Effects of urea types combination with alternate wetting and drying irrigation on rice yield in the Vietnam Mekong Delta

A field experiment was arranged with a split-plot design with three replications, two irrigation methods including continuous flooding (CF) and Alternate Wet Drying (AWD) combination with three amounts of two fertilizers, nitrogen White Urea and Black Urea. Actual yield, yield components, leaf N content, and chlorophyll index were investigated. The results showed that AWD application saved 9.5% of freshwater. Compared with FC, OM18 did not respond to AWD combination with white and black urea. Applying 100 or 120N of both nitrogen types gave better performances in panicles, filled grain, and yield under both FC and AWD. Regarding OM 18 yield, 100N (Black Urea)– 80 P2O5 – 60K2O in CF gave the best yield (8.0 tons/ha), but this treatment showed non-significant differences from 120N(Black Urea) – 80 P2O5– 60K2O in AWD as well as 120N (Black Urea) – 80 P2O5 – 60K2O in CF (7.2 tons/ha). OM 18 responded to Black Urea better than White Urea under AWD and FC. OM 18 showed the best yield in 100N(Black Urea) – 80 P2O5 – 60K2O under CF. AWD application saved up to 9.5% freshwater.


Introduction
Rice is the staple food for over 50% of the world's population.In 2020, Vietnam ranked second in rice export worldwide, while the Vietnam Mekong Delta (VND) is referred to as the country's rice bowl, providing more than 50% of rice production for domestic consumption and export.Unfortunately, this area faces two main issues: water shortage due to climate change and degraded soil quality by rice intensification.
Fertilizer plays a vital role in increasing rice yield.Of the three macronutrients N, P, and K in the Mekong Delta, N fertilizer increases yield by about 40-45%, phosphate contributes about 20-30%, and potassium contributes about 5-10% [1].However, the effectiveness of nitrogen application on rice is low, less than 50% [2], with an estimated rank of 20-40 kg NH3-N/ha in rice soils [3].According to [4], high urea fertilizer increases nitrogen loss.Recently, a new nitrogen fertilizer type, black urea, was produced.This type adds more essential elements such as SiO2, CaO, MgO, and 1.2 % organic matter.These would be useful for crops grown on degraded soil, like in the practices in the VMD.
Regarding water saving under climate change, alternate wetting and drying (AWD) is a helpful irrigation method for any freshwater shortage.In addition, applying AWD could increase root growth, facilitate higher nutrient uptake, and improve land and water use efficiency [5].However, previous results show that different trials led to varying results in the effectiveness of AWD in rice yield and water-saving levels for various reasons such as variety, soil type, cultivation technique, etc. Combining AWD with black urea in An Giang province seems to be a potential solution to sustain rice production.Consequently, it could improve farmers' income and protect water resources from climate change.
Water control: The width of the bunds between sub-plots was 0.3 m, and when bordering the irrigation canals, are 0.5 m.The irrigation canals have a width of 0.5 m.Plastic film was installed around AWD plots' bunds to control water loss from horizontal seepage.
Water management: The field water tube used to monitor the water depth in the field was made of a PVC tube, a diameter of 15cm, 20cm in height, with plenty of holes of 2mm in diameter to ensure that water can go into the tube throughout.The tube was installed into the soil ground equal 15 cm in depth in three AWD plots, and the soil inside the tube was removed.The water level was recorded daily by a ruler.AWD treatment was started at 20 DAS.After irrigation, the water depth gradually decreased when the water level dropped to about 15 cm below the soil surface, and irrigation was applied to re-flood the field to a depth of about 5 cm.The water level in CF treatment was always maintained at five cm in height from the soil surface.

Amount of irrigated water
During rice crop season, six irrigation times were applied to the AWD treatment (fig 3) to get the amount of water used from AWD and FC were 3,400 m 3 /ha and (3,757 m 3 /ha).In comparison, AWD could save up to 9.5% of water amount to CF (Figure 4).+ Index of chlorophyll on leaves: although there was little variation in the chlorophyll index between fertilizer dosage, fertilizer type, and water management, in general, there was no statistically significant difference in leaf SPAD among treatments at 20, 40, and 60 DAS stages.Similarly, no considerable difference in leaf nitrogen content among treatments was found at three sample times.The nitrogen content of leaves in the 60 DAS stage tended to be lower than in the previous period (Table 1).

Effect of N fertilizer formulation and water management method on yield components of rice
The number of panicles/m 2 : The number of panicles/m 2 ranged from 355.2 to 398.0.AWD_F4, CF_F5, and AWD_F1 treatments had the highest number of panicles/m 2 with 398.0, 394.7, and 393,3, respectively.However, the panicle number of these treatments showed a non-significant difference compared to AWD_F5 (382.3),CF_F4 (382.7), and CF_F6 (387.3).The treatment with the lowest number of panicles/m 2 was CF_F3 (355.3).
The number of filled grains/panicle and weight of 1,000 grains: Non-significant differences in the number of filled grains/panicle and weight of 1,000 grains among the dosages, type of nitrogen fertilizer, and irrigation methods were found.
Actual yield varied, ranging from 5.3 to 8.0 tons/ha.The group of treatments gave the highest values, including CF_F2 (8.0 tons/ha); AWD_F4 (7.2 tons/ha), and CF_F4 (7.2 tons/ha).The treatments with the lowest actual yield were AWD_F6 (5.3 tons/ha) and CF_F3 (5.4 tons/ha).A non-significant difference in actual yield between FC and AWD was found compared to the irrigation method.However, under FC condition, the actual yield of CF_F4, CF_F5, and CF_F6 were considerably higher than CF_F1, CF_F2, and CF_F3 (Table 2).
Thus, treatments under AWD conditions did not affect the yield of rice variety OM18 compared with CF treatment.Among the N fertilizer formulations, two fertilizers containing 120 N -Urea black and 100 N -Urea black gave the highest theoretical yield in the treatments of 100N (Urea black) -CF, 120N (Urea black) -CF and 120N (Urea black) -AWD.

Water saving efficiency
The results showed that using AWD saved 9.5% compared to CF, quite low compared to others.Previous studies showed that, water saving ranged 20-50% [5][6][7][8][9][10][11].Another result demonstrated that water saving was quite variable, i.e water saving about 9 -19% [12].This result is slightly consistent with our result.A question should be raised here is why we got various ranges of water saving.Our trial, could explain why we could not control horizontal water seepage between our experimental area and the surrounding fields of farmers (Figure 1).Water-saving efficiency could increase by over 9.5% if AWD is conducted in large areas under controlled water irrigation.
Effect of combination of fertilizer dosage and water management on the relationship between leaf nitrogen content and leaf chlorophyll index.The chlorophyll index is a constant indicating the nitrogen nutritional status of rice leaves [13].It is consistent with the result from [14] that showed a positive relationship between chlorophyll index and N content and rice leaf because the nitrogen element is a constituent of chlorophyll [15].Table 1 shows no relationship between leaf nitrogen content and chlorophyll index nor between leaf nitrogen content nor chlorophyll index and nitrogen amounts.The nitrogen uptake of rice from two types of Urea at three dosages (80, 100, and 120N) was the same.In other words, applying 80-120 kg.ha -1 of Back and White Urea on the OM18 variety did not positively affect its leaf nitrogen content and chlorophyll index under both FC and ADW.However, a previous result showed that AWD increased nitrogen mineralizing in the soil [16].However, our result is consistent with another result that showed a non-significant difference in rice nitrogen uptake between AWD and CF [17].

Effect of N fertilizer type and its dosage combination with irrigation method on yield components and yield of rice variety OM18
Filled grain number per panicle and 1000-grain weight: according to [18,19], nitrogen is important in contributing to the Filled grain number per panicle.Applying nitrogen enough at stages panicle initiationbooting stage of rice is crucial to ensure maximum filled grain number per panicle.Regarding grain weight, this parameter depends on genetic characteristics rather than others [20].So it is unsurprising that there is no significant difference in grain weight of 1000 among treatments from our trial (Table 2).However, there was not a considerable difference in filled grain number per panicle; a question should be raised here whether the amount of white and black Urea was enough for OM18 or not.The maximum nitrogen amount recommended for rice is about 120N per ha.It is deduced that other factors were uncollected from our trial to intervene in filled grain number, although nitrogen is admitted as the significant contribution of this parameter [18].
Actual yield shows the same trend with theoretical yield: CF_F5 gave the best yield compared to others except for AWD_F4 and CF_F4.CF_F5, AWD_F4, and CF_F4 treatments were applied to Black Urea.It could be explained that SiO2, CaO, MgO, and organic matter from black urea cause higher panicles number/m 2 , filled grain ratio, and 1000 grain weight (Table 2).Consequently, the yield in CF_F5, AWD_F4, and CF_F4 was higher than others.Nutrients play an important role in rice growth.Some are Silicon, Calcium, Magnesium, and Organic matter [21][22][23][24].In order, Silicon increases the thickness of the cell wall to help the rice plant to be hard, pest and drought-tolerant [21], soil pH neutralization, and stabilize the aggregate structure of some soils [22,23].At the same time, magnesium (Mg) also plays an essential role in maintaining good quality soil and sustainable crop production [23].Another factor is organic matter that increases fertility, as indicated by the increase of C, N, and other nutrient elements in the soil; the increase of water-stable aggregates and cation exchange capacity (CEC); and the increase of biological N-fixing activity as well [24].Generally, rice yield in Black Urea treatments under CF and AWD is higher than in White Urea ones.However, the effect of the Black Urea on yield under CF conditions is more evident than White Urea's.

Conclusion
Applying AWD saved up to 9.5% fresh water.OM18 performed well on panicles and filled grain, yielding higher nitrogen treatment (100N or 120N).The Black Urea showed its effects on yield and yield component of OM18 better than White Urea under both CF and AWD.

Figure 2 .
Figure 2. Field water tube used to monitor the water level 3.2 Data collection -Amount of water: accumulation of water irrigated during rice crop was calculated -SPAD index: SPAD meter -Rice height: from soil face to tip of the highest leaf -leaf protein content: applying the Kjeldahl method -Yield components: three frames of 1m 2 was randomized in a plot.-Actual yield: whole plot (25 m 2 ) -Data analysis: SPSS 21.0 software was used to analyze variance (ANOVA) and Duncan's multiplerange test.

Figure 3 .
Figure 3. Changes of water level in the field (cm) from the AWD treatment

Figure 4 .
Figure 4. Amount of irrigated water in AWD and CF treatment

Irrigation methodTable 1 .
Leaf SPAD index and nitrogen content at several stages Note: DAS: Date after sowing; AWD: Alternate Wet Drying irrigation; CF: continuous flooding irrigation; ns: non-significant difference.4.2 Leaf chlorophyll index and leaf N content response to fertilizer type, fertilizer amount and water management.

Table 2 .
Effects of type and amount of nitrogen fertilizer combination with irrigation method on yield components and yield of rice