2. MATERIALS AND METHODS

2.1 Study area

The QTP lies in the west of China, and has a long cold season (October-May) and short warm season (Lin et al. 2015b). The Beiluhe Basin is in central QTP where elevation ranges from 4500-4700 m a.s.l (Figure 1). Alpine meadow and alpine grassland are the main vegetation types, accounting for over 40% of the area (Yin et al. 2017). The vegetation communities are simple, and dominant plant species includeKobresia pygmaea, Carex moorcroftii, Stipa purpurea, and Littledalea racemose , etc. (Lin et al. 2019). Most plants are <15 cm tall and the growth period is short. Strong wind erosion on the plateau results in a fragile ecological environment (Li et al. 1996).
Data from Beiluhe Weather Station indicate that the annual mean air temperature was between -4.1 and -2.6 °C from 2005 to 2016, with an average value of -3.4 °C. Annual mean precipitation ranged between 229 and 467 mm (Figure 2), while the annual mean potential evaporation was ~1588 to 1626 mm in the same period (Lin et al. 2019). About 10% of annual precipitation falls as snow.
The Beiluhe basin is undulating covered with fine to gravelly surface sands. Surficial materials within 30 cm of the surface are predominantly aeolian sand or alluvial deposits (Yin et al. 2017). Thermokarst lakes are widely spread in the basin and have been eroding permafrost (Lin et al. 2010; 2011). Permafrost in the basin is continuous, relatively warm (near 0 ℃), and has high volumetric ground ice content with a mean value of ~16% (Lin et al. 2020; Fan et al. 2021). Active-layer thickness ranges from 1.8 to 3.0 m and the annual mean ground temperature is -1.8 to -0.5 °C. Sediment textures range from clay to sandy gravel, which overlies weathered mudstones and sandstones (Lin et al. 2010).

2.2 Sites descriptions

The study examined two sloping sites with opposing aspect (Figure 1). One site was at a south-facing sunny slope (34.8367°N, 92.9206°E) and the other is a north-facing shady slope (34.8486°N, 92.9268°E). The slope angle at the sunny slope is about 7.5°, and about 8.1° at the shady slope (Figure 3). The elevation is 4634 m at the sunny site and 4638 m at the shady site. The dominant plant species on the sunny slope are Stipa purpurea and Kobrecia parva , and the mean vegetation coverage is approximately 16.7%. On the shady slope the dominant plant species are Androsace tapete maxim, Carex moorcroftii , and the mean coverage is ~7.9%. The sediment profile to 5.0 m depth is presented in Figure 3, and includes information on gravimetric moisture content (GMC), excess ice content (EIC), and permafrost table depth (PT). The ground surface at both sites is typically covered by gravel or sandy silt. Below this layer lie coarse-grained deposits that are rich in flake gravel. The stratigraphy is similar up to 5 m depth at both sites, however the ice content differs (Lin et al. 2020). Soil texture information with depth is presented in Figure 4. The near-surface soil at the sunny slope was ~41% silt, 14% clay, and greater than 50% silt and clay combined. Most of the near-surface samples from the shady slope were sand dominated (63%), with only ~40% silt and clay combined. The soil at the sunny site was generally >60% silt at 3-4 m depth, and included high excess-ice content in this frost-susceptible layer. The soil organic matter (SOM) content in Beiluhe Basin soils is very low (Liu et al. 2014), and the measured SOM content at the shady slope was higher than that at the sunny slope (Figure 5).

2.3Temperatures

A HOBO Pro v2 (U23-004) external temperature data logger was used to measure air temperature (Ta) and ground surface temperature (Ts) at each site. The built-in sensor was installed in a solar radiation shield at 2.0 m height and an external temperature sensor was used to measure soil temperature at ~1-2 cm depth. The reported measurement accuracy of the sensors is ±0.21 °C from -40 to 100 °C. Data collection began in September 2016 and measurements were recorded every 30 minutes.
A drilling programme to install temperature sensors was conducted at the two sites in July-August 2016. A total of 18 boreholes were instrumented to 5 m depth to determine the variation in ground thermal conditions and associations between permafrost temperatures, soil moisture, and slope aspect. Each site included nine boreholes drilled 5 m apart in a 10 x 10 m rectangular grid. The multiple measurements at each site are meant to improve the evaluated accuracy of ground temperature characterization.
At each borehole, the drill core was extracted with a 10 cm diameter dry drill. Three HOBO soil temperature sensors (TMC20-HD; Onset Computer Corporation, Bourne, MA, USA) were installed at 5 cm, 250 cm, and 500 cm depth. The three measured depths represent the near surface temperature (Tns), the temperature near the permafrost surface (Tps), and the permafrost temperature (Tg), respectively. The three sensors were fixed to the outer wall of a polyethylene aluminium composite tube placed in each borehole. The holes were filled with dry sand and packed with a long rod in order to improve contact between the sensors and surrounding ground.
Ground temperatures were recorded by a HOBO UX120-006M 4-Channel Analog Logger. The reported measurement accuracy of the sensors is ±0.21°C from -20 to 70 °C. Data collection began in September 2016 and measurements were recorded every 4 h.

2.4 Soil moisture content

At each site a 1.5 m deep soil profile was excavated. HOBO soil moisture sensors (S-SMD-M005) were inserted directly into the soil profile at depths of 25, 50, 100, and 150 cm. The volumetric moisture content (m3/m3) was collected using a HOBO H21-002 Micro Station. The reported measurement accuracy of the sensors is ±0.031 m3/m3 (±3.1%) from 0-50 °C for mineral soil up to 8ds/m and ±0.020 m3/m3 (±2%) with soil specific calibration. Data collection began in September 2016 and measurements were recorded every 4 h.

2.5 Solar radiation

A CNR4 Net Radiometer (Kipp&Zonen, Delft-The Netherlands) was installed at 1.5 m height at each site to measure the energy balance between incoming short-wave and long-wave (Far Infrared, FIR) radiation versus surface-reflected short-wave and outgoing long-wave radiation. The CNR4 net radiometer consists of a pyranometer pair, one facing upward, the other facing downward, and a pyrgeometer pair in a similar configuration. All 4 sensors were calibrated individually for optimal accuracy.
The spectral range (50% points) of short wave measurements is 300 to 2800 nm and 4500 to 42000 nm in the long wave spectral range (50% points). The sensitivity of the sensors is 5 to 20 µV/W/m² and the temperature dependence of sensitivity (-10 to +40 ºC) is less than 4 %. The instruments can operate in temperatures of -40 to +80 °C and 0-100% RH. The instruments were factory calibrated. Two data loggers (CR1000, Campbell Scientific, Edmonton, AB, Canada) were separately employed to sample at 30 minute intervals and data were stored as 1 h averages for both sites. Data collection began in September 2016. Following collection, obviously erroneous measurements were removed and gaps were filled by interpolation.

2.6 Laboratory test of soil texture and SOM

Samples were collected for transport to the State Key Laboratory of Frozen Soil Engineering (Lanzhou), Chinese Academy of Sciences (CAS) to examine the soil texture and organic content at both sites. The dried soil samples were crushed and put through a 2 mm sieve. The particle-size distribution of soil that passed through the sieve (<2 mm) was determined using a Malvern Mastersizer 2000 Particle Size Analyzer (Malvern Panalytical Ltd, Malvern, UK). The resulting particle-size distributions were divided into three texture classes: (1) sand (2 mm ≥ sand > 75 μm), (2) silt (75μm ≥ silt > 5 μm), and (3) clay (5 μm ≥ clay).
The SOM of the pulverized homogenized samples were quantified by dry combustion using a Vario EL elemental analyzer (Elementra, Hanau, Germany). To measure the soil organic carbon content (SOC), 0.5 g air-dried soil samples were pretreated with HCl (10 mL, 1 mol L-1) for 24 h to remove carbonate.

2.7 Data processing

Annual mean air temperature, annual mean ground temperature, the surface offset, and freeze-thaw indices were calculated as outlined in Lin et al. (2019). Net short wave radiation (Rs, W·m-2), net long wave radiation (Rl, W·m-2), and net radiation (Rn, W·m-2) can be computed using four components (eq.1-3):
\(Rs=DR-UR\) (1)
\(R_{l}=DLR-ULR\) (2)
\(R_{n}=DR-UR+DLR-ULR\) (3)
Where DR and UR are downward and upward shortwave radiation, respectively. DLR and ULR are downward and upward long-wave radiation, respectively. The four parameters were measured at both sites using the CNR4 Net Radiometer.