Methane emission from a flooded rice field under pre-germinated system

: Local greenhouse gas flow measurement studies have been encouraged at a global level as a subsidy for national and state inventories. This study aimed to evaluate the seasonal methane emission during the 2008/2009 harvest, from an irrigated rice plantation, under pre-germinated system, in the municipality of Tremembé, State of São Paulo, using the static chamber technique and gas chromatography. The study showed high seasonal emission of methane (CH 4 ) for the studied area, probably due to the long flooding period. It was estimated the CH4 emission factor (6.51 kg CH 4 ha -1 dia -1 ), the partial global warming potential (pGWP, 27.2 Mg CO 2 eq growing season -1 ha -1 ) and the yield-scaled pGWP (YpGWP, 3.9 kg CO 2 eq kg -1 grain).

chemical characteristics at 0-20 cm depth: 41.4% sand, 36.0%silt and 22.6% clay; density: 1.31 g cm -3 ; total porosity: 48.91 %; pH in H 2 O= 5.55; total C=9.1 g kg -1 ; total N=0.88 g kg -1 ; P=3.63 mg dm -3 ; K, Ca, Mg,H+al=0.27,1.09,0.53,3.35cmol c dm -3 , respectively, and v=37.6%(Silva, 2009).total C (tC) and total N (tN) contents were determined using an elementary C and N analyzer (truspec-leco).the experiment was carried out under broadcast pre-germinated system, during the 2008/2009 harvest, with continuous water irrigation, and consisted of a single treatment with three repetitions in a one hectare block.this type of cultivation is characterized by the use of pre-germinated seeds in previously flooded soil.Straw from the previous crop was incorporated into the soil after harvesting.the soil was plowed three months in advance and flooded twenty days before sowing, which occurred on 02/09/2008 with the emergency observed on 09/09/2008.this previous flooding was due to control of the red rice.the long-cycle variety SCS 114 andosan (140 days) was used with a seed density of 140 kg per hectare.after flooding of the soil and the initial development of the seedlings, the height of the floodwater was maintained at an average of 16 cm.the agrochemical management was done according to regional technical recommendations.at 49 days after flooding (daF) were applied the herbicides ricer (150 ml), Basagran (2 l) and ally (10 g), and with the insecticide Curbix (200 ml) and vegetoil (1 l).Covering fertilization was carried out twice, with the first at 58 daF and the second at 85 daF, using the formulation 25-0-25 (30 kg N ha -1 and 30 kg K 2 O ha -1 ).Urea and KCl were used as N and K sources.Flowering and complete ripening occurred, respectively, at 148 and 179 daF. the static chamber method was used to collect the gas samples (Iaea, 1992).Gas sampling was conducted once a week on 18 occasions throughout the growing season (GS), beginning at 36 days after sowing (57 daF) and ending at 176 daF, before the harvest (10/02/2009), which was carried out with the soil still flooded.the gas samples were analyzed using an agilent model GC6890 gas chromatograph, equipped with a 6-way valve, and a flame ionization detector (FId).daily fluxes of CH 4 were calculated according to Bayer et al. (2014) and integrated to produce an estimate of the seasonal flow of CH 4 , representing the accumulated emission for each chamber used.the estimation of the CH 4 seasonal emission considered the period from the sowing, although the soil was flooded twenty days earlier, until the complete ripening.
The CH 4 emission factor was calculated from the division of the seasonal emission (eS) by the total of days of the GS, being expressed in kg CH 4 ha -1 d -1 .the partial global warming potential (pGWP), expressed in kg CO 2 eq ha -1 , was calculated by multiplying the accumulated emission (seasonal) of CH 4 throughout the GS and its radioactive forcing potential (pGWP CH4 = CH 4 * 28), according to MYHre et al. (2013).the yield-scaled global warming potential (YpGWP) was also calculated as the ratio of the pGWP and the yield of rice grains (Bayer et al., 2014).While collecting the gases, the temperatures of the air, the floodwater and the soil at depth of 5 cm were registered using a five point Full Gauge thermometer.the air temperatures on the inside of each chamber were registered using digital thermometers (MUltI-thermometer). the soil and water pH and oxidation-reduction potential (eh) were measured using a digimed digital pH-meter.Heights of the plants and of the floodwater were registered.the linear correlation coefficient between these variables and the CH 4 flow was determined using the procedure COrr of the SaS program (SaS, 2011).For a comparison of variability between the repetitions, the coefficient of variation (Cv) was determined for cumulative CH 4 fluxes.
Increasing CH 4 flows were observed during the rice tillering period (Figure 1B).Increase in the number of tillers resulted in a greater CH 4 transport capacity as a function of the greater density and amount of aerenchyme (KIM et al., 2018).the variety SCS 114 andosan is characterized by presenting high tillering, which could have contributed to the elevated CH 4 flows in this stage.Other emission peaks occurred near the panicle initiation stage (112 daF) and flowering (148 daF).GOGOI et al. (2005) also reported the occurrence of a high CH 4 flux at the panicle initiation stage, which was attributed to the decomposition of organic carbon in the form of root and leaf exudate and to the increased CH 4 transport capacity of the rice plant at this stage.the mean daily CH 4 emission was estimated as 616 mg of CH 4 m -2 d -1 (Cv: 17.15%) and the accumulated emission during the season was 93.60 g CH 4 m -2 (Cv: 17.15%), corresponding to a CH 4 emission factor of 6.51 kg CH 4 ha -1 d -1 , which is five times higher than the average indicated by the IPCC ( 2006), of 1.30 kg CH 4 ha -1 d -1 .the seasonal CH 4 emission evaluated in the present study is amongst the highest recorded in the measurement experiments conducted in the country.lIMa et al. ( 2007) also reported high seasonal CH 4 emission values in an experiment carried out in Itajaí, State of Santa Catarina, using the same cultivar (SCS 114 andosan) under the pre-germinated system, with seasonal emissions accounting 68.84 g CH 4 m -2 (Cv: 16.76%) and 138.21 g CH 4 m -2 (Cv: 8.53%) for rice cultivation in mineral and organic soils, respectively.the high CH 4 emissions observed in the present study could be probably attributed to the long time under inundation in the pre-germinated system and to the long cycle of the variety, totaling 176 days of inundation from sowing to complete ripening.Impact of the continuous flooding on CH 4 emission is well known in the literature (IPCC, 2006;MOterle et al., 2013).Moreover, the rice variety used presents high CH 4 efflux potential, according to SIlva et al. (2014), although this effect was not evaluated in this study.the correlation analysis between the measured soil, water and meteorological variables and the CH 4 fluxes is presented in table 1. Positive and significant correlations were reported between CH 4 emission and air and water temperatures and soil temperature at 5 cm depth.Such relation has registered in numerous studies (WaNG et al., 2017, FUMOtO et al., 2018).data of these variables are presented in the Figure 1C.Plant and floodwater height, soil and water pH, and oxide-reduction potential showed no significant correlations with CH 4 emissions.pGWP was evaluated as 27.2 Mg CO 2 eq ha -1 GS -1 .rice production was estimated as 6.8 t ha -1 , the value calculated for YpGWP being 3.9 kg CO 2 eq kg 1 of grains, a value much higher than those reported in the literature (table 2).this study resulted in a CH 4 emission factor (6.5 kg CH 4 ha -1 d -1 ) for an irrigated rice production system typically used in the state of São Paulo, thus contributing to national   (1,3)  7.9 (1,3)  1.8 (1,3)   No tillage 10.3 7.7 1.4 BaYer et al. ( 2015) Cachoeirinha, RS Fall tillage 8.6 (2,3)  8.1 (2,3)  1.1 (2,3) Spring tillage 10.9 (2,3) 7.7 (2,3) 1.4 (2,3)   zSCHOrNaCK et al. ( 2016) Cachoeirinha, RS CF 10.7 (4) 12.0 (4) 0.9 (4) II 6.8 (4) 11.8 (4) 0.6 (4) CF 7.6 (5) 10.7 (5) 0.7 (5) SII 2.0 (5) 10.4 (5) 0.2 (5) FII 2.6 (5) 10.9 (5) 0.2 (5) and regional databases on CH 4 emission factors, which are critical for improving greenhouse gas emission estimates.result also suggested that further studies should be conducted to investigate the impacts of the pre-germinated system on CH 4 emissions under different environmental conditions and varieties, as well as to identify emission mitigation options.

Table 1 -
Pearson's correlation coefficients and P-levels for the linear relationship between CH4 emission and the variables measured.

Table 2 -
Seasonal pGWP emissions, expressed as CO2eq ha -1 season -1 , grain yield in Mg ha -1 , and seasonal yield-scaled pGWP in kg CO2eq kg -1 grain, from flooded rice cultivation areas in the south and southeast of Brazil under different forms of management.
average of five agricultural years; 2 average of seven agricultural years; 3 N2O emissions included; 4 2009/10 agricultural year; 5 2010/11 agricultural year; 6 the low rice production was due to the occurrence of diseases; 1 7 average of four agricultural years; CF: Continuous flooding; II: Intermittent irrigation; SII: Sparse intermittent irrigation; FII: Frequent intermittent irrigation; CWr: Continuous water regime; IWr: Intermittent water regime; PG: Pre-germinated.