The CH 4 concentrations of the gas samples were analyzed using a gas chromatograph ( Agilent 7890A, Agilent Technologies Limited Co., USA) equipped with flame ionization detector (FID) using 60–80 mesh 13 XMS column (2 mm inner diameter and 2 m long), with an oven temperature of 55☌.
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East - Plainfield, Illinois) and a portable soil moisture measuring kit ML2x (ThetaKit, Delta-T Devices, Cambridge, UK), respectively. Simultaneously, the air temperature and pressure in the chamber were measured, and the soil temperature and moisture were measured at a 5 to 10-cm depth using a thermometer (Spectrum Technologies, Inc. Sixty-ml gas samples were collected at 10-min intervals for 30 min using sixty-ml syringes with airtight stopcocks. Two electric fans were installed inside the chamber to mix the air in the headspace continuously and thoroughly during the measurements (Fan et al. The grooves were filled with water for sealing the chamber. An opaque stainless-steel top chamber (50 cm height) with heat-isolation was installed over the base, with bottom in the groove. The chamber base was left in the field for one month before any flux measurement, avoiding potential disturbances. Stainless steel permanent bases (50 × 50 × 12 cm) with a 3-cm-deep groove for water sealing were inserted into soil down to a depth of 12 cm in the plots approximately one month before the experiment started in the first year. The measured vegetation variables included aboveground biomass, plant total carbon (PTC) and total nitrogen (PTN).Ī static chamber technique was used to measure the CH 4 flux (Song et al. It should be noted that no nitrite was detected at our field site. The measured soil variables included ammonium (NH 4 +) and nitrate (NO 3 -), soil pH, soil organic carbon (SOC), soil total nitrogen (STN), soil total phosphorus (STP), soil microbial biomass carbon (MBC) and soil microbial biomass nitrogen (MBN). Six soil cores were randomly taken in each plot and mixed completely. Soil samples were collected to a depth of 10 cm once per year during 2013-2016. Over the study period, soil and plant properties were measured (Supplementary Online Materials). Control plots were not fertilized, but rather watered with the same amount of water as used in the fertilizer solutions the water used to dissolve N and P was approximately 800 ml, which is equivalent to 0.8 mm for a 1 m 2 plot, a pre-treatment field experiment found that this small amount of water addition did not cause any significant changes in water supply, thus unlikely to have altered the ecosystem functions. The plots with both N and P additions received the same amounts of N and P as in the N-only or P-only addition treatments. The dose of P was 100 kg P ha -1 y -1, equivalent to previous nutrient addition experiments in grasslands (Phoenix et al. We monitored the atmospheric N deposition and estimated it to be 20.4 kg N ha -1 y -1 in our experimental site in 2012.
The dose of N addition was 100 kg N ha -1 y -1 as NH 4NO 3 solution this dose was selected because it meets the N required to sustain the local maximum vegetation productivity (Bai et al. The N and P were applied twice per month from May to September during 2013-2016 we sprayed the fertilizer solution to ensure that the application was evenly distributed in the plots. All blocks were separated with a 3-m buffer zone. Two chambers for greenhouse gas measurement were set up in each plot. There were four experimental treatments (i.e., control (CK), N addition (N), P addition (P), both N and P additions (N+P)), which were randomly assigned to four 6 m × 6 m plots in each block. A complete random design, with three block replicates, was adopted to address the high spatial heterogeneity. To assess the nitrogen (N) and phosphorus (P) impacts on CH 4 flux, a block design experiment with different combinations of N and P additions was established in early 2013 and run to 2016. The dominant plant species include Stipa krylovii, Artimesia frigida, Potentilla acaulis, Cleistogenes squarrosa, Allium bidentatum and Agropyron cristattum. Soil is Calcis-orthic Aridisol (the US Soil Taxonomy classification), with 62.75 ± 0.04% sand, 20.30 ± 0.01% silt and 16.95 ± 0.01% clay (Wu et al. The topography is characterized by low foothills at elevations of 1150-1800 m. The multiple-year mean annual precipitation and mean annual temperature are approximately 385 mm and 2.1☌, respectively. The field experiment was conducted in Duolun County (42☀2’N, 116☇0’E, 1324 m a.s.l.), a semiarid temperate steppe in Inner Mongolia, northern China.
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