Southwest China
2. Materials and Methods 1. Study Site
The study area was located near Maocun village, about 30 km southeastof Guilin (110◦3000”–110◦3345E, 25◦1011–25◦1230N), in a typical karst peak cluster depression and valley landscape, with a catchment of about 10 km2(Figure1). The climate in the area is mid-subtropical humid monsoon, hot and humid summer, characterized by spatio–temporal variations in rainfall;
the annual average temperature is 19.64–20.39◦C, and the annual average rainfall is 1160–1378 mm (Figure S1). The geology of karst areas consists of Upper Devonian Rongxian Formation (D3r) pure limestone. Vegetation in the studied sites was comprised of an evergreen broad-leaved forest dominated byCyclobalanopsis glauca,Loropetalum chinense,Alchornea trewioides, andNephrolepis auriculata, whose soil type was brown calcareous soil of the primosol order in the genetic soil classification of China (GSCC) [16], with a depth of 0.2–1 m. The geology of non-karst areas comprises iron clastic rock (D21). Vegetation in this studied site was comprised of an evergreen broad-leaved forest, dominated by Castanopsis fargesii,Schima superba,Itea chinensis, and a small amount ofMiscanthusspp.; the soil type was red soil of the ferralosol order in GSCC [16], with a depth of>1 m [9].
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Figure 1.Land use of study area and location of study sites.
2.2. Soil Physicochemical Properties
For each layer (10, 20, 30, 40, 50, 60, 80, and 100 cm) in the 1-m deep soil profiles, we recorded water volume concentration, conductivity, and temperature using a WET sensor and HH2 moisture meter (Delta, Wakefield, UK); the corresponding range and resolutions are 0–100%, 0.1%, 0–300 mS m−1, 1.0 mS m−1and–5–+50◦C, 0.1◦C. Soil pH was measured by a US IQ150 (0.00–14.00, 0.01) soil in situ acidity meter on 24 March 2015. The water volume concentration, conductivity, and temperature of the surface calcareous soil and red soil were measured monthly for one year (Table1; Figure2).
Soil organic matter was determined by the potassium dichromate volumetric method.
Table 1.Properties at different depths in calcareous and red soils.
Depth
(cm) pH Organic
Matter (%)
Water Content (%)
Conductivity (mS m−1)
Soil Temperature (◦C)
Calcareous soil (KP1)
10 7.32 4.57±0.05 26.7 92 16.3
20 7.29 2.57±0.03 22.6 106 16.4
30 7.35 1.92±0.04 23.4 112 16.1
40 7.27 1.78±0.03 22.3 115 16.1
50 7.25 1.89±0.08 22.2 118 16.0
60 7.13 2.02±0.05 20.8 123 16.0
80 7.21 1.34±0.01 26.2 117 15.8
100 7.24 0.86±0.00 32.2 116 15.7
Red soil (NKP1)
10 5.48 4.02±0.03 21.9 12 17.1
20 5.96 3.67±0.05 23.3 16 16.6
30 5.33 3.15±0.05 20.3 12 16.6
40 5.40 2.72±0.03 22.0 13 16.8
50 5.48 2.01±0.08 19.5 11 17.0
60 5.36 1.65±0.00 18.1 10 17.4
80 5.69 1.14±0.01 16.2 7 17.6
100 6.39 1.04±0.06 10.7 7 17.5
Data are the mean±standard deviation (SD). N=3.
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Figure 2.Temperature, water volume concentration and conductivity in the calcareous and red soils.
2.3. Soil CO2Concentrations and Emissions
We installed a CO2collection pipe in the calcareous (KP1) and red (NKP1) soil profiles in March 2015; two additional profiles in each soil type were included in January 2016 (KP2, KP3, and NKP2, NKP3), and monitoring points were separated by 5–50 m. Monthly monitoring continued from April 2015 to March 2018.
CO2 concentrations in the eight soil layers were measured and recorded monthly (April 2015–March 2018) using a self-made soil collection pipe, comprising a 100 mL test tube in which gas was collected using a vacuum pump (GAS-TECQ Kitagawa, Japan). If CO2concentration
>2.6%, we collected 50 mL of gas, and CO2concentration was doubled to give a concentration per 100 mL of gas.
Soil respiration CO2was collected from relatively flat ground, from which litter and weeds were removed to control for photosynthesis and the respiration of plants; the bare ground was immediately covered with a metal cylinder (25 cm diameter, 32 cm high), and the lower, open end of the cylinder was embedded 2 cm below the soil surface. A rubber tube was embedded in the soil within the cylinder and exposed to the atmosphere; a water stop clip was used to seal the end of the tube exposed to the atmosphere. Monthly atmospheric samples (April 2015–March 2018) were taken from the profiles at about 2 m above ground level to calculate background CO2levels; 1 h later, 200 mL of gas in the cylinder was continuously collected using a 100 mL medical syringe and injected into a vacuumed aluminum foil gas sampling bag. CO2concentration (ppm) was determined using an Agilent SP1 7890-0468 meteorological chromatograph (Agilent, Santa Clara, CA, USA) at the Ministry of Natural Resources/Guangxi Key Laboratory of Karst Dynamics, Guilin, Guangxi. Air pressure and temperature were simultaneously recorded. Sampling was done between 09:00 and 11:00, when soil respiration rates tend to reflect daily averages [17], to minimize diurnal variations in emissions. Soil CO2concentration and respiration in KP1 and NKP1 were sampled 33 times, and 23 times from the remaining profiles.
Theδ13C-CO2was measured from the eight layers in the soil profiles in June and December 2015, and July 2017, where we used a vacuum to extract soil CO2into sealed aluminum bags. Theδ13C-CO2 analysis was completed at the isotope laboratory of the Chinese Academy of Agricultural Sciences, Beijing, China using a MAT253 mass spectrometer. Samples were collected from KP1 and NKP1 three times, and once from the remaining profiles.
2.4. Dissolution Rate of Carbonate Rock
The rate was acquired through the standard carbonate tablet method [18]. Carbonate tablets were determined from high purity, 4 cm diameter×0.3 cm thick calcareous tablets that were washed and subsequently dried at 70◦C to constant weight after cooling, through a repeated dry-weigh process.
In March 2015, in KP1 and NKP1, three tablets were placed 150 cm above the ground, at the soil surface, and inserted at the 20, 50, and 100 cm soil layers. In June, September, and December 2015, and March 2016, the tablets were recovered, analyzed, and re-buried following the washing and drying process described above.
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2.5. Data Analysis
The units of CO2measurement (ppm) were converted to mg m−3using:
CO2= M
22.4×ppm× 273
273+T× Ba
101325 (1)
where M is the molecular weight of the gas; ppm is the measured volume concentration; T is the atmospheric temperature (◦C); and, Ba is atmospheric pressure (Pa).
Soil respiration rate (VR; mg C m−2h−1) was calculated as:
VR=(C1−C0)×V S×h ×12
44 (2)
where C1is CO2concentration in the cylinder (mg m−3); C0is the corresponding background CO2 concentration (mg m−3); V is the sampling box volume (m3); S is the area of sampled soil (m2); and, h is the monitoring time (h).
Annual soil CO2emission flux (F; t C km−2a−1) was calculated as:
F= n
i=1Vn×(Tn−Tn−1)×24×10−3
(Tn−T1) 365
(3)
where Vnis the nth measured soil respiration rate (mg C m−2h−1); Tn−Tn−1and Tn−T1are the nth and (n−1)th, the nth and 1st sampling intervals (d), respectively.
The annual dissolution rate of the carbonate tablet (ER; mg cm−2a−1) was calculated as:
ER= (W1−W2)×1000× T
365×S (4)
where W1is the initial weight of the tablet (g); W2is the weight after embedding (g); T is the embedding duration (d); and S is the tablet surface area (about 28.9 cm2).
CO2recovery in calcareous soil (CRC; t C km−2a−1) was calculated according to the stoichiometric coefficient ratio of the carbonate dissolution reaction:
CaCO3+CO2+H2O↔Ca2++HCO3− (5)
CRC=ER×97%× 12
100×10 (6)
where ER is the annual dissolution rate of the carbonate tablet (mg cm−2a−1); 97% is the purity of the standard carbonate tablet.
The dissolution rate of carbonate rocks in calcareous soil is 23.8 times that of clastic rocks in red soil at the study site [19]. According to the clastic rock dissolution reaction:
2CO2+3H2O+CaSiO3=Ca2++2HCO3−+H4SiO4 (7) the CO2recovery in red soil (CRR; t C km−2a−1) was calculated this way:
CRR=CRC
23.8×2 (8)
We used one-way analysis of variance (ANOVA) to analyze the differences in soil respiration rate, CO2concentration, andδ13C-CO2among the soil layers between the two soil types atp=0.05.
A Pearson correlation analysis was used to test for association between CO2concentration andδ13C-CO2
in different layers. Analyses were performed in Statistical Package Social Science (SPSS ver. 20.0; IBM Crop., Armonk, NY, USA).
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