Soil respiration and environmental factors in the Wenchuan earthquake-affected area

1.1 Measurement of Soil RespirationSR was measured on clear days during five sampling phrases: September 2015, December 2015, March 2016, June 2016, and September 2016. In each plot, one PVC collar (20 cm diameter, 10 cm height) was installed near the center by embedding it into the soil by 5 cm. SR measurements were conducted using an LI-8100 Automated Soil CO₂ Flux System (LI-COR Biosciences, Lincoln, NE, USA) in connection with the PVC collar. A total of six collars were deployed per plot and remained undisturbed across the study period with the remove of any vegetation or litter within collars. Meanwhile, soil temperature (ST) and soil moisture (SM) at 5 cm depth were measured dynamically between 10:00 and 18:00 on each sampling date using the integrated temperature and moisture sensors of the LI-8100 system. Each variable was measured three times continuously at six different time points during the monitoring window, obtaining six replicate recordings per day per sensor (Davidson et al., 2002).1.2 Soil sampling and analysisIn each plot, at least four surface soil samples from the depth of 0–20 cm were collected adjacent to the soil collars with 100 cm3 stainless steel cutting rings, following the removal of litter and debris from the soil surface. The collected soils were thoroughly mixed to form a composite sample for the analysis of chemical properties. Additional intact soil cores were collected from the same plots using cutting rings for the determination of physical properties. A portion of fresh soil was stored at 4 °C for biological analysis.Soil physicochemical properties were determined according to methods described by Lu (2000). Soil bulk density (Bd) and soil porosity (SP) were measured using the cutting ring method. Electrical conductivity (EC) was determined using a conductivity meter (Starter 3100C), and soil pH was measured in a 1:2.5 soil: water suspension with a pH meter (Starter 3100). Soil organic carbon (SOC) and total nitrogen (TN) were analyzed using an elemental analyzer (EA3000, Euro Vector S.P.A., Italy). Total phosphorus (TP) was measured by the Mo-Sb Anti spectrophotometric method (Olsen et al., 1982).Soil microbial properties, including microbial biomass carbon content (MBC), microbial nitrogen content (MBN), bacterial number (Bac), actinomycetes number (Act), fungus number (Fun), sucrase activity (SUC), cellulase activity (CEL), urease activity (URE), alkaline phosphatase activity (ALP), and catalase activity (CAT), were determined following the methods of Lin (2010) and Wu (2006). Microbial biomass was determined using the chloroform fumigation method. MBC was analyzed by the potassium dichromate oxidation-external heating method. MBN was determined using the semi-micro Kjeldahl method. Microbial abundances were quantified via the dilution plate method. Bac, Act, and Fun were cultured on beef extract peptone, Gao’s No. 1, and potato sucrose agar media, respectively. Sucrase and cellulase activities were measured using the 3,5-dinitrosalicylic acid colorimetric method. Urease activity, alkaline phosphatase activity, and catalase activity were determined by the phenol-sodium hypochlorite colorimetric method, the disodium phenyl phosphate colorimetry, and potassium permanganate titration, respectively.1.3 Plant Sample Collection and MeasurementHerbaceous and shrub plants were collected in a 1 m × 1 m subplot adjacent to each soil sampling point. After removing attached soil and debris, plants were separated into aboveground and belowground parts using scissors. Fresh weights of two parts were measured immediately. The samples were then placed in labeled envelopes, oven-dried at 80 °C until constant weight (approximately 48 hours). Dry weights were determined using an electronic balance with a sensitivity of 0.001 g to calculate total biomass (TB) and underground biomass (UB).1.4 Relationship between Soil Respiration and Related FactorsA classical parametric exponential function with soil temperature (ST) as the driving variable has been commonly used to characterize the relationship between soil respiration (SR) and ST (Chen et al., 2003; Deng et al., 2019). The function is expressed as follows:                                        R = aebT                                                                       (1)where R represents the measured SR rate (μmol m⁻² s⁻¹), T is the soil temperature at 5 cm depth (°C), a indicates the basal respiration at 0 °C, and b is used to calculate the temperature sensitivity coefficient Q10, which reflects the factor that SR increases per 10 °C rise in temperature .