Effect of Direct-seeding with Non-flooding and Wheat Residue Returning Patterns on Greenhouse Gas Emission from Rice Paddy

Zichang ZHANG, Lijun LIU, WANG Zhiqin, Jianchang YANG*, Yongfeng LI

1. Institute of Plant Protection, Jiangsu Academy of Agricultural Sciences, Nanjing 210014, China;

2. Key Laboratory of Crop Genetics and Physiology of Jiangsu Province, Yangzhou University, Yangzhou 225009, China

CO2,CH4and N2O are important greenhouse gases, playing a critical role in global warming.Rice is one of main stable food crop in the world, accounting for 1/3 in terms of planting area and the produced greenhouse gases,such CO2,CH4and N2O, account for high proportions in discharge of greenhouse gas worldwide[1]. China is the largest rice producing and consuming country and the area of paddy field represents 23% of global rice planting area. Therefore, it is of great significance for relief of greenhouse effect to further explore discharge rules of CO2, CH4and N2O and take countermeasures. In China,researches available on discharges of greenhouse gas on paddy fields are much more, but most concentrate on effects of water management[2-4], fertilization[5-6], and planting methods[7-9]ondischarges of CH4and N2O from transplanted rice. Less attention is paid to effects of straw returning methods on discharges of CO2, CH4and N2O from direct seeding.In the research,hence,discharge characters of greenhouse gases on paddy fields were observed by direct seeding and straw burying and coverage in order to further evaluate effects of direct seeding and straw returning on environments, which provides references for effective control of greenhouse gas on paddy fields with simplified cultivation.

Materials and Methods

Materials and sowing methods

The experiment was conducted at a farm cultivation and physiology laboratory belonging to Yangzhou University, Jiangsu Province, Two cultivars,Yangdao 6 (an indica cultivar) and Yangjing 4038 (a japonica cultivar),were used as materials.The preceding crop was wheat and soils were sandy loam with 2.15%organic matter,104.2 mg/kg available nitrogen, 37.8 mg/kg Olsen-P, and 86.4 mg/kg exchangeable K. After harvest of wheat, wheat straws were piled on fields and border field was 1.5 m of width, and 15 cm of depth, and 20 cm of width of border ditch. Basal fertilizers were then applied on border fields, including urea(N 46%) at 120 kg/hm2, superphosphate (P2O513%) at 750 kg/hm2, and potassium chloride (K2O 60%) at 375 kg/hm2. After two days of fertilization,rice soaked for 48 h were sown as per test design with a quantity of 75 kg/hm2.

Treatments

The experiment comprised four treatments in a complete randomized block design with four replicates and a plot size of 1.5 m ×10 m. The four treatments were non-flooding and wheat straw mulching (NSM), nonflooding and wheat straw incorporation into soil (NSI),wheat straw incorporation into soil and traditional flooding(SIF) and tranditional flooding (TF).NSM prepared wheat straws on border fields with a amount of 6 000 kg/hm2(dry weight) and non-flooding cultivation with water only in border ditch as per non-flooding mulching cultivation.NSI cut wheat straws and ploughed into fields before rice sowing with a returning amount of 6 000 kg/hm2(dry weight) as per non-flooding cultivation with buried straws. SIF cut wheat straws and ploughed into fields before rice sowing with a returning amount of 6 000 kg/hm2; rice was irrigated occasionally after earing.TF was the treatment as per local cultivation method,without straw returning and rice was intermittent irrigated after heading.In 1-3 d of sowing,herbicides were sprayed when none accumulated water can be found on border fields. Subsequently,urea was applied at 10, 20, 30 and 55 d after sowing with the rate of 75 kg/hm2and at 75 d with the rate of 90 kg/hm2.It is notable that total N applied in different treatments should be 234.6 kg/hm2. Plots were separated by a 0.5 m wide alley using plastic film inserted into soil. At fertilization, a shallow water layer should be maintained on border fields in different teatments.

Sampling and analysis

CH4, N2O and CO2were determined by using the method of static chamber-gas chromatographic techniques and the chamber was 50 cm×50 cm. Specifically, the chamber was made of PVC and the height increased upon rice height. For example, the chamber height was 0.5 m before jointing and 1.1 m after jointing. The chamber was then covered with aluminized paper in case of significant changes of inner temperature of the chamber by solar radiation, and the PVC base was fixed on soil. Since the 10thd of rice sowing, the gases were sampled once every 5 d and once every 10 d after heading. Specifically,before gas sampling, the top two minifans in the chambers (12 V) were opened to well mix the inner gases and the sample chamber was vertically placed a groove with depth of pedestal at 5 cm. Then, the chamber was added with water and sealed. The gases were sampled at 50 ml with a polyethylene needle during 8:00-9:00,totaling 5 times, namely, 0, 10, 20, 30 and 40 min after the chamber closed.The concentrations of sampled CO2,CH4and N2O were analysed by gaschromatograph (7890A)produced from Agilent. The Standard sample concentration of CO2, CH4and N2O were 351, 8.5 and 4.9 ppm, respectively. Based on standard peak area and sample peak area, the concentrations of the sample can be computed as per the equation of gas concentration of samples=Concentration of reference material ×Peak area of sample/Peak area of reference material. Furthermore, sample gases were analyzed on that day. In addition, emission flux was concluded of F=M/Vo×P/Po×To/T×H×dc/dt, where F refers to emission flux of gases (mg/(m2·h));M is molar mass (g/mol); V0represents molar volume (22.41×10-3m3) with temperature at 273.15 K and atmospheric pressure at 101.325 kPa; Toand Poare absolute temperature and pressure in standard state; P is the pressure of sample sites;T is absolute temperature at sampling; H is net height of sample chamber; dc/dtis change rate of gas concentration in the chamber. The emission fluxes of CH4,CO2and N2O were represented by averages of the repetitions,and it can be concluded the total discharge of CH4,CO2and N2O. According to previous researches, one molecule CH4=25 molecules CO2, and one molecule N2O=298 molecules CO2. Additionally,global warming potential(GWP)=CO2+25×CH4+298×N2O[10].

Yield components measurements and harvesting

After maturity, thirty hills of rice were sampled from each plot to determine the number of seed per panicle,setting percentage and 1 000-seed weight, and grain yield was determined from all plants from a 5 m2site(except border plants)in each plot.

Data analysis

Analysis of variance was conducted using the DPS and mapped with Sigma Plot 10.0.

Results and Analysis

Yield and components

As shown in Table 1, compared with that under traditional flooding(TF), wheat straw incorporation into soil and traditional flooding(SIF)significant increased yield by 10.19% -11.48%, while the non-flooded (NSI and NSM) reduced grain yield by 2.96%-6.61% and the ruduction was not significant. Considering from yield components, SIF significantly increased grain yield due to the increase in setting percentage and 1 000-seedweight, and the non-flooded decline the grain yield due to decrease in total spikeletes(Table 1).

Table 1 Effect of direct-seeding with non-flooding and wheat straw returning patterns on the grain yield and its components of rice

Discharge of greenhouse gas on paddy fields

As shown in Fig.1, direct seeding with non-flooding and straw returning had significant effects on discharge of CH4. In NSI and NSM, the discharge flux proved significantly higher when compared with that under TF and SIF Specifically, under the condition of TF and SIF, the discharge of CH4grew in the process of rice development, and reached the peak on the 55thd. the discharge of CH4averaged 22.37 mg/(m2·h) after wheat residue returning, which was significantly higher compared with the TF of 13.01 mg/(m2·h). Comparatively speaking,the average discharge in the same treatment performed higher with nonglutinous rice, than that of japonica rice. By non-flooding cultivation, only few CH4was discharged in treatments NSI and NSM, showing insignificant differences.

As shown in Fig.2, water management significantly influenced N2O discharge. For instance, average discharges in the NSI and NSM were considerably higher compared TF and SIF. Under, SIF, the discharge of N2O averaged 12.59 μg/(m2·h), which was a little lower compared with TF. Furthermore, in different treatments, the discharges of N2O all showed peaks after fertilization, which was higher in early stage, and the peak values in NSI and NSM were significantly higher compared with TF and SIF.

As shown in Fig.3, as rice grew,the discharge of CO2kept increasing and reached a peak, followed by declining. Under NSI and NSM, average discharge rates of paddy rice improved dramatically compared with TF and SIF, and the discharge actually advanced discharge of CO2.

In the ecosystem of paddy fields,the discharges of CH4, N2O and CO2play a crucial role in global warming and GWP is generally used to represent relative radiation effects of differ-ent greenhouse gases at the same quantity on greenhouse effects. It can be concluded from Table 2 that SIF significantly increased the comprehensive CO2effects,ranging from 47.4% to 53.7% when compared with TF,. Under NSI and NSM, the comprehensive greenhouse effects significantly declined in the range of 24.3%and 35.5%. What’s more, non-flooding cultivation of rice also significantly reduced GWP per unit area (Table 2),which indicated that non-flooding of rice would effectively control the discharge of greenhouse gases and reduce greenhouse effects.

Table 2 Greenhouse warm potentials(GWP)of CH4,N2O and CO2 from rice field under direct-seeding with non-flooding and wheat residue returning patterns

Discussions

The practice of returning straws to fields has good effects on soils and farmland ecology, which would promote rice growth and yield increase.Ge et al. indicated that the practice of returning maize straws to fields would improve rice yield ranging from 18.9% to 32.0% without N applied and increased in 0.5%-11%with N applied[11]. Yang et al. showed that the practice would enhance rice yield, averaging 7.2%[12]. Xu et al. suggested that the practice increased the number of seed per panicle,setting percentage and 1 000-seed weight, and the yield increased by 2.7%[13]. In contrary, Zhu et al. found that the practice reduced the number of panicles by 12.14% ,thousand-seed 5.68% and yield 7.68%[14]. Our study showed that SIF significantly increased grain yield in the range of 10.2%-11.5%, while NSI and NSM showed some reduction.The reduction in grain yield was 3.0%under NSI and 6.0% under NSM. The difference in grain yield was insignificant between NSI and TF or NSM and TF.On basis of analysis of yield components, after wheat residue returning to fields, the decline of rice yield was mainly caused by the decrease of seed number per panicle, which demonstrated that NSI and NSM have adverse effects on glumous flower of rice. Therefore, the increase of water supply in the growth stage of glumous flower would stimulate differentiation of glumous flower or reduce degeneration to guarantee a high yield by nonflooding cultivation.

The discharge of CH4is a result of methane bacteria’s effects on methane matrix under extreme anaerobic conditions. Hence, soil moisture becomes a key factor affecting CH4discharge on paddy fields. It is researched that drying paddy field in sunshine would increase soil permeability, allowing O2in atmosphere can be expanded to soils,destroying original state, so that soil Eh would grow rapidly, affecting activity of methane bacteria and inhibiting discharge of CH4[15].Cai et al.proposed that the discharge of CH4from paddy fields by long-term flooding irrigation was far higher compared with the drying fields or fields by alternate treatment of flooding and non-flooding irrigation[16].Towprayoon et al.explored the effects of field drying days and frequency on discharge of CH4, and the results showed that the drainage of water in rice growing stage significantly reduced discharge of CH4. For example,the discharges declined by 29% and 36% by one drainage and twice drainage respectively[17]. The research indicated that compared with that of non-flooding cultivation, traditional flooding considerably advanced CH4release in vegetative period when photosynthesis performs strong and more photosynthates are secreted from roots, supplemented by more O2transported to roots released by photosynthesis,promoting multiplication of facultative anaerobes which decom-pose organic matters into H2, CO2and acetic acid. These actually provide abundant carbon source and energy for CH4. Meanwhile, the multiplication of facultative anaerobes consumes O2in soils,which provides a better anaerobic condition for CH4also. Subsequently,rice roots began degenerating and root exudate dropped, resulting in decrease of CH4release[18]. On the other hand, non-flooding cultivation considerably improved aeration of paddy fields, destroying anaerobic condition of bacteria producing CH4and reducing the discharge accordingly. The research investigated that SIF released more CH4, possibly caused by decomposition of microorganisms in soils by buried straws,which produced carbons[19].

More researches available are on the effects of soil moisture on discharge of N2O in rice growing period,and the results indicate that almost the release of N2O on paddy fields in soil drying phase.By long-term flooding irrigation, nitrifying process is significantly limited, and denitrification released more N2,and less N2O[20].Xu et al. researched that by field drying and alternate treatment of flooding and non-flooding cultivation, the discharge of N2O grew considerably[21].Li et al.also investigated that after rice transplanting, soils were by flooding irrigation for a long term before field drying and the discharge flux of N2O always maintained a lower level,with N fertilizers,taking up to 80%of total N fertilizers, applied[22]. Nevertheless, previous researches concentrate on transplanted rice and few attention is paid to discharges of N2O.The research incorporated that the release of N2O would reach a peak every time after fertilization and discharge flux by non-flooding treatments performed higher compared with flooding-cultivation treatments. Specifically, during rice growing period by non-flooding cultivation,the maximal discharge flux of N2O appeared before rice growth term, and the causes are as follows:Firstly, in seedling stage, rice grew mainly depending on nutrients supplied by seeds,and absorbed few N.Secondly,sandy soils performed poorly in fixing or absorbing N fertilizers.Thirdly,good soil permeability by non-flooding cultivation stimulated release of N2O. Furthermore, the research also demonstrated that after straws returning to fields, N2O in SIF reduced by 14.1%-19.2% compared with TF, possibly caused by declining of denitrification rate under anaerobic condition[23].Additionally,in the process of straw decomposition, the released allelochemicals would inhibit activity of soil microorganisms, further preventing N2O releasing from soils[24].

Compared with CH4and N2O, few researches are available on discharge of CO2. On paddy fields, CO2are mainly produced from respiration or decomposition of organic matters of microorganisms, driven by atmosphere and soil temperatures. However,it is also explored that the discharge of CO2from paddy fields with participation of plants is considerably lower compared with that without plant participation[25-26],of which the former is mainly driven by temperature and the latter by water[25].The research demonstrated that as rice grew,the discharge of CO2was growing to a peak, because rice was in a tillering stage when O2transportation capacity performed stronger and root systems grew exuberantly. Meanwhile, temperature is suitable for soil microorganism multiplication, improving soil respiration, and more secreta produced with rice growing, accelerating soil microorganism activity, so that increasing CO2was discharged[27].Still,it is notable that the increased carbon sources by straw returning also advanced the release of CO2.

The research made evaluations on greenhouse effects of CH4, N2O and CO2under four cultivations patterns and the results showed that cultivations patterns changed greenhouse effects of paddy fields’ecosystems, as follows:SIF＞TF＞NSI＞NSM.In terms of yield, SIF significantly improved rice yield, as well as discharge of carbon per unit area,and rice yields of NSI and NSM declined a little, but discharge of carbon per unit area reduced substantially. On the other hand, GWP of yield per unit area reflects environmental and economic effects, which is considered as a key index of modern agricultural sustainability. The lower GWP per unit area,the lower environmental cost per unit yield. Hence, it can be concluded that NSI or NSM is a simplified,water saving and high yielding cultivation,should be widely promoted. Still, the research just investigated the effects of wheat residue returning on discharges of CO2, CH4and N2O, for direct-seeding rice in a specific season,and the effects in following seasons requires further exploration.

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