductivity detector (TCD) and flame ionization detector (FID) simultaneously determined the concentration of CO2 and CH4, respectively [44], while the electron capture detector (ECD) determined the concentration of N2O [45]. Standard curves were generated using mixtures of gases at standard concentrations of CO2 (360 ppm), CH4 (1.6 ppm) and N2O (1.0 ppm) (Praxair) and used to calculate the headspace concentrations of respective gases.