Site description and experimental design

The study was conducted at the experimental station of the Rio Grandense Rice Institute (IRGA) in Cachoeirinha, RS, Brazil (29° 57’ 02” S and 51° 06’ 02” W), during the rice growing seasons of 2009/2010, 2010/2011, and 2011/2012, and the non-rice seasons of 2010 and 2011. The region has a humid subtropical climate (Cfa), a mean annual temperature of 20 °C, and mean annual rainfall of 1,394 mm. The field experiment was performed on a Gleysol (Gleissolo Háplico) with pH(H₂O) of 5.3, 170 g kg⁻¹ clay, 13 g kg⁻¹ OM, and 6.7 mg dm⁻³ P, and 29 mg dm⁻³ K (Mehlich-1) in the 0.00-0.20 m layer.

The experiment followed a randomized complete block design with three replicates. Treatments involved three management levels for the rice crop (medium, high, and very high) differing in seeding and fertilizer rates, the beginning of flooding, and pesticide application (Table 1). The medium level was closest to “business as usual” in most rice farms in southern Brazil (^{IRGA, 2006}). For all management levels, no-till rice seeding was performed by using ryegrass (Lolium multiflorum Lam.) as a winter cover crop (Table 1), which was desiccated with glyphosate in early spring.

Air sampling and gas analysis

Air was sampled on a weekly basis during the flooded rice season (spring-summer), biweekly during the drained non-rice season (fall-winter), and daily after the N application events, using the static closed chamber method (^{Bayer et al., 2014}). Each chamber consisted of an aluminum base (0.60 × 0.60 × 0.20 m) and an aluminum top of the same size. The bases were driven 0.05 m into the soil before permanent flooding in the rice season and after rice harvest in the non-rice season, and left in the soil throughout the seasons.

Each base had an open bottom and sealable channels on the sides to facilitate free flowing of irrigation water in the rice season. The channels on the sides were sealed during air sampling events. Each base covered three rows of rice plants. In the rice season, additional 0.20 or 0.30 m aluminum extensions were stacked on the bases as the plants grew taller. The chamber volume was considered in estimating all GHG emissions. Each chamber top had a rubber septum sampling port, a stainless steel thermometer, and a battery operated fan to circulate and homogenize air within the chamber (^{Bayer et al., 2014}). Chamber closing and initial air sampling began at 9:00 am, which was followed by five air samplings at intervals of 5 min (^{Bayer et al., 2014}). Air samples were withdrawn with polypropylene syringes, transferred to the Biogeochemical Laboratory at Ufrgs and analyzed for CH₄ and N₂O on the same day in a gas chromatograph (Shimadzu Corp. 2014) equipped with flame ionization (250 °C) and electron capture (325 °C) detectors.

Calculations

Methane and N₂O fluxes were calculated according to equation 1:

in which: f is the gas production rate (g m⁻² h⁻¹); ΔQ/Δt, the change in gas concentration (mol h⁻¹); P, the atmospheric pressure in the chamber (1 atm); V, the chamber volume (L); R, the ideal gas constant (0.0825 atm L mol⁻¹ K⁻¹); T, the chamber temperature (K); M, the gas molar mass (g mol⁻¹); and A, the chamber basal area (m²).

The flux rate of GHG as estimated from air samples collected from 9:00 to 11:00 a.m. was used as a measure of mean daily flux (^{Costa et al., 2008}). Seasonal emissions (rice and non-rice periods) were calculated by trapezoidal interpolation of the daily CH₄ and N₂O flux rates throughout each period (^{Bayer et al., 2014}). Annual GHG emissions were obtained by combining the emissions for the rice and non-rice seasons. A partial global warming potential was calculated according to the equation 2:

in which pGWP is the partial global warming potential (kg CO₂eq ha⁻¹), CH₄ and N₂O are the seasonal emissions of the gases (kg ha⁻¹), and 25 and 298 are the radiative forcing potential of CH₄ and N₂O, respectively.

Yield-scaled pGWP was calculated as the ratio of pGWP to rice grain yield, according to equation 3:

in which YpGWP denotes yield-scaled GHG emission (kg CO₂eq kg⁻¹ rice); pGWP, the partial global warming potential (kg CO₂eq ha⁻¹ season⁻¹); and Y, the rice grain yield (kg ha⁻¹).

Meteorological variables and supplementary determinations

Figure 1 shows daily solar radiation, average air temperature, and rainfall during the study period obtained from an automatic meteorological station installed at the experimental site. The temperature at a soil depth of 0.05 m was monitored at the site by using a digital thermometer in each air sampling event.

Figure 1 Rainfall, air temperature, and solar radiation during the period studied. 

Aboveground biomass of ryegrass was assessed at the flowering stage in the 2010 and 2011 non-rice seasons. The rice biomass was evaluated at the R₄ stage [anthesis, according to ^{Counce et al. (2000)}] in the 2009/10 and 2010/11 rice seasons. In both determinations, a 0.5 m² area was sampled and the biomass was oven dried at 60 °C to constant weight. Grain yield was measured in a 15 m² area and expressed at 13 % moisture.

Statistical analysis

Data were visually analyzed for normality and constant variance of errors. Visual analysis of normality consisted of constructing a stem and leaf plot and a histogram of the model predicted values by the residual values (observation minus predicted). The shape of the data points indicated whether the data set is normally distributed. Constant variance of errors was assessed by making a box plot of the residuals for each treatment. Again, the shape of the data points (heights of the box plot) indicated whether the errors are constant and homogenous among treatments. Appropriate transformations were applied if either assumption was violated. Analyses of variance for the dependent variables (seasonal N₂O and CH₄ emissions, and pGWP) were conducted separately for each period (rice and non-rice seasons, and annual results), whereas grain yield and YpGWP were only analyzed in the rice season. Treatment, year, and their mutual interaction were used as fixed effects, and block was the random effect. The general linear model of the GLM Procedure in the SAS software suite was used in all cases. Differences between treatment means were evaluated via Tukey's test at p<0.1.

Soil CH₄ and N₂O fluxes

Methane fluxes occurred mainly during the rice season and ranged from −0.80 to 855.50 g ha⁻¹ h⁻¹, and their behavior at that time was similar for all crop management levels (Figure 2). For the 2010/11 and 2011/12 rice seasons, CH₄ fluxes were low (<40 g ha⁻¹ h⁻¹) during the first six weeks after rice seeding, but gradually increased from the fourth week after flooding and remained high thereafter - especially during the second half of the rice cycle (reproductive stage). For the 2009/10 rice reason, no analysis of the initial emissions was possible, because flux measurements started only after flooding. The gradual increase in CH₄ emissions after flooding are consistent with anoxic conditions in flooded soil, once pH and E_h have leveled off and oxidized species such as NO₃^–, Mn⁴⁺, and Fe³⁺ have been microbially reduced (^{Silva et al., 2011}). In addition, the presence of rice plants influences CH₄ production by exudation of photoassimilated C from their roots for use by methanogens (^{Lu et al., 2000}; ^{Das and Baruah, 2008}) and by transfer of the CH₄ - primarily through the aerenchyma (^{Nouchi et al., 1990}).

Figure 2 Methane and N₂O fluxes from rice paddy soil over three cropping seasons (unshaded bands) and two non-rice seasons (shaded bands) under medium, high, and very high crop management levels in southern Brazil. Vertical bars represent standard errors. 

Our results also showed that soil drainage before harvest significantly increased CH₄ fluxes to levels above 500 g ha⁻¹ h⁻¹ for most treatments in all three rice seasons (Figure 2). As previously suggested (^{Yagi et al., 1996}; ^{Liang et al., 2013}), the increased emissions may have resulted from the release of CH₄ trapped in the soil during flooding. After peaking during soil drainage, CH₄ fluxes decreased to near-zero levels until harvest and remained low during the non-rice season. Soil CH₄ fluxes for the two non-rice seasons (2010 and 2011) ranged from −8.27 to 90.26 g ha⁻¹ h⁻¹. The lower CH₄ fluxes relative to the rice season were related to unfavorable conditions for CH₄ production and emission (e.g., the absence of an anaerobic environment) (^{Zhang et al., 2014}) and also to the milder prevailing temperatures (Figure 1).

Soil temperatures ranged from 6 to 28 °C, and soil CH₄ fluxes were virtually zero below 20 °C during the period studied (2009-2012). In the rice seasons, soil temperature ranged from 18 to 28 °C (Figure 3) and CH₄ emissions increased exponentially with increasing temperature (p<0.0001). The increase was more marked in the reproductive stage than in the vegetative stage, probably because of the effect of C exudation from roots and increased development of plant aerenchyma (^{Das and Baruah, 2008}). The ability of rice plants to transport CH₄ increases along with the increasing number and size of tillers, leaves, and roots (^{Gogoi et al., 2005}; ^{Das and Baruah, 2008}), and also of the structures involved in CH₄ transport from the soil to the atmosphere. In addition, a substantial fraction of photoassimilated C is translocated to roots in the reproductive stage, and they provide a source of labile C for methanogens (^{Lu et al., 2000}).

Figure 3 Relationship between soil temperature and CH₄ fluxes in the vegetative and reproductive stages of rice in southern Brazil. 

Unlike CH₄, N₂O fluxes peaked during the non-rice season and before flooding in the rice season; furthermore, they were unrelated to soil temperature or fertilizer N added to the rice or ryegrass crops (Figure 2, Table 1). The average soil N₂O flux was 30.2 g ha⁻¹ day⁻¹ for the non-rice season and 2.08 g ha⁻¹ day⁻¹ for the rice season. Flooding is known to reduce N₂O fluxes to near-zero levels (^{Johnson-Beebout et al., 2009}; ^{Liu et al., 2010}); and nitrification is restricted by the absence of O₂, which precludes NO₃^– production and denitrification. Also, prolonged flooding results in an increased proportion of N₂O being reduced to N₂ (^{Reddy and DeLaune, 2008}; ^{Liang et al., 2013}).

Seasonal soil CH₄ and N₂O emissions

Cumulative soil CH₄ emissions in the rice season were significantly influenced (p<0.001) by the management level and differed over the years (Table 2), ranging from 250.9 to 671.5 kg ha⁻¹ (Table 3), within the range previously reported for southern Brazil (^{Moterle et al., 2013}; ^{Bayer et al., 2014}, ²⁰¹⁵). The considerable differences in cumulative soil CH₄ emissions between rice seasons may have been related to the specific weather conditions and to differences in rice crop development (^{Liang et al., 2013}) and ryegrass biomass in the previous winter.

On average for the three rice seasons, the highest soil CH₄ emissions occurred at the high crop management level (546.6 kg ha⁻¹), and were 34 and 69 % higher than those for the very high and medium level, respectively, with no significant difference between the last two (Table 3). The combined effect of the inputs and their respective amounts at the high crop management level probably provided better development conditions for the rice plants and methanogenic activity through more extensive allocation of C to the root system (^{Liang et al., 2013}). This assumption is strengthened by the significant positive relationship (p<0.01) between the amount of aboveground biomass of the rice crop and cumulative CH₄ emissions during the crop season (Figure 4), mainly upon comparing the medium and high crop management levels. Increased plant growth probably boosted production and release of organic compounds through the root system.

Figure 4 Relationship between aboveground rice biomass (R₄ stage) and CH₄ emissions during the rice season at different crop management levels in southern Brazil. 

The increase in crop management level from medium to high, and the resulting impact on the production of rice biomass (Figure 4), may also have contributed to the increased CH₄ emissions observed during the next non-rice season (Tables 2 and 3). Thus, the combined cumulative soil CH₄ emissions at these two levels in the two non-rice seasons were 609 % higher (p<0.01) than they were at the medium management level. In a previous study, ^{Xu and Hosen (2010)} found that rice straw input significantly increased CH₄ emissions during the fallow period, but only when the soil had water content exceeding 79 % of its retention capacity.

Another factor that significantly (p<0.1) influenced CH₄ emissions from the soil in the rice season was ryegrass biomass produced in the previous non-rice season (fall-winter, Figure 5). Based on the results, adding 1 Mg ha⁻¹ ryegrass to the soil increased CH₄ emissions by more than 210 kg ha⁻¹ in the subsequent rice crop. The addition of residues of a cover crop to soil is known to boost CH₄ emissions by increasing the supply of labile C to methanogens and enhancing soil reduction as a result (^{Kim et al., 2013}).

Figure 5 Relationship between aboveground ryegrass biomass in the previous winter (non-rice season) and cumulative soil CH₄ emissions during the subsequent rice season in southern Brazil. 

We found a decrease of 25 % in cumulative CH₄ emissions (average of the three rice seasons) from high to very high crop management levels. This decrease in CH₄ emissions may be at least partially related to the lower production of biomass by rice observed under very high crop management in the two seasons that this variable was evaluated (Figure 4). The lower crop development may result in lower allocation of C to the root system, with a negative impact on methanogenic activity, as discussed previously. However, it is not clear why rice development decreased under very high management compared to high crop management.

An additional factor that may be related to the lower CH₄ emissions under very high management compared to high crop management is the higher amount of inorganic N fertilizer applied. Although controversial, recent field studies (^{Dong et al., 2011}; ^{Liang et al., 2013}) and extensive literature reviews (^{Xie et al., 2010}; ^{Linquist et al., 2012b}) have found that an increased inorganic N rate (especially one from ammonium-based fertilizers or amide forms) can help mitigate CH₄ emissions through CH₄ oxidation by methanotrophic microorganisms. However, the potential effect of urea application on CH₄ fluxes was not evident in the subsequent days after fertilizer application in our study nor in previous studies carried out in this same region (^{Bayer et al., 2014}, ²⁰¹⁵). So, this specific issue should be addressed in future studies.

The similarity of N₂O emissions among management levels suggests that increasing N input by fertilization (base and topdressed) had no effect on soil N₂O emissions (Tables 2 and 3) in any of the periods studied (rice and non-rice season, and annual results). The average of cumulative N₂O emissions for the three rice seasons were 0.41, 0.32, and 0.33 kg ha⁻¹ at the medium, high, and very high crop management levels, respectively (Table 3). Our results are consistent with those of ^{Zou et al. (2007)} and ^{Liu et al. (2010)}, who found no relationship between N₂O emissions and N fertilizer applied during flooded rice cultivation - not even under increased amounts of N. The negative emission values obtained in the 2009/2010 season (-0.16 to −0.32 kg ha⁻¹) indicate that the soil acted as a sink for atmospheric N₂O. However, this N₂O influx may have resulted from the fact that gas measurements during that crop season were carried out only after flooding. Therefore, as in the other crop seasons, N₂O emissions peaked after the 4-week period between seeding and flooding, when the soil was dry or alternatively dry and moist because of rainfall events (Figures 1 and 2).

Annual GHG emissions, calculated as a combination of the results for the rice and non-rice season, highlighted the importance of expanding studies beyond the rice season. Thus, cumulative soil CH₄ emissions for the annual period ranged from 290.3 to 677.2 kg ha⁻¹ yr⁻¹, the non-rice season accounting for 0.1-19 % (Table 3). By contrast, cumulative soil N₂O emissions ranged from 0.57 to 6.14 kg ha⁻¹ yr⁻¹, the non-rice season accounting for over 90 % (Table 3). In a study conducted in California (USA) to evaluate the effect of applying different amounts of N during rice farming on CH₄ and N₂O emissions, ^{Pittelkow et al. (2013)} found the non-rice season accounted for 16-30 % of the annual CH4 emissions and 22-79 % of the annual N2O emissions, and stressed the importance of GHG measurements beyond the rice season.

Partial global warming potential (pGWP), rice yield, and yield-scaled pGWP

Averaged across rice seasons, the high management level resulted in the highest pGWP value: 13,761 kg CO₂eq ha⁻¹, which is 67 and 34 % higher than the value for the medium and very high crop management levels, respectively (p<0.01, Tables 2 and 3). Methane emissions during the rice season accounted for more than 98 % of the annual pGWP, which is consistent with previous results of ^{Bayer et al. (2014)} for the same soil and climate conditions.

The average annual pGWP was 12,084 kg CO₂eq ha⁻¹ yr⁻¹ and largely (>90 %) the result of CH₄ emissions in the rice season - which accounted for more than 80 % of the annual figure. The average pGWP for the non-rice season was 1,410 kg CO₂eq ha⁻¹ and, unlike the rice season, it consisted mainly of soil N₂O emissions (≈65 %). Maintaining the soil drained during the non-rice season provided better conditions for N₂O production by nitrification and denitrification but attenuated CH₄ production owing to the anaerobic character of methanogenic microorganisms (^{Reddy and DeLaune, 2008}; ^{Xu and Hosen, 2010}).

Our results suggest that strategies to mitigate GHG emissions from Southern Brazilian rice fields are more effective if measures are taken in the rice season, and also that the focus should be placed on CH₄ emissions. In fact, GHG emissions, in CO₂eq, were almost 10 times lower in the non-rice season than in the rice season; however, if measures are also implemented in the fall-winter non-rice season, mitigation of emissions requires shifting the focus to N₂O.

Rice grain yields increased significantly (p<0.01) along with increasing management levels, from an average value for the three crop seasons of 9,851 kg ha⁻¹ at the medium level to 11,484 kg ha⁻¹ at the very high level (Tables 2 and 3). These results are consistent with those obtained by ^{Mariot et al. (2009)} in a similar study at the same experimental site, in which they found that increasing the crop management level increased rice yield by 65 % and increased profits.

Yield-scaled pGWP (YpGWP) was significantly higher (p<0.1, Tables 2 and 3) at the high crop management level in two of the three crop seasons (Table 3). The YpGWP at the medium and very high crop management levels was similar in the three crop seasons (0.71-1.09 and 0.52-1.36 kg CO₂eq kg⁻¹ rice, respectively). The effect on rice yield of increasing the crop management level from medium to high was less pronounced than the effect on soil GHG emissions, resulting in an increase of 60 % in YpGWP. Thus, due to this larger impact on GHG emissions than on crop yields, a future increase in rice yield as a result of adoption of improved crop management levels, along with no negative impact on GHG emissions in the regional production systems, may require adopting other additional agricultural practices that have a mitigating effect on GHG emissions, such as intermittent irrigation.