The effects of aeration and irrigation regimes on soil CO2 and N2O emissions in a greenhouse tomato production system

CHEN Hui, HOU Hui-jing, WANG Xiao-yun, ZHU Yan, Qaisar Saddique, WANG Yun-fei, CAI Huan

jie1

1 College of Water Resources and Architectural Engineering, Northwest A&F University/Key Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Ministry of Education, Yangling 712100, P.R.China

2 School of Hydraulic, Energy and Power Engineering, Yangzhou University, Yangzhou 25127, P.R.China

1. Introduction

Carbon dioxide (CO2) and nitrous oxide (N2O) are significant greenhouse gases (GHGs) that contribute to stratospheric ozone layer depletion and global warming, with their concentrations increasing by 40 and 20%, respectively,since global industrialization (IPCC 2013). Worldwide,anthropogenic sources of CO2and N2O are dominated by agriculture and have increased by approximately 17% from 1990 to 2005 (Robertson and Grace 2004; Forsteret al.2007). China accounts for 45% of the world’s total vegetable production, occupying 11.6% of the country’s cultivated land(FAOSTAT 2009). Unlike the conventional staple grain crops(i.e., rice, wheat, and corn), vegetable cropping systems are subjected to high nitrogen (N) fertilizer inputs, frequent irrigation, high temperatures, and multiple harvests within a year. For instance, greenhouse vegetable cropping systems in Beijing suburbs utilize N fertilization rates of approximately 1 000 kg N ha–1crop–1(Chenet al.2004), over 2 800 kg N ha–1yr–1in Huiming, Shandong Province (Juet al.2006) and on average 600 kg N ha–1crop–1in the suburban area of Xi’an City, Shaanxi Province (Zhouet al.2006). These values are much greater than the average national N fertilizer rate of approximately 180 kg N ha–1yr–1or 120 kg N ha–1crop–1(Zhu and Chen 2002) that has been the cause of serious pollution (Sunet al.2017). Furthermore, the constant irrigation of these vegetable crops plays an increasingly important role in Chinese agriculture (Sunet al.2013).Irrigation influences the soil organic matter decomposition,microbial biomass and activity, root biomass, soil aeration and nitrogen turnover, which influence soil CO2and N2O production and emission (Rethet al.2005; Huanget al.2007; Scheeret al.2008, 2013). Excessive N fertilization along with abundant soil moisture undoubtedly generates high CO2and N2O emissions measured from vegetable cropping systems. Thus, continuous field measurements of CO2and N2O emissions from vegetable cropping systems are necessary to fully understand crop-specific CO2and N2O emissions (Flynnet al.2005).

Although CO2and N2O emissions from Chinese grain crops have been extensively documented, research on CO2and N2O emissions from vegetable systems in China are sparse and lead to imprecise values for national CO2and N2O emissions inventory from Chinese croplands(Heet al.2009). Previous studies on GHGs in vegetablefields have extensively analyzed fertilizer application(Riyaet al.2012; Denget al.2013; Liet al.2015a).However, the effect of water management coupled with GHGs in vegetable fields remains uncertain, especially when considering aerated irrigation (AI) practices. As early as the 1994s, AI application has and continues to be employed in root zone soils to overcome hypoxia.The benefits of this technique have been verified in crops including pineapple (Chenet al.2011), soybean and cotton(Bhattarai and Midmore 2009), cucumber (Ehretet al.2010; Niuet al.2013), tomato (Bhattaraiet al.2006; Houet al.2016) and rice (Zhuet al.2012). The majority of AI studies have concentrated on the economic benefits (e.g.,crop production and water use efficiency) and overall fruit quality (Bhattarai and Midmore 2009; Ehretet al.2010;Abuarabet al.2013; Niuet al.2013). Very few studies on AI-derived soil properties (i.e., soil aeration, NO3–and electrolytic conductivity (EC)) have been performed (Niuet al.2012; Ben-Noah and Friedman 2016), especially for soil CO2and N2O emissions. Soil respiration caused by AI has been studied in wheat, cotton and pineapple (Chenet al.2011) but the effect of AI on vegetable-planted soils is insufficiently documented. Furthermore, variations of N2O emissions under AI have been demonstrated in the laboratory (Hwang and Hanaki 2000; Castro-Barroset al.2015) while field experiments are extremely limited.During our previous study, only gas emissions and soil water contents were measured after AI treatment during the autumn-winter season, and the experiment did not capture emissions at the tomato seedling stage and part of the blooming and fruit setting stage (Houet al.2016).Unfortunately, little information was gathered regarding factors influenced by AI and the relationship between gas fluxes and environmental variables. The effects of soil aeration and irrigation regimes on GHGs were not recorded. Thus, a study on the combined effects of soil aeration and irrigation regimes on CO2and N2O emissions from vegetable fields is important to perform a comprehensive assessment and to develop greenhouse gas emission reduction measures.

In this study, the closed chamber and gas chromatography technique was employed to measure CO2and N2O emissions from the soil of a greenhouse tomato production system in Northwest China. The soil temperature, soil water content, soil organic carbon, soil nitrate content and electrolytic conductivity were monitored concurrently. The objectives of this study were to (1) evaluate soil CO2and N2O emissions under different aeration and irrigation regimes and to (2) discern the primary factors influencing soil CO2and N2O fluxes.

2. Materials and methods

2.1. Experimental site

A field experiment was conducted from 4 April 2015 to 17 January 2016 in a solar greenhouse at the Key Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Areas of the Ministry of Education, Northwest A&F University, Yangling, Shaanxi Province of China (34°20′N,108°04′E). The greenhouse was 36 m×10.3 m×4 m in dimension. The site was in a semi-humid and liable-drought climate zone with an annual average sunshine duration of 2 163.8 h and frost-free period of 210 d. The experimental site had a Lou soil type in a brown soil zone. The soil texture was silt clay loam and the groundwater depth was at least 100 m below the surface. Physical properties of the top 20 cm soil layer are shown in Table 1.

Table 1 Physical properties of soil in testing site

2.2. Field management

Experimental designTwo irrigation regimes were carried out in the experiment, full irrigation (1.0W) and deficit irrigation (0.6W). Here, W represented the irrigation volume of sufficient water supply which was calculated according to Houet al.(2016). Each irrigation regime contained aeration and non-aeration. Hence, four treatments with three replications were performed with a completely randomized design: (1) aerated deficit irrigation (AI1), (2) non-aerated deficit irrigation (CK1), (3) aerated full irrigation (AI2) and(4) non-aerated full irrigation (CK2).

Each plot (size: 4.0 m×0.8 m) represented one replication.Eleven plants were planted in each plot with a spacing of 35 cm. During the experimental period, all plots were covered with plastic films. Subsurface drip irrigation was used with drip irrigation tape buried at a depth of 15 cm and a dripper spacing of 35 cm. The Mazzei air injector model 287(a Venturi, Mazzei Injector Company, America) was installed for aeration at the head of each irrigation pipe with an inlet and outlet pressure of 0.1 and 0.02 Mpa, respectively, and set to inject 17% air by volume of water (Zhuet al.2016).Vegetable cropHumus pots of tomato seedling (variety:Feiyue) were adopted in the greenhouse for two consecutive growing seasons. In the spring-summer season, tomatoes were transplanted on 4 April 2015 and harvested on 29 July 2015. In the autumn-winter season, transplantation occurred on 30 August 2015 and the harvest occurred on 17 January 2016.

Fertilization and irrigationThroughout the tomato growing stage, only base fertilizer was applied in the spring-summer season, which was comprised of organic fertilizer (N-P2O5-K2O≥10%, organic matter≥45%) and compound fertilizer(total nutrients≥45%, including N, P2O5and K2O each at 15%). They were applied on 22 January 2015 at a rate of 5 937.5 and 3 437.5 kg ha–1, respectively. During the autumnwinter season, they were applied on 5 August 2015 at a rate of 3 437.5 and 2 187.5 kg ha–1, respectively. However,the topdressing (spraysven potassium sulfate, K2O≥52%,SO3≥46%) in the autumn-winter season was applied at a rate of 236.8 kg ha–1on 27 October, 2015 and on 22 October,2015 for deficit irrigation and full irrigation, respectively.

An E601 evaporation pan was placed in the greenhouse to determine the irrigation volume as performed by Houet al.(2016). Irrigation occurred at 8:00 a.m. Throughout the tomato crop growth, with the exception of transplanting irrigation, the irrigation treatments were applied 27 and 35 times at an interval of 3–5 days in the spring-summer season and autumn-winter season, respectively. The total irrigation volume in the spring-summer season for deficit irrigation and full irrigation was 180.1 and 300.1 mm,respectively. In the autumn-winter season, the value for deficit irrigation and full irrigation was 116.2 and 194.6 mm,respectively.

2.3. Sampling and measurements

The CO2and N2O fluxes were simultaneously measured in three replications for each treatment by using the static closed chamber method (Houet al.2016). The static chamber with 6-mm thick PVC material covered an area of 25 cm×25 cm, with a height of 25 cm. The bases of the chambers, also made of PVC, were installed in the middle of each plot on the day of transplanting and remained there until tomato harvest. A 3-cm-deep groove on the top edge of the bottom layer and on the base of the chamber was to be filled with water to seal the rim of the chamber.All chambers were wrapped with layers of sponge and aluminum foil to minimize air temperature changes within the chamber that could be caused by sunlight during gas sampling. The inside of each chamber was equipped with an electric fan near the top for mixing air. Gas samples forflux measurements were collected between 10 to 11 a.m. at an interval of approximately 6 and 9 days during the early and late growing stages, respectively. Chamber air samples were collected using a 50-mL syringe following 0, 10, 20 and 30 min after the chambers were placed on pre-fixed plastic frames. A 30-mL air sample was drawn each time with a syringe. Gas samples in the syringes were analyzed within a few hours. Sample sets were discarded unless they yielded anR2linear regression value greater than 0.85.

Gas samples were analyzed for CO2and N2O concentrations using a gas chromatograph (7890A GC System, Agilent Technologies, America) equipped with aflame ionization detector (FID) and an electron capture detector (ECD) for measuring CO2and N2O, respectively.Fluxes of CO2and N2O from the greenhouse tomato production soils were calculated based on the equation given by Houet al.(2016).

The soil/air temperatures were measured when air samples were collected. The soil temperature was measured at a depth of 10 cm near the chamber bases using a geothermometer (RM-004). The temperature inside the chamber was measured using mercury thermometers(WNG-01) and the air temperature outside the chamber was recorded using a mercury thermometer (WNG-01) placed 1.5 m above the ground.

Soil sample cores from 0 to 10 cm were taken between two plants at the head, middle and end of each plot through a diameter gauge at an interval of 8, 18, 10 and 13 days on average for soil water-filled pore space (WFPS), soil organic carbon (SOC), nitrate (NO3–) content and EC,respectively (Chenet al.2016). Part of the fresh soil was used to measure the soil water contentviaoven-drying at 105°C for 12 h and then converted to WFPS using methods by Dinget al.(2007). To determine NO3–, fresh soil samples were mixed thoroughly and sieved through a 5-mm mesh.Subsequently, each fresh soil sample (corresponding to 5 g dry soil) was extracted by 50 mL of 2 mol L–1KCl for 30 min. The NO3–contents were analyzed using an ultraviolet spectrophotometer (EV300PC, Thermo Fisher, England)following the methods of Zhanget al.(2015). Another dried soil sample was passed through a 1-mm sieve to measure SOC using the oil bath heated potassium dichromate oxidation volumetric method (Zheng 2013) and the EC was determined in a 1:5 of soil:H2O (g mL–1) suspension using an EC meter (FE30K Plus, METTLER TOLEDO, China).

All mature tomato fruits in each plot were manually harvested and individual fruit yields were determined at each picking.

2.4. Data analyses

All statistical analyses, including correlations, were computed using SPSS Statistics 22.0. Figures were plotted in Sigmaplot 12.5. A Pearson correlation test was used to further explain the relationships between gas fluxes (CO2and N2O) and environmental variables (i.e., soil temperature,WFPS, SOC, NO3–, and EC).

3. Results and discussion

3.1. Tomato yield

The soil aeration and irrigation regimes influenced tomato yields, with spring-summer season yields ranging from 36.83 to 55.10 t ha–1and autumn-winter season yields ranging from 21.77 to 33.17 t ha–1(Table 2). Aeration under each irrigation regime over the two seasons increased tomato yields compared to the non-aeration treatments and the difference was significant under the full irrigation treatment (P＜0.05).This was attributed to the oxygation benefit to enhance availability of dissolved oxygen to the root system (Bhattarai and Midmore 2009), enhancing crop yields for different plant species (Bhattarai and Midmore 2009; Abuarabet al.2013; Niuet al.2013). Moreover, full irrigation during the two seasons enhanced crop yields significantly compared to the deficit irrigation treatments (P＜0.05, Table 2). Similarfindings have also been reported in other studies (Patanè and Cosentino 2010; Zhanget al.2017). These results were determined to be an effect of water stress on plant cells’ multiplication and expansion throughout all growth stages (Zhanget al.2017), in turn influencing the crop yield.

Variance analysis on aeration and irrigation treatments showed that both had a significant effect on total tomato yields (P=0.000 for irrigation,P=0.021 for aeration), while their interaction effect on total crop yield was not significant(P=0.881, Table 2).

3.2. CO2 and N2O emissions from greenhouse tomato production soils

Seasonal dynamics and cumulative CO2 emissionsSoil CO2fluxes over all treatments showed fluctuated patterns(Fig. 1-A) that varied from 8.52 to 518.76 mg m–2h–1and from 91.80 to 495.75 mg m–2h–1during the spring-summer season and autumn-winter season, respectively. The maximum total cumulative CO2emission from the soil was 14 836.07 kg ha–1for the AI2 treatment, resulting in a 4.2,29.7 and 20.4% increase compared to AI1, CK1 and CK2 treatments, respectively (Table 2).

In contrast to non-aerated soils, soil aeration under different irrigation regimes increased the cumulative CO2emissions for each of the two seasons (Table 2) and the difference was found to be significant in the autumn-winter season (P＜0.05). Other studies have seen this effect where soil respiration has been shown to increase under aerated irrigation (Chenet al.2011; Houet al.2016).However, throughout typical growing seasons, only a few measurements are conducted and thus some flux peaks might be missed. Such few measurements fail to estimate the variations of greenhouse gases accurately and could severely underestimate cumulative gas emissions (Liuet al.2010), providing inaccuracies in the CO2emissions national inventory from Chinese vegetable cropping systems.Hence, the static closed chamber and gas chromatography technique were used to systematically and continuously study the effect of aerated irrigation on greenhouse gas emissions from greenhouse tomato production soils.Previous studies had found that crop root and soil microbial respiration were the main sources of soil CO2emissions.The higher soil CO2emissions under aerated irrigation were potentially resulted from the effect of aeration on increased oxygen (Guadieet al.2014), soil microbial/enzyme activity(Liet al.2015b) and root respiration (Bhattaraiet al.2008).In addition, a slight higher soil temperature under aerated irrigation may also be responsible for the higher CO2emissions compared to non-aerated irrigation (Fig. 2).

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Fig. 1 Variation of CO2 fluxes (A) and N2O fluxes (B) for different treatments from greenhouse tomato production soils over two seasons. AI1, aerated deficit irrigation; CK1, non-aerated deficit irrigation; AI2, aerated full irrigation; CK2, nonaerated full irrigation. Solid arrows and dotted arrows denote time of topdressing for full irrigation and deficit irrigation, respectively. Bars indicate standard errors of three replications.

In our study, the cumulative CO2emissions under the full irrigation regime during the two seasons were slightly higher than those under deficit irrigation(P＞0.05, Table 2), which was consistent with other studies (Scheeret al.2013;Zornozaet al.2016). Several explanations may be given for this conclusion.First, deficit irrigation has been shown to reduce soil organic matter breakdown and enhanced SOC accumulation (Zornozaet al.2016). Second, the application of higher water quantities has been shown to increase soil enzyme activity (Zhang and Wang 2006; Wanet al.2008), serving as an indicator of microbial activity and soil biochemical intensity (Mersi and Schinner 1991).

Variance analysis on aeration and irrigation showed that aeration had a significant effect on total cumulative CO2emissions (P=0.021), while the effect of irrigation on total cumulative CO2emissions was not significant (P=0.447,Table 2). Meanwhile, the aeration and irrigation did not have a significant interaction effect on total cumulative CO2emissions (P=0.881).Seasonal dynamics and cumulative N2O emissions The soil N2O fluxes showed clear seasonal patterns (Fig. 1-B). In the spring-summer season,higher N2O fluxes were observed before 30 April, followed by relatively low N2O flux levels. In the autumn-winter season, soil N2O fluxes slowly decreased before 15 October. Peaks that occurred around 1 November 2015 were strongly ascribed to the topdressing. Additionally, soil N2O fluxes changed minimally after 10 November. Similar patterns were observed by previous researchers who concluded that fertilization coupled with wetted soil resulted in extremely high N2O emissions (Heet al.2009; Liet al.2015a; Zhanget al.2015). This short peak emission and dissipation phenomenon can be strongly attributed to the fact that soil N2O fluxes have been shown to be affected by the life cycle of plants with more and more soil available N taken up by plants as plants grew up gradually (Xiaet al.2013).

Soil N2O fluxes over all treatments ranged from 2.90 to 311.77 μg m–2h–1and from 5.47 to 136.59 μg m–2h–1over the two seasons, respectively (Fig. 1-B). The highest total cumulative N2O emission was 1.78 kg ha–1measured in the AI2 treatment, a 61.8, 95.6 and 11.3% increase compared to AI1, CK1 and CK2, respectively (Table 2).

In general, N2O is primarily produced during soil nitrification and denitrification processes (Paulet al.1993;Skiba and Smith 1993), which are highly dependent not only on the water regime, soil aeration and soil temperature but also on fertilizer inputs (Weieret al.1993). The soil oxygen content variation caused by aerated irrigation must inevitably affect the condition of nitrification and denitrification(Hwang and Hanaki 2000; Liikanen and Martikainen 2003;Bhattaraiet al.2006) and has been seen to influence soil N2O emissions (Houet al.2016). In the present study,we found that there was no significant difference between aeration and non-aeration in the cumulative N2O emissions during the spring-summer season (P＞0.05, Table 2). This might be because the soil moisture throughout the growing period was relatively low most of the time. The increased soil porosity and the reduced soil moisture due to aeration were bound to reduce soil N2O emissions. Moreover, N2O emissions in the autumn-winter season increased with increasing oxygen (P＜0.05, Table 2), which was similar to previous researches (Liikanen and Martikainen 2003;Houet al.2016). Compared to the control treatment,higher WFPS under aerated irrigation (57.6%vs. 57.0%on average, Fig. 2) during this period was beneficial to N2O production. However, the effects of the aeration regimes on N2O emissions predominantly focused on laboratory experiments (Hwang and Hanaki 2000; Castro-Barroset al.2015) and field experiments regarding the effects of different modes of aeration and irrigation combinations on soil N2O emissions have not been reported. In this study,the Mazzei air injector was used for soil aeration and the lack of knowledge about aerated irrigation technology practices could attribute to the differences observed.

The full irrigation treatment over the two seasons showed significantly increased cumulative N2O emissions relative to the deficit irrigation treatment (P＜0.05, Table 2),which was similar to the results reported by Scheeret al.(2013). The full irrigation treatment could have prompted the denitrification processes causing hypoxic conditions that in turn promoted elevated emissions of N2O from the soil. However, the significance of the treatment effects on N2O emissions varied and could be a result of different experimental conditions. Scheeret al.(2013) showed that under all treatments, the highest N2O emissions occurred following heavy rainfall, overriding the effect of irrigation treatments. The present study was conducted in a controlled greenhouse setting and was not skewed by rainfall events.

Variance analysis on aeration and irrigation showed that irrigation had a significant effect on total cumulative N2O emissions (P=0.000), while the effect of aeration on total cumulative N2O emissions was not significant (P=0.102,Table 2). Meanwhile, the aeration and irrigation did not have a significant interaction effect on total cumulative N2O emissions (P=0.931).

Fig. 2 Seasonal changes of soil/air temperature (A) and water-filled pore space (WFPS, B) under different treatments in a greenhouse tomato production system. AI1, aerated deficit irrigation; CK1, non-aerated deficit irrigation; AI2, aerated full irrigation; CK2, non-aerated full irrigation; Ta, air temperature.Bars indicate standard errors of three replications.

3.3. Environmental variables influencing CO2 and N2O emissions

Soil temperature and WFPSThe soil temperature showed clear seasonal dynamics, increasing as the air temperatures increased (Fig. 2-A). Aeration and full irrigation increased the average soil temperature slightly, but not significantly compared to non-aeration and deficit irrigation over the two seasons (P＞0.05).

The WFPS was slightly influenced by aeration and irrigation regimes (P＞0.05, Fig. 2-B). Aeration has been shown to affect WFPS through: (1) changing the soil structure due to the shrinking and movement of soil particles(Ben-Noah and Friedman 2016), inducing evaporation; (2)encouraging soil moisture uptake by the root, a process of water consumption; and (3) affecting animal, microbial and root respiration (Bhattaraiet al.2008), producing water.

Soil organic carbon, nitrate and ECSimilar patterns of SOC during each season were observed over all treatments(Fig. 3-A). Aeration and irrigation regimes did not have a significant influence on SOC (P＞0.05). SOC fluctuated from 7.43 to 9.56 g kg–1(average=8.27 g kg–1) for the AI1 treatment, from 7.31 to 8.80 g kg–1(average=8.10 g kg–1) for the CK1 treatment, from 7.75 to 8.90 g kg–1(average=8.32 g kg–1) for the AI2 treatment and from 7.51 to 8.57 g kg–1(average=8.17 g kg–1) for the CK2 treatment during the two seasons.

No significant differences in NO3–content were found between the aeration and irrigation regimes (P＞0.05,Fig. 3-B). The mean value of the NO3–contents during the two seasons for AI1, CK1, AI2 and CK2 were 118.53, 122.34 112.64 and 113.13 mg kg–1, respectively.

EC varied slightly throughout the growing tomato period(Fig. 4). The treatment effects between aeration and irrigation regimes on EC during the two seasons were insignificant (P＞0.05). In the 0–10 cm layer, the value of EC during the two seasons was, on average, 1.90, 1.87, 1.98 and 1.92 mS cm–1for AI1, CK1, AI2 and CK2, respectively.

3.4. Correlation of gas fluxes with environmental variables

Correlation of CO2 with environmental variablesThe Pearson correlation test was used to further explain the relationships between the gas fluxes (CO2and N2O) and environmental variables (i.e., soil temperature, WFPS, SOC,NO3–and EC) (Table 3). The CO2flux was significantly and positively correlated with soil temperature under different irrigation modes (Table 3, Fig. 5). This relationship between CO2emissions and soil temperature has been demonstrated to be a result of increased temperatures causing increased microorganism activities and organic matter decomposition(Tonget al.2015). Similar results were obtained at Three Gorges Reservoir area (Iqbalet al.2010), in California’s Central Valley tomato fields (Kallenbachet al.2010), in a Chinese mountainous area (Liet al.2008), as well as in the humid temperate grasslands of Japan (Wanget al.2009).Furthermore, a significant positive correlation between the CO2flux and WFPS under aerated irrigation was observed(Table 3), calling for further study to investigate the correlations between the CO2flux and other environmental variables.

Fig. 3 Seasonal changes of soil organic carbon (A) and soil NO3– content (B) under different treatments in a greenhouse tomato production system. AI1, aerated deficit irrigation; CK1, non-aerated deficit irrigation; AI2, aerated full irrigation; CK2, non-aerated full irrigation. Bars indicate standard errors of three replications.

Correlation of N2O with environmental variablesThe dependence of soil N2O fluxes on WFPS and NO3–showed a positive exponential correlation under all treatments(Table 3, Fig. 6). This trend was in alignment with numerous studies. For example, in an intensively managed vegetable cropping system, Zhanget al.(2015) found that N2O flux was significantly affected by WFPS and the NO3–content.Weslienet al.(2012) reported that N2O fluxes were positively correlated with WFPS (R=0.50) in a carrot cropping system.These findings suggested that nitrogen fertilizer application and moisture are the main factors influencing soil N2O emissions (Xuet al.2004; Gaoet al.2014). Moreover, the highest N2O flux in this study was observed at a WFPS of 58.5 and 69.3% under aerated irrigation and non-aerated irrigation, respectively (Fig. 6-A). This was in accordance with the previous findings reporting that N2O production has a maximum at WFPS of roughly 60% (Linn and Doran 1984;Schmidtet al.2000; Gaoet al.2014). Once, WFPS reaches more than 60%, the availability of O2and CO2substrates for nitrification declines due to severely restricted diffusion rates (Davidson and Schimel 1995). However, these values were higher than previous results of approximately 50%WFPS (Dinget al.2007; Houet al.2016). This could be attributed to fertilization and irrigation field management during transplantation. Abundant substrates based on base fertilizer and large amounts of water irrigated into the soil during transplantation were conducive to gas emissions.

Fig. 4 Seasonal changes of electrolytic conductivity (EC)for different treatments. AI1, aerated deficit irrigation; CK1,non-aerated deficit irrigation; AI2, aerated full irrigation; CK2,non-aerated full irrigation. Bars indicate standard errors of three replications.

Fig. 5 Correlations between soil CO2 fluxes and soil temperature under aerated irrigation and non-aerated irrigation among the two seasons. T, soil temperature. ＊ indicates significance at the 0.05 level.

Table 3 Pearson correlation coefficients between gas fluxes and environmental variables under different irrigation modes1)

The N2O fluxes from the soil as a function of the soil temperature at a depth of 10 cm was normally distributed,with 33.6% of the total N2O emitted within 16–23°C under all treatments (Fig. 7). Similar results were obtained by Schmidtet al.(2000) and Gaoet al.(2014). Furthermore, N2O fluxes peaked under all treatments when the soil temperature was approximately 18°C (Fig. 7). An exponential positive correlation was observed between the soil N2O fluxes and soil temperature when the soil temperature was below 18°C,while a linear negative correlation was observed when the soil temperature exceeded 18°C (P＜0.01).

Fig. 6 Correlations between soil N2O fluxes and WFPS (A) and between soil N2O fluxes and NO3– (B) under aerated irrigation and non-aerated irrigation among the two seasons. AI, aerated irrigation; CK, non-aerated irrigation. ＊＊ indicates significance at the 0.01 level.

Fig. 7 Correlations between N2O fluxes and soil temperature under aerated irrigation (A) and non-aerated irrigation (B) among the two seasons. T, soil temperature. ＊＊ indicates significance at the 0.01 level.

4. Conclusion

Our results showed that aeration under full irrigation over the two seasons significantly increased tomato yields by 21.9%on average. Soil CO2and N2O emissions were significantly increased by aeration during the autumn-winter season. Full irrigation over the two seasons increased tomato yields and soil N2O emissions significantly. Moreover, soil temperature was the primary factor influencing CO2emissions, while soil temperature, moisture and NO3–were the primary factors influencing N2O emissions. Our results provide a theoretical foundation and scientific basis for evaluating aerated irrigation techniques on farmland soil ecology. Irrigation coupled with particular soil aeration practices may offer a desirable balance between crop production yields and greenhouse gas mitigation in greenhouse vegetable fields.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51309192), the National Key Research and Development Program of China (2016YFC0400201)and the Fundamental Research Funds for the Central Universities, China (Z109021510).

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