*3.2. Experimental Design*

Based on the irrigation amount of full irrigation (W) calculated as Equation (1), three irrigation levels were set: 60%, 80%, and 100% of W. Non-aerated SDI (S) was used as a control for aeration (O). Therefore, six treatments were designed (W0.6O,W0.6S,W0.8O,W0.8S,W1.0O, andW1.0S). Three replicates of each treatment were used (18 total plots), and the experiment was arranged using a randomized block [31]. Each plot with one row was 4 × 0.8 m in size, with eleven tomato plants of cultivar "JINGPENG SEED" planted on 6 August 2017. The plants were spaced 35 cm apart. All plots were mulched with a layer of low-density polyethylene to minimize surface evaporation [42]. SDI was applied in the experiment, which was buried at a depth of 15 cm below the soil surface with a dripper interval of 35 cm [9,10]. Additionally, a Mazzei air injector Model 287 (Mazzei Injector Company, LLC, Bakersfield, CA, USA) was installed at the head of each irrigation line for AI (inlet pressure: 0.1 MPa; outlet pressure: 0.02 MPa) [42]. Definitively, the air injectors were set to inject 17% volumetric air concentration in the water [25].

Daily evaporation measured by an E601 evaporation pan is shown in Figure 8. In all growth stages, 20 irrigation events were applied every seven days, with a total irrigation amount for W of 19.80 L per plot [31]. Irrigation amount was determined following the Equation (1) [9,10,42]:

$$\mathcal{W} = k\_{\text{cp}} \times E\_{\text{pan}} \times A \tag{1}$$

where *kcp* is the crop-pan coefficient, being 1.0; *Epan* is the total evaporation quantity following the last irrigation event (mm); and *A* is the area controlled by one irrigation dripper in this experiment, being 0.14 m<sup>2</sup> (0.35 m × 0.4 m).

Only basal fertilizer, including organic fertilizer (N–P2O5–K2O ≥ 10%, organic matter ≥ 45%) and compound fertilizer (total nutrients ≥ 45%, N, P2O5 and K2O each at 15%), was applied for all plots. The application was achieved at a rate of 1875 and 1250 kg·ha−<sup>1</sup> on 3 August 2017 for organic and compound fertilizer, respectively. Other agronomic managements were consistent with local production practices [42]. The experiment ended on 25 December 2017 with a total growth period of 142 days.

#### *3.3. Measurement Index and Methods*

Soil samples from 0 to 10 cm depth were collected when gas samples were collected except on 9, 20, 62, 83, and 104 DAT. Soil samples were taken through a diameter gauge with the three-point sampling method to measure soil water content via oven drying at 105 ◦C for 12 h, and then converted to WFPS by the following equation:

$$\text{WFFPS}(\%) = \frac{\text{gravitationalwater content}}{\text{totalsoil porosity}} \times \text{soilbulkdensity} \times 100\tag{2}$$

where total soil porosity = 1 – soil bulk density/2.65, with 1.35 <sup>g</sup>·cm<sup>−</sup><sup>3</sup> as the assumed particle density of the soil.

Soil temperature at a depth of 10 cm was recorded using a geothermometer (RM-004, Hengshui, China) when gas samples were collected, excluding on 9 and 62 DAT.

Soil samples of top-soil (0–20 cm) were collected to measure soil microbe and enzyme activity on 35, 53, 77, 98, 119, and 141 DAT. The *cfu*b, *cfu*f, and *cfu*a were estimated using the plate dilution counting method in beef extract and peptone medium, Martin's medium, and the improved Gao's No. 1 medium, respectively. Media plates were incubated at 37 and 25 ◦C, and the number of colonies after approximately 3 to 5 d was counted [23]. MBC was measured by the chloroform fumigation–K2SO4 extraction method. MBC in the extracts was determined by the K2Cr2O7–FeSO4 additional heating method. Detailed measurement steps regarding CA and DHA are described by Хaзиев[43].

On 42, 68, 104, and 142 DAT, one plant from per plot was sampled to measure dry biomass and root morphology (total root length, surface area, and volume). All plant samples were first separated into leaves, stems, fruits, and roots. The roots collected from soil by digging were gently washed, scanned (Epson Perfection V700 photo, Seiko Epson Crop., Nagano-ken, Japan) to obtain a gray-scale JPG image, and then analyzed with the WinRHIZO Pro image processing system (Regent Instrument Inc., 2672 Chemin Sainte-Foy, Quebec City, Quebec G1V 1V4, Canada) to obtain root morphology [18]. After that, every part of the tomato plant including root was put into a 105 ◦C oven for 1 h to deactivate enzymes and then dried at 75 ◦C until the parts reached a constant weight. The dry biomass of each part was weighed on an electronic scale [31].

Gas samples of soil respiration was measured using the static closed chamber method described by Hou et al. [10]. All chambers, which were made of polyvinyl chloride (PVC) materials and wrapped with sponge and aluminum foil, were 25 × 25 × 25 cm in dimension. The bases of the chambers were installed between two plants in the middle of each plot on the day of transplanting and remained there until the end of the experiment. There was a 3-cm-deep groove on the top edge of the bottom layer and on the base of the chamber that was filled with water to seal the rim of the chamber. A mercury thermometer (WNG-01, Hengshui, China) at the top of each chamber was equipped to measure air temperature when gas sampling for calculating gas emission flux. Gas samples at an average interval of seven days were collected at 10:00, 10:10, 10:20 and 10:30 a.m. of each sampling time. A 30-mL air sample was drawn each time with a syringe. Gas samples in the syringes were analyzed for CO2 concentrations using a gas chromatograph (7890A GC System, Agilent Technologies, Santa Clara, USA) within a few hours. Sample sets were discarded unless they yielded an R<sup>2</sup> linear regression value higher than 0.90. Then, soil CO2 fluxes (soil respiration), which were the sum of autotrophic and heterotrophic respiration, were calculated following the equation given by Hou et al. [10]:

$$F = \rho \cdot h \cdot \frac{273}{273 + T} \cdot \frac{dc}{dt} \tag{3}$$

where *F* is the soil respiration (mg·m<sup>−</sup>2·h−1); ρ is the gas density at standard state (1.964 kg·m<sup>−</sup>3); *h* is the height of chamber above the water surface (m); *dcdt* is the gas mixing ratio concentration (μL·L−1·h−1); and *T* is the mean air temperature inside the chamber during sampling (◦C).

## *3.4. Statistical Analysis*

A two-way analysis of variance (ANOVA) followed by an LSD test (95% confidence level, *p* < 0.05) was used to test for the e ffects of irrigation, aeration, and their interaction on soil respiration, soil physical and biotic properties (WFPS, temperature, *cfu*b, *cfu*f, *cfu*a, MBC, CA, DHA, root morphology, dry biomass). Regression analysis of soil respiration with soil physical and biotic variables was conducted. All statistical and regression analysis were performed using the software SPSS Statistics 22.0 (SPSS Inc., Chicago, IL, USA), and figures were generated using SigmaPlot 12.5 (Systat Software, Inc., Chicago, IL, USA).
