1. Summary of Paper Submitted to Journal of Soil Science and Plant Nutrition
Japan, 2017

2. Impacts of alternate wetting and drying on greenhouse gas emission from paddy field in Central Vietnam
Dang Hoa Tran, Trong Nghia Hoang, Takeshi Tokida, Agnes Tirol-Padre, Kazunori Minamikawa

3. Progress Submitted to the Journal: 16 May 2017 Reviewed: 2 August 2017
Reviewer response: on going; 15 September 2017

4. Introduction Global warming: big issue for the human
Major attributor: C02, CH4, N20 emissions from land use including agriculture
Rice cultivation: a major CH4 source

5. CF is a traditional rice cultivation, enhance CH4 emission
AWD: reduced water input, kept grain yield, reduced CH4 emission

6. The world’s fifth largest rice producer
Rice cultivation is the largest GHG source in agriculture. CH4 emission from paddy fields was estimated to be 50.5% of the agricultural GHG emission
AWD adoption: contribute to the reduction of GHG emission
Some researches on GHG emission from paddy fields in the Northern and Southern areas were reported
Limited data for Central region
Vietnam

7. Objectives
To establish the baseline GHG emission from a paddy field in Central Vietnam
To investigate the feasibility of AWD in term of GHG emission, rice productivity and water use.

8. Material and method

9. Site and experiment description
16˚28’16’’N; 107˚31’26’’E
- Huong An commune, Huong Tra district, ThuaThien Hue Province, Central Vietnam during six consecutive cropping seasons from 2013 to 2016

10. Experimental layout R 1 R2 R3 CF AWD AWDS
Soil: classified as Fluvisol (Table 1)
Area: 30 m2(5 m x 6 m)
Bank: 30 cm
Harvest area: 5 m2

11. Table 1.Soil properties of the experimental site
Property
pH (H2O)
4.18
Total carbon (%)
1.92
Total N (%)
0.19
Total P2O5 (%)
0.013
Total K2O (%)
0.40
Available P2O5 (mg 100g-1)
3.50
Available K2O (mg 100g-1)
2.40
Texture
Loam
Clay (%)
17.8
Sand (%)
49.3
Silt (%)
32.9
Bulk density (g cm-3)
1.01
Active Fe (%)
1.23
Active Mn (%)
0.0027

12. Treatments CF: Conventional flooding, floodwater depth 3 - 5 cm
AWD: Safe AWD (Irrigate when water level is at 15 cm below soil surface)
AWDS: Site specific AWD (flexible, AWD that is more adapted to specific site)

13. Table 2. Rice cropping calendar in the winter-spring (WS) and summer-autumn (SA) seasons
DAS, days after sowing.
In parentheses for chemical fertilizer, the application rates of urea (kg N ha-1), super phosphate (kg P2O5 ha-1), and potassium chloride (kg K2O ha-1).

14. Measurements Closed chamber Base - Diameter: 50 cm, - Height: 30 cm
Top chamber has Volume: 120 litters; height: 70cm
Thermometer, fan

15. Measurements Gas sampling: - Weekly in mid-morning (8:00-10:00 AM).
1, 2, 3, 4, and 5 days after nitrogen (N) fertilizer application.
The gas samples were collected using a 60-mL syringe fitted with a stopcock at 0, 6, 12, 20, and 30 min after chamber closure and used a 19-mL evacuated glass vials

16. Analysis gas sampling
Gas chromatograph (8610C, SRI Instruments, CA, USA) equipped with a flame ionization detector (FID) for the analysis of CH4 and an electron capture detector (ECD) for the analysis of N2O.
The columns for the analysis of CH4 and N2O were packed with Porapak Q (50-80 mesh) and the carrier gas was nitrogen (N2)
N2O and GWP were recorded for WS3 and SA3. Only 2 years (4 seasons) data

17. Statistical analysis
ANOVA using a split-plot design, where cropping season (CS) was treated as the whole-plot factor and treatment (water management: CF, AWD, AWDS) as the split-plot factor, with three replications.
Using the Box-Cox transformation was conducted for CH4, N2O, GWP and yield-scaled GWP using the "powerTransform" function in the "car" package of R.
To test differences among water managements, Tukey's HSD test was performed with a significance level of 0.05.

18. RESULTS

19. Fig 1. CH4 and N2O emission in Winter – Spring season

20. Fig 2. CH4 and N2O emission in Summer – Autumn season

21. Table 3. Seasonal CH4 and N2O emissions, GWP, rice grain yield, yield-scaled GWP, total water use, and water productivity as affected by cropping season and water management

22. Discussion The magnitude of GHG emission in this site
The magnitude of CH4 emission observed in our study was relatively high ( kg CH4 ha-1)
The application of organic fertilizer may have enhanced the CH4 emission irrespective of treatment
The CH4 emission was variability between WS and SA in Vietnam, although not statistically significant due to difference in air temperature and short fallow period between WS and SA
The N2O emission played a negligible role in terms of the GWP of CH4 and N2O because of the relatively large CH4 emission in this site.

23. Effects of cropping season and water management on GHG emission
The effect of cropping season on the CH4 emission was mainly attributed to the change in rice stubble management between the first two seasons and the later four seasons
AWD is effective in reducing the CH4 emission.
CH4 fluxes did not sharply decline to be negligible level after drainage events.
Water management showed no significant effect on N2O emission

24. Feasibility and limitations of AWD in Central Vietnam
AWD did not reduce rice grain yield (rather tended to increase), and water saving was achieved as expected
The GWP of CH4 and N2O was reduced by 26-29% under AWD and AWDS compared to that under CF. AWD is a powerful tool in reducing CH4 emission.
The response of CH4 flux to it was not distinct throughout the six cropping seasons differed between AWD and AWDS.
The development of mitigation options for the GHG emission in fallow seasons will contribute to the reduction in anthropogenic GHG emission in Vietnam.

25. Conclusion
CH4 emission ranged from 500 kg CH4 ha-1 in WS to 644 kg CH4 ha-1 in SA. Rice paddy in Central Vietnam can contribute to the national GHG budget.
GWP of CH4 and N2O of AWD reduced 26-29% compared to CF
A possibility of AWD’s performance on increasing rice productivity. Thus, it will be key to spread AWD to local farmer

26. Acknowledge We would like thank:
The Ministry of Agriculture Forestry and Fisheries (MAFF) of Japan through MIRSA 2 project
Prof. Kazuyuki Inubushi (Chiba University, Japan), Dr. Reiner Wassmann, Dr. Bjorn Ole Sander (IRRI, Philippines), and Dr. Kazuyuki Yagi (NIAES, Japan) for their valuable comments

27. Thanks for your attention