Carbon dioxide fluxes in a farmland ecosystem of the southern Chinese Loess Plateau measured using a chamber-based method

Study site

This study was conducted at the Changwu Agro-Ecological Experiment Station on the Chinese Loess Plateau (35°12′N, 107°40′E, 1,200 m a.s.l.). Regional climate is driven by the semi-arid continental monsoon. The annual mean temperature is 9.1 °C, and the mean annual precipitation is 584 mm, with an annual mean potential evapotranspiration of 949 mm (1957–2012; Han, Liu & Lin, 2015). The Loess Tableland is a typical rain-fed agricultural area; hence, precipitation is the sole water resource for crop growth. The soils are Cumuli-Ustic Isohumosols, according to the Unified Soil Classification System (Gong, Zhang & Chen, 2007), a type that is porous and has a high-water holding capacity. However, this soil type suffers from moisture deficits under uneven rainfall and high evaporation conditions. The soil pH value is approximately 8.4, with a soil organic matter content of approximately 3% (Han et al., 2016).

Experimental design

There are three types of fluxes in the field under observation: (1) NEE of carbon in the maize field (NEE-maize, with maize plants in the chamber); (2) soil respiration in the maize field (SR-maize, no plants in the flux box and covering of only the inter-row soil); and (3) soil respiration in the bare field (SR-bare field, with no tillage and regular weed removal). CO₂ flux is the general term used for the NEE and SR of the maize ecosystem. The soil organic carbon (SOC) of 0–20 cm depth layers in the maize and bare fields were not significantly different, and the average values were 6.85 ± 0.61 gkg⁻¹ and 6.34 ± 0.68 gkg⁻¹, respectively (n = 6).

The spring maize variety used in the experiment was Xianyu 335, and its sowing time was April 28, 2011. The growth stages of corn primarily include the seedling (May 7 to June 10), jointing (June 11 to June 30), heading (July 1 to July 15), grain filling (July 16 to July 31), and ripening (August 1 to September 15) periods. The growth stages were classified according to the climatic and maize growth characteristics of this region (Liu et al., 2010). Fertilization is conducted every year during the sowing period. The fertilization standard applied was 300 kg ha⁻¹ of nitrogen and 750 kg ha⁻¹ of phosphate fertilizer.

Field measurements of CO₂ fluxes

The CO₂ fluxes were continuously measured in the maize field and bare field plots (10 m ×10 m) at intervals of one hour using a multi-channel automated chamber system (Fig. 1). There were three chambers per treatment, as described in our previous study (Han et al., 2016). The chambers (50 ×50 × 50 cm, L × W × H) were made of transparent plexiglass, with lids hinged at the sidewalls so they could be opened. In each chamber, there was a telescopic cylinder driven by high pressure from a compressor that opened and closed the lid, with a small fan used to mix the air when the lid was closed (Liang, Inoue & Fujinuma, 2003; Han et al., 2016). The air samples were pumped from one closed chamber into an infrared gas analyzer (IRGA, Li-820, Li-Cor, Lincoln, USA) to measure their CO₂ concentration through a multi-channel valve connected with a desiccant tube, a filter, and a flow controller. A programmable logical controller (PLC, Master-K120S, LG, Korea) was used to control the solenoid valves, to open and close the target chambers, and to circulate the gas samples (Zhang et al., 2011; Han et al., 2016). Each chamber was closed for three minutes for measurements, and the flow rate was controlled at 1 L min⁻¹ (Drewitt et al., 2002). After nine chambers were closed in sequence, the next measurement cycle was performed. The CO₂concentration was measured continuously using an infrared gas analyzer (IRGA) and recorded using a data logger at ten second intervals (CR1000, Campbell Scientific, Logan, USA).

The automated chamber system has been tested repeatedly and has been proven to work well under field conditions with high precision and stability (Steduto et al., 2002; Liang, Inoue & Fujinuma, 2003; Zhang et al., 2011; Zhou et al., 2012). First, the chambers were made of transparent plexiglass with approximately 95% transmission, which allows plants in the chamber to perform photosynthesis normally, thus ensuring the accuracy of the NEE measurements. Second, the chambers were operated automatically. This provides the ability to collect averaged data of continuous observations during both the daytime and nighttime, as well as measurements during extreme weather, such as rainfall. Third, the collected gas was directly transferred from the infrared gas analyzer without the need for subsampling, which is more ideal than gas chromatography methods. Moreover, the small fan in the chamber was able to adjust the temperature and mix the gas inside the chamber during measurements, thus reducing the effect of temperature on CO₂ flux.

Soil temperatures (Ts) at depths of 10- and 20-cm were measured using a thermocouple thermometer. The soil water content (SWC) at a depth of 10 cm was measured using a Hydra Probe (SDI-12, Stevens, USA). Other variables, such as air temperature (Ta), photosynthetically active radiation (PAR), and precipitation (P), were simultaneously monitored and recorded at the Changwu Agro-Ecological Experiment Station located approximately ten meters from the field site.

Data processing

Statistical analysis

Multiple comparisons and a one-way analysis of variance (ANOVA) was performed to evaluate the significance level of the differences in the CO₂ fluxes among the treatments and different growth stages. The Pearson correlation coefficients were calculated to determine the significance of the correlations between CO₂ fluxes and the influencing variables (Ta, Ts, PAR, P, and SWC). These statistical analyses were conducted using SPSS19.0 software (IBM SPSS Statistics, Chicago, USA). A principal component analysis (PCA) was conducted using CANOCO 5.0 to quantify the relative contribution of the influencing factors to the changes in the soil CO₂ fluxes. All figures were drawn using SigmaPlot 12.5 software (Systat Software, San Jose, USA).

Variation in microclimatic variables

The environmental conditions varied widely during the measurement period (Fig. 2). The mean Ta was 20.3 ± 2.9 °C, while the mean Ts at 10 cm were 22.3 ± 2.3 and 21.1 ± 2.1 °C in the bare land and the maize field, respectively (p >0.05). The corresponding mean soil moisture levels in the bare land and the maize field at the 10 cm depth were 14.0 ±1.0% and 15.4 ± 1.5%, respectively (p > 0.05). The total precipitation during the measurement period was 388.4 mm.

Diurnal variation characteristics of the CO₂ fluxes

There were noticeable diurnal variations in the NEE in the spring maize ecosystem (NEE-maize) during each growth period, with positive values in the nighttime (19:00–6:00) and negative values during the day (7:00–18:00) (Fig. 3A). This was consistent with the trend in the PAR, and a short “noon break” occurred when the PAR intensity was at its maximum. The minimum daily NEE-maize values in the seedling, jointing, heading, grain filling, and ripening stages were –0.37, –4.74, –5.92, –4.07, and –1.36 μmol CO₂ m⁻² s⁻¹, respectively. The diurnal variation in soil respiration (SR-maize) rates were best described with peak values in the heading, grain filling, and ripening stages of 3.44, 4.20, and 2.75 μmol CO₂ m⁻² s⁻¹, respectively. The peak value of soil respiration rates in the bare field (SR-bare field) was 1.66 μmol CO₂ m⁻² s⁻¹, which was lower than that for the SR-maize (Fig. 3B).

Seasonal trend in CO₂ fluxes during the maize growth period

Overall trend

Throughout the observation period, the daily NEE-maize ranged from –6.04 to 3.33 g C m⁻² d ⁻¹, while the daily SR-maize and SR-bare field had values of 0.77–4.86 g C m⁻² d⁻¹ and 0.41–1.63 g C m^{− 2} d⁻¹, respectively. The cumulative NEE-maize (i.e., carbon uptake) and SR-maize (i.e., carbon emission) were –68.61 and 245.69 g C m^{− 2}, respectively, with corresponding soil carbon emissions in the bare field of 114.08 g C m⁻². These results indicated a larger amplitude in the flux of the SR-maize than the SR-bare field, and the spring maize ecosystem functioned as a weak carbon “sink” throughout the growing season (Fig. 4). The carbon in the grain (C_gr) was calculated as 234.60 g C m ⁻². This figure considered the carbon released by the grains, and hence the maize field turned into a slight carbon source.

Carbon flux characteristics during different growth stages

The cumulative NEE-maize values in the seedling, jointing, heading, grain filling, and ripening stages were 2.63, –19.85, –32, –16.76, and 65.26 g C m⁻², respectively. To enable a more intuitive comparison, the averaged carbon fluxes during each growth stage were also calculated. The NEE-maize and SR-maize showed significant variations among stages. The NEE-maize during the heading stage was −2.13 ±  0.25 g C m⁻² d⁻¹ (Fig. 5A), and the SR-maize during the grain filling stage was 3.12 ± 0.70 g C m⁻² d⁻¹ (Fig. 5B). Therefore, they were significantly smaller and larger than those during other stages (p < 0.05). However, the SR-bare field showed no significant variations according to the above stages (Fig. 5C).

The temperature sensitivity of NEE and soil respiration

As Fig. 6 shows, the SR-maize values had significant positive correlations with soil temperature, for which the Q₁₀ values were 2.10 and 2.27 at the 10 cm and 20 cm depths, respectively (Fig. 6A). The corresponding Q₁₀ values were 1.36 and 1.37, respectively, for the SR-bare field (Fig. 6B). This suggested that the SR-maize was more sensitive to temperature than the SR-bare field, and also more sensitive to changes in soil temperature at the 20 cm depth.

The relationship between soil respiration and soil temperature varied with the measurement depth, which was caused by an attenuation and phase shift in the soil temperatures with depth (Fig. 7). As the air temperature reached a daily maximum before the SR-maize and SR-bare field, the hysteresis loops rotated clockwise (Figs. 7A, 7B). However, when the soil temperatures peaked after the SR-maize and SR-bare field, the loops rotated counterclockwise (Figs. 7C–7F). This indicated that although the soil respiration was affected by temperature, the trend was not consistent with the single-peak trend of temperature, thereby resulting in the hysteresis.