## 2 Materials and Methods  
### Soil Collection and Gas Sampling  
#### 2.1.1 Soil Sample Collection and Soil Physicochemical Properties Analysis  
From June to September 2019, rice fields from a typical karst area (KA) (25\({}^{\circ}\)08\({}^{\prime}\)30\({}^{\prime\prime}\) N, 110\({}^{\circ}\)31\({}^{\prime}\)28\({}^{\prime\prime}\) E) and a non- karst area (NKA) (25\({}^{\circ}\)10\({}^{\prime}\)51\({}^{\prime\prime}\) N, 110\({}^{\circ}\)31\({}^{\prime}\)35\({}^{\prime\prime}\) E) were selected as studying fields within the karst experimental site in Maocun village of Guangxi Zhuang Autonomous Region, China. The soil type in the KA is a limestone brown soil, whereas the soil type in NKA is a silicate red soil (Soil forming parent material are mainly sandstone and granite). Each field was divided into three replicate plots with the same area (KA: 42.20 m\({}^{2}\) and NKA: 46.67 m\({}^{2}\)) using by ridge. Each plot was subjected to uniform irrigation, fertilization, and management practices. Three random sampling points were used in each plot. Paddy fields were planted with single-season rice, and the rice growing period was 94 d. Compound fertilizers (N-P\({}_{2}\)O\({}_{5}\)-K\({}_{2}\)O) containing 18% each of nitrogen, phosphorus and potassium were applied as base fertilizer to the fields at 7.56 kg (KA) and 8.53 kg (NKA) before transplanting seedlings. In addition, 5.70 kg (KA) and 6.40 kg (NKA) of compound fertilizer were applied in each plot as follow-up fertilizer on 14 July. A total of 1 kg of soil was collected from each sample point at a sampling depth of 0-20 cm; the soil samples from the three sampling points were equally and uniformly mixed into a single 3 kg sample. A total of 48 soil samples were collected on eight dates with intervals of about 14 d (with the exception of the first sampling interval of 7 d). The sample from 4 June represented the rice planting date, 12 June was from the seedling stage, 26 June and 10 July were from the tillering stage, 24 July was from the modulation stage, 7 August was from the gestation stage, 21 August was from the tasseling stage, and 4 September was from the maturity stage.  
Soil physicochemical properties were analyzed by routine methods [33]. Briefly, soil moisture content was measured using the drying method. pH was measured using CO\({}_{2}\)-free distilled water as the leaching agent after mixing soil with water at a 1:2.5 ratio, followed by measurement with a precision pH meter (model: IS128C). SOC was measured using the sulfuric acid potassium dichromate external heating method. Soil total nitrogen (TN) and hydrolyzed nitrogen (AN) measured using the concentrated sulfuric acid digestion-Kjeldahl method and the alkali hydrolysis diffusion method. Total phosphorus (TP) and available phosphorus (AP) determined by the sodium carbonate melting method and the hydrochloric acid-ammonium fluoride method. Soluble organic carbon (DOC) was extracted by shaking a soil-water mix (at a ratio of 10:1), filtering through a 0.45 \(\upmu\)m fiber membrane, and analysis on a multi N/C3100 total organic carbon analyzer (Analytik Jena, Jena, Germany). Cation exchange capacity (CEC) was measured using the EDTA-ammonium fast method. An optima 7000 DV inductively coupled plasma emission spectrometer (Perkin Elmer, Waltham, MA, USA) (ICP-OES) was used to measure the exchangeable calcium (Ca\({}^{2+}\)) and magnesium (Mg\({}^{2+}\)) ion concentrations.