Comparison of Carbon Dioxide (CO2) Fluxes between Conventional and Conserved Irrigated Rice Paddy Fields in Myanmar

Rice (Oryza sativa. L.), a major food crop widely grown in Myanmar, is the most prominent cause of greenhouse gas (GHG) emissions in agriculture. Moreover, as a result of modification in agricultural management practices (such as soil tillage), the soil organic matter is exposed to more oxidizing conditions, releasing CO2 into the environment, contributing to global warming. Therefore, we studied the effects of both conventional and conservation soil tillage management practices on CO2 fluxes on an experimental rice paddy field in Myanmar. Total CO2 emissions during the night from paddies farmed under conventional practices were significantly higher than those from paddies farmed under conservation practices; however, no net CO2 flux differences were found between practices. Total net CO2 fluxes ranged from −59 to 1614 mg CO2 m−2 h−1 in conventional practices and from −282 to 1082 mg CO2 m−2 h−1 in conservation practices, respectively. Significantly higher rice biomass and grain yields were observed in conventional practices when compared to those in conservation practices, causing a significant rise in both CO2 uptake and emissions during the day and night, respectively. In addition, the results of this study revealed that CO2 emissions in rice fields could be much higher than expected, requiring further study to elucidate key factors driving the dynamics of CO2 in rice paddy systems.


Introduction
Global warming and increased emissions of anthropogenic greenhouse gases (GHGs) has become an international issue of great concern. Comprehending the dynamics of global climate change requires an understanding of the exchange of greenhouse gases between terrestrial ecosystems and the atmosphere [1]. Carbon dioxide (CO 2 ) is considered to be the major contributor to anthropogenic GHGs, accounting for 76% of total emissions in 2010 [2]. Based on various population growth and energy use scenarios, the current CO 2 concentration of 379 ppm is expected to rise to a concentration between 485 and 1000 ppm by 2100 [3]. The carbon cycle in cropland ecosystems is strongly affected by human activities. Emissions of CO 2 from agricultural systems can occur via plant respiration, the oxidation of organic carbon in soil (soil respiration) and crop residues, the use of fossil fuels in agricultural machinery, and the use of fossil fuels in production of agricultural production inputs [4,5]. Organic carbon in soil is the largest of the terrestrial carbon pools [6]; therefore, despite relatively small changes in soil CO 2 , it can significantly affect both the atmospheric CO 2 concentration and soil carbon sequestration processes [7]. Despite significant changes of CO 2 between the atmosphere and agricultural land, net flux is considered to be approximately balanced, although there is limited evidence for this presumption [8]. Many studies investigated CO 2 emissions from various terrestrial

Material and Methods
Trials were conducted from mid-January through June 2018 at the experimental paddy fields of the Department of Agronomy, Yezin Agricultural University, Myanmar. These fields are situated in the Yezin area, Pyinmana Township, Naypyitaw district (19 • 50 03" N, 96 • 16 03" E), and have been used for rice production for several years. The area is characterized by annual average temperature and rainfall of 21-33 • C and 1000 mm, respectively. The fields in our study were located in an area vulnerable to drought and, therefore, the vast majority of rice paddy fields in Myanmar are comparable to current experimental field conditions. In this region, summer rice cultivation uses irrigation water, which is normally initiated from January to May, depending on water availability and type of cultivars. Rice seeds of Oryza sativa. subsp. indica (Manawthukha, Masuri-M) (135 days), a commonly grown cultivar in Myanmar, were sown in the nursery during the second week of January 2018 and transplanted manually in the first week of February. Rice seedlings were prepared in the nursery before land preparation and transplanted as 25-day-old seedlings.
Six experimental plots, three under conventional and three under conservation practices, were constructed adjacent to one another. Plot size covered an area of 6 × 3 m, separated by bunds covered with plastic film serving as a barrier to prevent contamination of applied inorganic fertilizer from conventional plots to non-treated conservation plots. In a corner of every plot, a small area was left without rice plants for measuring bare soil respiration ( Figure 1). The entire duration of the experiment lasted 155 days, including the fallow period. Carbon dioxide (CO 2 ) gas sampling started one week after transplanting the rice plants and lasted throughout the fallow period.
Manual tillage was conducted using a spade, up to a maximum of 20 cm depth. The conventional practice (Conv) treatment was tilled three times and prior to transplanting, compound fertilizer (NPK 15:15:15) was added as a basal application at the rate of 61.75 kg ha −1 . Urea fertilizer (46% N) was applied two times (Tillering and Panicle Initiation/Heading-see Table 1) as a split application at the rate of 50 kg N ha −1 [44]. Other standard practices such as application of pesticides and weed control were conducted using conventional concentrations and rates. By contrast, in conservation practice (Cons) treatment, tillage was carried out only once and without application of chemical fertilizer, pesticides or herbicides. Water was drained two weeks before harvesting, and the fallow paddy field was flooded for four weeks before the next planting season started.
Two airtight gas chambers with frosted acrylic sheets were constructed, one chamber for each treatment (Conv vs Cons) for the sampling of CO 2 gas emitted from the paddy soil and rice plants. Acrylic sheet was used because it is non-reactive with the CO 2 trapped inside the chambers. Each chamber (1 × 0.4 × 0.6 m) was equipped with a fan for mixing air inside the chamber, and the CO 2 sensor was installed at the top of the chamber [29]. The fan was operated by a power bank device (2600 mAh), while carbon dioxide gas was measured immediately, using a SenseAir ® CO 2 sensor module K33 ELG, designed to measure and record data for environmental parameters such as temperature (T/ • C), relative humidity (%) and carbon dioxide (CO 2 ) concentration (up to 5000 ppm) [45]. CO 2 flux was measured once a week during the entire summer paddy growing season (from transplanting to throughout the fallow period). Due to the high variability of CO 2 fluxes by photosynthetic activities during the day [46], CO 2 fluxes were consistently measured at a fixed time for all treated plots: between 9:00 and 12:00 for the day, and 21:00 and 24:00 for the night. CO 2 values were recorded simultaneously for each replication under Conv and Cons management practices, with 2 min logging time for 30 min. To distinguish between respiration from the rice plants and paddy soils alone for the calculation of the CO 2 gas balance, CO 2 flux was measured separately with and without rice plants for each treatment. There were five rice growth periods (T-Tillering,    Daily mean ambient air temperature and relative humidity (%) were recorded by an automated weather station nearby the experimental fields. The meteorological data for this study year (2018) is presented in Figure 2 and daily data were downloaded from the server of the Agro-Meteorological Department of Yezin Agriculture University [47]. Soil temperature ( • C) was recorded at three different depths of 0-5 cm, 0-10 cm, and 0-20 cm using a T&D TR-7wf/nw series soil temperature data logger. Soil samples for soil organic carbon (SOC%) analysis were taken before soil preparation and at the time of harvest. The sub-samples were analyzed at Palacký University Olomouc, Czech Republic. Redox potential (Eh) and soil water pH were measured using a Hanna Instruments HI83141 portable meter before and after rice harvest on the same date as CO 2 flux measurements. For vegetation analysis, plant characteristics such as plant height (cm) and leaf area (cm 2 ) were recorded throughout the experiment, whereas characteristics such as fresh and dry biomass weight, yield and yield component characteristics were measured at harvest. Three randomly selected hills (plants) were used for plant height and five hills for leaf area measurements for each treatment. For leaf area measurements, a CI-203 handheld portable Laser Leaf Area Meter (CID Bioscience, Inc., USA) was used. Rice quality was evaluated based on measurements of the head rice, chalky rice, amylase content, gel consistency and protein content. After the rice harvest, 1000 grains were evaluated for each treatment. Grain quality analysis was conducted at the Department of Agricultural Research (DAR, Yezin) by using UV-Vis Spectrophotometer, Jenway-6305 with the KI solution method for Amylose (%), by the Kjedahl digestion and distillation method for protein %, and by the Gel Flow rate Method for Gel Consistency (mm) [48].

Flux Calculation
CO2 emission flux, namely the change of the gas quantity over the soil covered by a chamber per hour per unit area, was calculated using the following equation [49]: where F is the total flux density of gas (mg CO2 m −2 h −1 ); ∆ ∆ is the slope of regression obtained by plotting concentration of CO2 vs time recorded during sampling; V is the total volume of the chamber (m 3 );  For vegetation analysis, plant characteristics such as plant height (cm) and leaf area (cm 2 ) were recorded throughout the experiment, whereas characteristics such as fresh and dry biomass weight, yield and yield component characteristics were measured at harvest. Three randomly selected hills (plants) were used for plant height and five hills for leaf area measurements for each treatment. For leaf area measurements, a CI-203 handheld portable Laser Leaf Area Meter (CID Bioscience, Inc., USA) was used. Rice quality was evaluated based on measurements of the head rice, chalky rice, amylase content, gel consistency and protein content. After the rice harvest, 1000 grains were evaluated for each treatment. Grain quality analysis was conducted at the Department of Agricultural Research (DAR, Yezin) by using UV-Vis Spectrophotometer, Jenway-6305 with the KI solution method for Amylose (%), by the Kjedahl digestion and distillation method for protein %, and by the Gel Flow rate Method for Gel Consistency (mm) [48].

Flux Calculation
CO 2 emission flux, namely the change of the gas quantity over the soil covered by a chamber per hour per unit area, was calculated using the following equation [49]: where F is the total flux density of gas (mg CO 2 m −2 h −1 ); ∆C ∆t is the slope of regression obtained by plotting concentration of CO 2 vs time recorded during sampling; V is the total volume of the chamber (m 3 ); A is the area of the chamber's base (m 2 ). The CO 2 flux produced by the rice plants only was calculated by using the equation: where F Total is total CO 2 flux (which is the sum of CO 2 respired by the rice plants and CO 2 respired by the soil), F soil is CO 2 respired by the bare soil only, and F Rice is the CO 2 emission flux from the rice plants only. The net CO 2 flux refers to a difference in total CO 2 fluxes measured during the night and day, and annual flux was estimated by extrapolating each measurement based on this flux for rice paddy fields in Myanmar.

Statistical Analysis
Analysis of variance (ANOVA) was performed for total CO 2 fluxes and soil respiration from bare paddy soil in each of the sampling dates separately, to evaluate the differences between land management practices and time on CO 2 fluxes during day and night. Significance was tested at the p < 0.05 level. In addition, differences in mean values were calculated by using Tukey's Honest Significance Test pair-wise comparisons at a significance level of p ≤ 0.05. Statistical software (R version 3.6.3) was used to perform ANOVA and plot graphs. Linear regression analysis was performed in Microsoft Excel to determine the relationship between plant height and CO 2 fluxes influenced by the rice plants and paddy soil.

Ambient Air Temperature, Relative Humidity and Soil Temperature
Mean ambient air temperature ( • C) and relative humidity (%) showed only slight fluctuations during the day and night. The highest air temperature during the day of 33.13 • C was recorded on May 10, 2018 and the lowest temperature at night of 22.75 • C on February 8, 2018 ( Figure 3). Air temperature ( • C) and relative humidity (%) recorded inside the gas chambers containing rice plants and bare paddy soil closely matched the same patterns during day and night-time periods Air temperature ranged between 37.26 and 65 • C during the day in the chambers with rice plants, the maximal and minimal temperatures at night measurement showed 26.83 • C and 21.53 • C, respectively. Mean air temperature from bare paddy soil during the day ranged between 35.95 and 63.97 • C and 17.46 and 30.48 • C during the night. The relative humidity (%) with rice plants ranged between 22.14% and 47% during the day and 76.08% and 96.06% during the night. By contrast, relative humidity in the bare soil treatment ranged between 20.4% and 44.2% during the day and 79.3% and 97.6% during the night (Figure S1a-d; where "S1" denotes supplementary material). maximal and minimal temperatures at night measurement showed 26.83 °C and 21.53 °C, respectively. Mean air temperature from bare paddy soil during the day ranged between 35.95 and 63.97 °C and 17.46 and 30.48 °C during the night. The relative humidity (%) with rice plants ranged between 22.14% and 47% during the day and 76.08% and 96.06% during the night. By contrast, relative humidity in the bare soil treatment ranged between 20.4% and 44.2% during the day and 79.3% and 97.6% during the night ( Figure S1. a,b,c,d; where "S1" denotes supplementary material). There were no significant differences among mean soil temperatures measured at three different depths (5, 10, and 20 cm) during the day and night. As expected, the highest mean soil temperature There were no significant differences among mean soil temperatures measured at three different depths (5, 10, and 20 cm) during the day and night. As expected, the highest mean soil temperature during the day and night was observed at the 5 cm depth (30.97 • C and 31.17 • C, respectively), whereas the lowest temperature during the day and night was recorded at a depth of 20 cm (13.9 • C and 21.53 • C, respectively) (Figure 4a,b).

Patterns of CO2 Fluxes under Different Soil Management Practices
CO2 emission fluxes measured during the day were always negative for both soil management practices, except during the first 2 weeks after transplanting the rice seedlings and the period after harvesting ( Figure 5a). As expected, CO2 emission flux at night was highly positive for both soil management practices during the entire rice growing season (Figure 5b

Patterns of CO 2 Fluxes under Different Soil Management Practices
CO 2 emission fluxes measured during the day were always negative for both soil management practices, except during the first 2 weeks after transplanting the rice seedlings and the period after harvesting ( Figure 5a). As expected, CO 2 emission flux at night was highly positive for both soil management practices during the entire rice growing season (Figure 5b). The highest fluxes were observed in Conv practice on April 26 and May 10, peaking at 1254 mg CO 2 m −2 h −1 .

Patterns of CO2 Fluxes under Different Soil Management Practices
CO2 emission fluxes measured during the day were always negative for both soil management practices, except during the first 2 weeks after transplanting the rice seedlings and the period after harvesting ( Figure 5a). As expected, CO2 emission flux at night was highly positive for both soil management practices during the entire rice growing season (Figure 5b). The highest fluxes were observed in Conv practice on April 26 and May 10, peaking at 1254 mg CO2 m −2 h −1 .   Table 1 for abbreviations of different growth stages).
Due to the absence of photosynthesis, CO2 fluxes from bare soil respiration under both Conv and Cons practices were always positive during the day and night (Figure 7a,b). Soil CO2 emission fluxes showed no significant differences between management practices except for the night measurements on February 8, April 19, May 3, and May 17 (Figure 7b). a b Figure 6. Effects of different soil management practices on net CO 2 fluxes (±SE) from various rice growth stages (see Table 1 for abbreviations of different growth stages).
Due to the absence of photosynthesis, CO 2 fluxes from bare soil respiration under both Conv and Cons practices were always positive during the day and night (Figure 7a,b). Soil CO 2 emission fluxes showed no significant differences between management practices except for the night measurements on February 8, April 19, May 3, and May 17 (Figure 7b). growth stages (see Table 1 for abbreviations of different growth stages).
Due to the absence of photosynthesis, CO2 fluxes from bare soil respiration under both Conv and Cons practices were always positive during the day and night (Figure 7a,b). Soil CO2 emission fluxes showed no significant differences between management practices except for the night measurements on February 8, April 19, May 3, and May 17 (Figure 7b). Total CO2 emission fluxes from Conv practices were significantly higher than for Cons during night (Table 2). However, a non-significant difference between practices was observed during the day. In contrast, total CO2 flux from bare soil respiration showed significant differences between the two management practices for both the day and night measurements. The average contribution of a b Total CO 2 emission fluxes from Conv practices were significantly higher than for Cons during night (Table 2). However, a non-significant difference between practices was observed during the day. In contrast, total CO 2 flux from bare soil respiration showed significant differences between the two management practices for both the day and night measurements. The average contribution of soil respiration to the total CO 2 emission fluxes during the night was 31.4% (ranging from 27% to 36%) in Conv, and 29.3% (range 28% to 33%) in Cons practice. Higher uptake of CO 2 by rice plants during the day was observed in Conv practices compared to Cons practices, as well as higher net emissions of CO 2 observed in Conv practices. During the fallow period, significantly higher CO 2 flux was observed in the Conv versus Cons practices during the day, but non-significant differences of the CO 2 fluxes were observed during the night between the practices ( Table 2).

Effects of Different Agricultural Tillage Practices on Plant and Soil Characteristics
Plant height increased over time as the rice growing stage extended and peaked at maturity. A statistically significant difference was observed in plant height between Conv (range 31.2-97.5 cm) and Cons practices (range 26.2-82.9 cm at the early growth stages, Feb 22 and March 1) ( Figure 8); however, no apparent difference was found during late growth stages. Maximum plant heights of 97.5 and 82.9 cm were observed in Conv and Cons practices, respectively.
Despite different management practices, we found no significant differences between soil organic carbon (SOC%), phosphorus, and nitrogen contents in the soil. Nevertheless, a slight decrease in SOC was observed during the harvest, as compared to before planting. Soil water pH and redox potential (mV) before and after harvesting of the paddy field showed no significant differences among Conv and Cons practices. However, soil water pH showed a slight increase after harvest (Table S1; S1 denotes supplementary appendix). Total CO 2 fluxes from the Conv and Cons practices were positively correlated with plant height, while the coefficient of determination (R 2 ) in Conv practices accounts for 69% which had more fitted values than Cons practices (43%) (Figure 9a,b). However, no significant difference was found when comparing the two different slopes. ** * * *, significant at the 0.05 probability level; **, significant at the 0.01 probability level; ns, not significant.

Effects of Different Agricultural Tillage Practices on Plant and Soil Characteristics
Plant height increased over time as the rice growing stage extended and peaked at maturity. A statistically significant difference was observed in plant height between Conv (range 31.2-97.5 cm) and Cons practices (range 26.2-82.9 cm at the early growth stages, Feb 22 and March 1) ( Figure 8); however, no apparent difference was found during late growth stages. Maximum plant heights of 97.5 and 82.9 cm were observed in Conv and Cons practices, respectively. Despite different management practices, we found no significant differences between soil organic carbon (SOC%), phosphorus, and nitrogen contents in the soil. Nevertheless, a slight decrease in SOC was observed during the harvest, as compared to before planting. Soil water pH and redox potential (mV) before and after harvesting of the paddy field showed no significant differences among Conv and Cons practices. However, soil water pH showed a slight increase after harvest (Table S1; S1 denotes supplementary appendix). Total CO2 fluxes from the Conv and Cons practices were positively correlated with plant height, while the coefficient of determination (R 2 ) in Conv practices accounts for 69% which had more fitted values than Cons practices (43%) (Figure 9 a,b). However, no significant difference was found when comparing the two different slopes. Significant differences for the harvestable yield were observed between Conv and Cons practices, with 5.52 metric tons (Mt) ha −1 and 2.63 Mt ha −1 , respectively (Table 3). However, the nutritional, edible and visual quality of rice such as amylose (%), protein (%), gel-consistency (mm), head rice rate (%), and chalky rice rate (%) was not affected by the different soil management practices as presented in Table 4. Plant height (cm) and other yield component characteristics of rice such as number of spikelets per panicle, filled grain (%), unfilled grain (%), and 1000 grain weight also showed no significant differences between the practices. However, we found the number of effective tillers per hill to be significantly higher than in Cons practices (Table 3). At harvest time, fresh and dry biomass weight (g hill −1 ) was significantly different between the different practices. According to the crop growth stages, leaf area during the tillering stage (43 DAT) showed significant differences between Conv (36.22 cm 2 per hill) and Cons practices (28.82 cm 2 per hill). Leaf area was significantly different between different practices during the flowering stage (88 DAT) ( Table 3). a b Significant differences for the harvestable yield were observed between Conv and Cons practices, with 5.52 metric tons (Mt) ha −1 and 2.63 Mt ha −1 , respectively (Table 3). However, the nutritional, edible and visual quality of rice such as amylose (%), protein (%), gel-consistency (mm), head rice rate (%), and chalky rice rate (%) was not affected by the different soil management practices as presented in Table 4. Plant height (cm) and other yield component characteristics of rice such as number of spikelets per panicle, filled grain (%), unfilled grain (%), and 1000 grain weight also showed no significant differences between the practices. However, we found the number of effective tillers per hill to be significantly higher than in Cons practices (Table 3). At harvest time, fresh and dry biomass weight (g hill −1 ) was significantly different between the different practices. According to the crop growth stages, leaf area during the tillering stage (43 DAT) showed significant differences between Conv (36.22 cm 2 per hill) and Cons practices (28.82 cm 2 per hill). Leaf area was significantly different between different practices during the flowering stage (88 DAT) ( Table 3).

Discussion
Previous measurements of CO 2 emission fluxes from paddy fields were reported in other studies in various Asian countries such as China, Japan, and Thailand. Rice fields in Myanmar occupied 34% of the total sown area [21]; however, relatively few studies have investigated the effects of land-use practices in rice paddy cultivation on CO 2 fluxes. In the present study, we focused mainly on the effects of Conv and Cons agricultural tillage practices on CO 2 emission fluxes from the paddy fields during the summer season in Myanmar. Whereas a broad range of paddy fields in Myanmar are farmed under similar conditions as our experimental field, the current CO 2 emissions from Myanmar may be highly underestimated.
The CO 2 concentration inside of the acrylic chamber was negative during the day, indicating prevalent uptake of CO 2 via plant photosynthesis [5,[50][51][52][53]. Variation of the CO 2 gas exchange pattern can be influenced by leaf photosynthesis, which is strongly affected by high temperatures or light conditions [54]. Nevertheless, there is little temperature effect on leaf photosynthesis in rice from 20 to 40 • C [55]. High temperatures can reduce photosynthetic rate by 40%-60% at different growth stages. The photosynthetic rate of leaves under light dependence conditions is highly correlated with atmospheric CO 2 , and also varies with the growing temperature [56].
During the day, CO 2 is consumed from the ambient atmosphere and CO 2 is emitted by flooded soil. However, CO 2 fluxes from respiration in bare paddy soil were positive during both the day and night, and lower during the day. This was likely due to CO 2 uptake/release by aquatic weeds and algae present in the overlying paddy water [6]. Net soil CO 2 flux throughout the growing season was generally positive, indicating the dominance of respiratory CO 2 release by the soil microorganisms as well as by aquatic weeds and algae in paddy water. Generally, flooded bare paddy soil acted as a CO 2 source throughout the day (Table 2). Similar results were reported by Nishimura et al., (2015) [5], who found the net soil CO 2 flux was generally near zero during the submerged period, with paddy rice cultivation having a slight CO 2 influx in the daytime and efflux at night-time.
Plant respiration in the absence of photosynthesis at night always resulted in a positive flux (efflux), suggesting that the field overgrown with rice plants was acting as a CO 2 source during the night (Figure 5b). In this study, a peak of net CO 2 emission fluxes from rice plants was observed in both the Conv and Cons practices during the grain filling period ( Figure 6). On the contrary, Dutta and Gokhale (2017) [29] found the peak of net CO 2 emission fluxes earlier, during the flowering period, probably due to higher ambient air temperature and development of root growth. On the other hand, a decrease in net CO 2 emissions occurred during the maturity or ripening period. Due to the maturation of leaves such as leaf rolling, senescence, and yellowing, CO 2 uptake rate gradually declined during the late growth stage or ripening/mature stage [57]. In addition, respiration of plants also decreased at night-time during this period (Figure 5b).
In the fallow period after removal of all aboveground biomass, CO 2 flux was mediated only by the soil itself. Nevertheless, average values of CO 2 emissions were higher compared to those from bare soil respiration (see Table 2). This could be associated with higher root residues and the decomposition of organic litter after removal of the aboveground biomass [49] and to intensive aerobic respiration initiated just after the drainage of water. Typically, higher CO 2 emissions from Conv practices during  [58].
Greenhouse gas emissions, including CO 2 from Conv tillage are usually reported as higher than emissions from Cons agricultural practices with minimal tillage [59][60][61][62][63][64][65]. Data from the present study were consistent with the observations from the previous findings mentioned above. Total CO 2 emission fluxes from the Cons practice were significantly lower than those from the Conv practice at night ( Table 2), but there was no significant difference during the day. Practitioners of Conv practices often apply nitrogen fertilizer (urea), resulting in increased plant biomass and subsequent stimulation of biological activity and increased CO 2 emissions [66].
In the present study, higher biomass yields (Table 3) and net CO 2 emissions ( Table 2) were observed in Conv versus Cons practices, indicating that aboveground biomass was an important factor influencing CO 2 fluxes in this field experiment. Similar findings were noted by Maraseni et al., (2009) [64] who found that higher GHG emissions were directly linked to increasing rice productivity by using higher farm inputs. The aboveground biomass also contributes organic matter to the soil [67,68]. A quantity of soil CO 2 emission is often linked to the amount of aboveground biomass produced [69] because plant biomass is a primary source of the soil C pool. Moreover, Conv tillage practices also increase CO 2 emissions by exposing organic matter to increased aerobic conditions, thus enhancing the soil organic matter decomposition process [70,71]. The results of our experiment revealed that fresh biomass from the Conv practice was higher than that from Cons practice, and the soil CO 2 fluxes released by Conv practice were also always higher than those produced by Cons practice for both day and night measurements. The peak CO 2 fluxes in soil from the Conv practice observed during flowering (FL) and grain filling (GF) stages ( Figure 6) were likely due to increased substrates derived from root exudation and microbial decomposition of the remaining residues from previous crops [72] and photosynthates translocated from the aboveground biomass [73].
Plant height usually exhibits a highly significant relationship with leaf area, aboveground biomass, and yield [74]. In our study, the mean plant height from the Conv practice was greater in the Conv versus Cons practice. Consequently, higher biomass weight (g hill −1 ) and leaf area (cm 2 ) was observed in the Conv versus Cons practice. These findings are similar to the results of Tilly et al., (2013) [75], indicating that increased plant height is followed by higher plant biomass. As expected, rice grain yield was also affected by the different soil management practices, with the Conv practice yielding twice as much as the Cons practice. Additionally, a significantly higher number of effective tillers per hill was observed in the Conv practice compared to the Cons practice. Wu et al., (2013) [76] pointed out that the Cons practice using minimum tillage significantly reduces the ratio of effective tillers to the total number of tillers. Lower grain yield might be also be affected by the decreased uptake of nitrogen by the rice plants due to weed infestation and high loss of nitrogen fertilizer [77,78]. On the other hand, some researchers suggested that no tillage practice improves both physical and chemical soil properties [79]; hence, the soil condition would favor germination and plant growth, consequently increasing rice grain yields [80].

Annual CO 2 Flux from Various Rice Paddy Fields in Asian Countries
We estimated average emissions of CO 2 from summer rice fields at 2347 g CO 2 m −2 yr −1 . Compared to CO 2 fluxes from other rice fields under different water management, our findings are within the range of the previously found values presented in Table 5, despite the fact that most of the studies were focused mostly on CO 2 emission from the soil only. It is also important to note that our findings are consistent with these measurements although they were carried out using different methodologies under flooded/irrigated conditions. Annual CO 2 flux was extrapolated from the summer rice area based on Department of Agriculture (MoALI, 2016) [21] data, and resulted in 28.2 ± 3.97 Tg CO 2 yr −1 (7.6 Tg C yr −1 ). This value is 21.3% higher than estimated CO 2 emissions from Myanmar rice cultivation from both dry and wet seasons, as assessed from global emissions in the FAO Statistical Database [81].

Conclusions
Our field experiment demonstrated that the Conv practice of rice cultivation produced higher aboveground biomass and, also, grain yield, compared to the Cons practice (Table 3). However, this Conv management also emitted significantly higher CO 2 fluxes than Cons management using minimum tillage and inputs of agrochemicals. Although a positive relationship between plant height and total CO 2 emission was observed (Figure 9), fluxes indicated that the rice plant biomass is associated with CO 2 production; we also found significantly higher production of CO 2 from bare paddy soils managed under the Conv practice.
These findings suggest that management of soil is a primary factor with influence on resulting rice biomass as well as final CO 2 flux. In comparison with methane, another greenhouse gas emission very often studied from rice paddies, the atmospheric CO 2 concentration above a rice field shows a conspicuous diurnal pattern, with the lowest values during the day and highest during the night. Unsurprisingly, both plant photosynthesis and respiration are responsible for these diurnal changes in CO 2 concentrations as contributions from soil to total CO 2 emissions were generally less than 40%. Hence, CO 2 flux was affected by the metabolic activity of rice plants rather than aerobic/anaerobic conditions of the soils, as is typically the case for methane.
In the context of global climate change and ongoing mitigating approaches aiming to reduce emissions of GHGs from rice fields focusing mostly on methane [86][87][88], it is worth noting that modification of current cultivation systems toward Cons practices that emit less CO 2 requires farmers to be motivated, as this practice results in lower plant biomass as well as lower grain yields. Another noteworthy finding from our study was that the emissions of CO 2 by rice fields may be much higher than previously expected [87], requiring verification from further studies. Therefore, additional studies are also needed to incorporate a range of multi-year/season assessments to determine seasonal variation of CO 2 fluxes exchange from rice production in Myanmar.

Supplementary Materials:
The following are available online at http://www.mdpi.com/2071-1050/12/14/5798/s1, Table S1. Effects of different agricultural tillage practices on soil characteristics of the experimental field, Figure S1. Relative humidity (%) and mean temperature ( • C) inside the chamber with rice plants (a,b) and without rice plants (c,d) recorded day and night during the experimental period (February-May 2018).
Author Contributions: S.M. and M.R. conceived the study design, S.M. implemented the field research collected and analyzed the field data, S.M. wrote the paper with the help of M.R., M.R. commented on and edited the manuscript. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by Palacký University Olomouc, Czech Republic, grant number IGA_PrF_2018_020. And the APC was funded by the Department of Ecological and Environmental Sciences, Palacký University Olomouc, grant number IGA_PrF_2020_020.