Effects of plastic mulching on soil CO2 efflux in a cotton field in northwestern China

Site description

In 2012, a field experiment was conducted in the Aksu National Experimental Station of Oasis Farmland Ecosystem²⁷ (40°37′ N, 80°45′ E, altitude 1028 m) (Fig. 1), located in the west of Tarim River Basin in Xinjiang Province, China. The experimental area had a typical temperate arid climate. During the study period (May to October), the average minimum and maximum temperatures varied between 16.7 and 34.8 ℃ respectively.

The cotton fields where the experiment conducted were public land, belong to Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, China. With the permissions of Xinjiang Institute of Ecology and Geography, we conducted experiments in the cotton field of the Aksu National Experimental Station of Oasis Farmland Ecosystem.

Experimental design

Two treatments, each 10 m × 10 m in size, were established on one of cotton fields at the Aksu National Experimental Station of Oasis Farmland Ecosystem on April 5, 2012.One treatment planting cotton with TC method, the other with MC method. For the MC method, a high-density and air-tight transparent polythene film (0.01–0.02 mm thick, 1.25 m wide) was placed over the soil surface before sowing. Small holes (0.02 m × 0.02 m, at 0.1 m intervals within a row) in the plastic film were made to place cotton seeds. Four rows were sown on each strip of plastic film. For the TC treatment, the plants were sown as that for the MC treatment. The planting density (266 667plant ha⁻¹) and irrigation pattern (frequency and volume of irrigation) for the TC method were entirely consistent with those for the MC method.

Half-hourly measurements of soil CO₂ efflux, soil temperature and moisture were made on 6 June 2012. The whole experiment was completed on 4 November 2012. According to irrigation, the whole experiment can be divided into three stages: stage before irrigation (from 6 to 24 June), during irrigation (from 25 June to 10 October) and irrigation stop stage (from 11October to 4 November). During the irrigation period, we conducted seven times of irrigation (once in two week). The water-soluble compound fertilizer (N + P₂O₅ + K₂O ≥ 51%) was used for fertilization in the experimental field, and the application rate was 30 g m⁻². We dissolved water-soluble compound fertilizer in water and sprayed into the field by sprayer. During the irrigation period, the fertilizer was applied for 5 times.

The cottonseeds we used in this study comply with the provisions of the regulations of the People’s Republic of China on Seed Administration and the detailed rules for the implementation of crop seeds. The fertilization we used in this study comply with the provisions of the People’s Republic of China on Chemical fertilizer standard. All the experiments we conducted in the cotton field of Aksu oasis farmland ecosystem National Experimental Station met the provisions of the agricultural law of the People’s Republic of China. We also carried out the experiment of this study under the guidance of the provisions of the measures for the administration of national field scientific observation and research stations.

Field measurement of soil CO₂ concentration

Solid-state CO₂ sensors (GMM221 and GMM222, Vaisala, Finland) were installed in the midpoint of each treatment to measure soil CO₂ concentration. A cable connected each soil probe with a transimitter body placed on the ground. The transimitter sent output signals from the probe to a data logger (CR1000, Campbell Scientific Inc., Logan, UT, USA) and to an optional LCC display on the transmitter.

In each treatment, four CO₂ concentration sensors were buried at depths of 0 cm, 5 cm, 10 cm and 15 cm. Soil CO₂ concentrations were recorded once in 30 min. The measurement of soil CO₂ concentrations were conducted from 6 June 2012 to 4 November 2012.On 8 November, these sensors were excavated and recalibrated in the laboratory. We found no change in the slope or offset.

Environmental and soil CO₂ efflux measurements

The soil water content and temperature at the same soil depth with solid-state CO₂ sensors were measured on the cotton fields at the Aksu National Experimental Station of Oasis Farmland Ecosystem^27,28, respectively. Soil volumetric water content and soil temperature were measured using soil moisture probes (pF-Meter, EcoTech GmbH, Bonn, Germany)²⁶ and temperature probes (PT100,Heraeus Sensor Technology, Kleinostheim, Germany)²⁶, respectively.

Bulk density was determined by core method²⁹. Briefly, a cylindrical metal sampler (volume of 100cm³) was inserted into the soil and carefully removed to preserve the sample. The sample was oven-dried at 105 °C and weighed. The ratio between dry weight of the soil sample and the cylinder volume was applied to provide the bulk density.

Half-hourly soil CO₂ efflux measurements were conducted using a closed dynamic chamber method²⁶ (CIRAS-1 PP Systems, Hitchin, UK) on the TC treatment, beginning on 6 June 2012. A chamber, with a diameter of 9.96 cm and a volume of 1, 170 cm³ was inserted into the soil at depth of 3 cm. Soil CO₂ concentrations were measured by infrared gas analyzer. The collecting of CO₂ from each sampling point took 120 s to get reliable estimates of soil CO₂ efflux.

Data analysis

In order to calculate CO₂ efflux in soil, Fick’s first law of diffusion was used:

where F_i is the CO₂ efflux at depth z_i, D_s the CO₂ diffusion coefficient in the soil, and d_c / d_z the vertical soil CO₂ gradient. In this study, the vertical CO₂ gradient (dC/dz) was approximately a constant at different depths of soil in our site for the field conditions experienced in the TC treatment during study period. However, a quadratic function of depth to concentrations fitted to soil CO₂ concentration gradients in the MC treatment.

D_s can be estimated as

where ξ is the gas tortuosity factor and Da is the CO₂ diffusion coefficient in free air. The effect of temperature and pressure on D_a is given by

where T is the temperature (K), P the air pressure (kPa), D_ao a reference value of D_a at 20 °C (293.15 K) and 101.3 kPa, and is given as 14.7 mm² s^–130 .

There are several empirical models in the literature for computing ξ³¹. We used the Millington–Quirk model³²:

where a is the volumetric air content (air-filled porosity), Φ is the porosity. Note,

where ρ_b is the bulk density, and ρ_m is the particle density for the mineral soil.

Soil surface CO₂ efflux was calculated using the CO₂ gradient flux method based on CO₂ concentrations within the soil profile¹. Briefly, the flux of CO₂ between any two layers in the soil profile was calculated using the Moldrup model³³.

In order to determine soil CO₂ storage, the equation for CO₂ was performed.

where C (ppm) is the concentration of CO₂ within the soil pores, (a) is the aerial porosity of the soil layer, D is the molecular diffusivity of CO₂ with the soil, and S(µmol m⁻³ s⁻¹)is the source strength in the soil layer at depth.

We determined temperature responses for soil CO₂ efflux using the van’t Hoff equation³⁴ (Eq. 7);

where R is soil CO₂ efflux, T is soil temperature (°C) at 10 cm depth, and R₀ is the soil respiration rate at a reference temperature of 0 °C (µmol m⁻³ s⁻¹).

The Q₁₀ value for Eq. (8) was calculated according to definition as:

where R_T and R_T+10 are R_r or R_d rates at temperature T and T + 10, respectively. The Q₁₀ value is independent of temperature in Eq. (8).

Source: Ecology - nature.com