Data for paper "CH4 and N2O emissions from smallholder agricultural systems on tropical peatlands in Southeast Asia"

Abstract

This data paper contains greenhouse gas flux and environmental data from 48 sites across four regions in Malaysia (North and South Selangor) and Indonesia (West and Central Kalimantan) corresponding to the paper “CH4 and N2O emissions from smallholder agricultural systems on tropical peatlands in Southeast Asia”. Gas flux sampling was conducted monthly between March 2018–February 2019 in Malaysia and between May 2018–April 2019 in Indonesia using two complimentary methods.

Gas samples were collected using static and dynamic chamber methods. Both methods used circular chambers with an inner diameter of 28 cm and a headspace capacity of 11.5 L and they were made of opaque polypropylene. The top of each static chamber was fitted with a 19 mm rubber Suba-Seal Septa. Each measurement consisted of four 25 mL gas samples taken at 4 min intervals through the septa using plastic syringes and hypodermic needles. After each sampling event, 5 mL of gas was flushed out and the remaining 20 mL of sample was injected into 12 mL pre-evacuated glass vials. In contrast, the dynamic closed chamber was connected to a Los Gatos Ultraportable GHG analyser This was complementary to the vial sampling and consisted of series of intensive gas sampling campaigns (between March–June 2018 and October–January 2019 in both Malaysia and Indonesia). The dynamic chamber was equipped with two push-fit connectors (instead of the rubber septa) that acted as an inlet and outlet allowing the gas to flow from the chamber to the analyser and then back into the chamber. The soil CH4 concentration (in ppm) increment inside the chamber was measured during 6 to 10 minutes. Gas samples contained in the glass vials were analysed for CH4 and N2O using a gas chromatograph with two separate channels fitted to a thermal conductivity detector, a flame ionization detector, and an electron capture detector. The GC was connected to a custom-made single-injection auto sampler. Each injection consisted of 5 mL of sample flushed into a non-polar methyl silicone capillary column and a porous polymer packed column using nitrogen as the carrier gas.

Simultaneously with each gas sampling event, air temperature was measured on top of the chamber. Once the gas sampling was concluded, soil temperature was measured vertically at depths of 5 and 10 cm next to each plot. Additionally, water table depth was measured inside dipwells. A negative scale was used to record belowground water table depth values and a positive scale for values above the ground surface. Finally, between 120 mL and 250 mL of water was collected from the dipwells in each site for the determination of total dissolved nitrogen (TDN). Water samples were filtered through 0.45 µm cellulose nitrate membrane filters using a vacuum pump and stored in the dark at around 4°C until further analysis. TDN was measured in a TOC-V CSH Total Organic Analyser with TNM-1 Total Nitrogen Measurement unit using 2mL of water sample diluted with ultrapure Milli-Q® water in a 1:10 ratio. A calibration curve containing a blank and five known concentrations of TDN (2, 4, 6, 8 and 10 mg L-1) was used to measure TDN concentrations.

Flux calculations were automated using R 3.6.3 and the flux v0.3-0 package. This package converts trace gas concentrations to mass units using the ideal gas law. Thereafter, the package calculates CH4 and N2O fluxes using the slope of the linear change in gas concentration over time.