Methane transport through an agricultural soil following pulse injection of isotopically-enriched methane in the sub-surface

The field experiment was conducted from June to July 2012 at Bunny Park, Nottinghamshire, UK (52.863°N, 1.126°W) which is part of the University of Nottingham farm. The soil at the experimental site is a stagno-gleyic brown earth (loamy sand texture) of the Newport series, overlying a geology of Triassic mudstone.

Twelve plots were established in 3 × 4 grid (dimensions 45 m × 25 m) in the centre of a ~1 ha area planted with spring wheat (Triticum aestivum cv. Tybalt – Redigo). Wheat plants were removed from six randomly selected experimental plots in a circular area of 1.5 m diameter. Gas sampling tubes (Huxtable et al., 2017) of appropriate length were inserted at a 45°angle from the soil surface such that their perforated sampling tips were positioned in a vertical line at 10, 20, 30, 40 and 50 cm depths from the surface. The lowermost (50 cm) sampler was used to inject a pulse of CH4 (15 cm3 at ambient temperature and pressure – see details of isotopic composition below). Four randomly selected control plots (two with and two without wheat) did not receive CH4 injections. A shallow, circular PVC collar (50 cm diameter) was inserted to a depth of 5 cm above each plot to provide a gas-tight seat for headspace chambers. Samples of soil gas were taken immediately prior to injecting the CH4 pulse (antecedent samples) and at times ranging from 15 minutes to 7 days after injection. A close-fitting clear PVC headspace chamber 50 cm in diameter and either 50 or 100 cm in height (depending on the growth stage of the crop) was placed over the experimental plots at intervals from 3 hours to 168 hours post-injection. These intervals were carefully timed to avoid interference with soil gas sampling. Samples of head space gas were taken at 30 minute intervals over a 120 minute period during which the headspace gas was continuously stirred with a small electric fan. After 120 minutes the chambers were removed from the plots and the headspace gas samples used to derive soil surface CH4 fluxes. During the experiment, gas injections were carried out on three separate occasions during the periods of 11th-14th June, 25th-28th June and 16th-19th July. The four day periods were needed to administer single (instantaneous) gas injections to individual plots in sequence and to take the ensuing soil and head space gas samples in all eight treatment plots.

Isotopically-enriched methane was prepared by mixing 12CH4 (99.95% 12C) with 13CH4 (99.0% 13C) at a ratio of 7:3 (gases were supplied by CK gases, Hook, UK). The resulting delta-13C value (with respect to PD belemnite) was 37,139 ‰. After preparing the mixed 12/13CH4 in the laboratory, 15 ml aliquots were dispensed prior to injection in the field experiment.

Analysis of gas samples was carried out in the laboratory by both GC-FID (GC-2014, Shimadzu Corp., Japan) for bulk methane concentrations and by GC-IRMS (DeltaplusXP, ThermoFinnegan) for simultaneous 12CH4 and 13CH4 measurements. For GC-IRMS analysis, chromatographic separation of gases was performed on a Varian CP-PoraPLOT Q-HT column with helium as the carrier gas at a temperature of 30°C held isothermally for 20 minutes.

During the experiment a full suite of meteorological data was collected automatically over 30 minute intervals using a Davis Vantage Pro 2 wireless weather station located within 30 m of the centre of the plot. Soil temperatures were measured at depths of 10, 20, 30 and 40 cm using two ‘strings’ of thermocouples located close to the centre of the 45 m × 25 m experimental sub-plot. Soil temperatures were recorded automatically over 15 minute intervals using a Campbell data logger.

Soil moisture was measured in situ at depths of 10, 20, 30 and 40 cm using a Delta-T PR2 profile probe and an HH2 moisture meter (Delta-T Devices, Cambridge, UK). In situ soil moisture measurements were taken manually each week at 15 monitoring points. At the end of the experiment, soil pits were excavated as close as possible to each in situ soil moisture monitoring point. From these pits, intact soil samples of fixed volumes were taken at 5 cm intervals from the soil surface to a depth of 50 cm. These were used to measure gravimetric water content, saturated water content (used as a measure of soil porosity) and dry bulk density.