Dear Mofijul,

Surface broadcasted urea may contribute to faster and greater release of NH4 + and greater potential for NH3 volatilization losses (relative to deep-placed Urea).  As such (if NH3 losses were lower from deep-placed urea), there may be relatively more mineral N available for denitrification in the ‘deep-placed urea’ treatment than under ‘surface broadcasted’ urea treatment.  Therefore, there may be a possibility to observe more N2O emissions from the ‘deep-placed urea’ treatment than from the ‘broadcasted urea’ treatment.

A classical example for potential ‘trade-offs’ between different loss pathways for applied N.  We have seen evidence from other research, when we try to mitigate one loss pathway of applied N, increases from other loss pathways.

guess you have reducing conditions under d.eep placement

Dear Mofijul,

In rice fields, even under intermittent irrigation, soil at deeper layer gets reduced unlike oxidized surface soil. And denitrification takes place in the reduced soil. Therefore it is no wonder that you got more nitrous oxide from deep placed urea than from urea broadcast on the surface soil.

Dear Mofijul

Water logged soil pores are susceptible to anoxic conditions. Surface soil is more exposed to atmospheric oxygen even under flooded conditions where oxygen can be available from the water layer exposed to the atmosphere.

Denitrification occurs at low oxygen levels where heterotrophic denitrifiers use oxidised N species (i.e. NO3, NO2, NO and N2O) as oxygen sources for respiration. Your applied urea has to be hydrolysed to become ammoniacal-N before oxidised to NO2 or NO3 forms (nitrification). Urea hydrolysis can occur under most conditions and reasonably rapid. However, nitrification is strictly an aerobic process which requires O2. Without nitrification there won't be any N2O emissions. It appears your intermittent irrigation appears to promote NO3 generation from applied urea. You are observing more N2O from deep placement because of intermittent anoxic conditions are conducive for denitrification. Another possibility is N2O reduction to N2 may be low under your experimental conditions.

Kind regards

Selva

Maybe your N2O-formation mainly stems from the denitrification-pathway. Here reductive conditions and available carbon-sources will promote denitrification. Maybe you have data of carbon-sources (DOC, TOC, etc.) at hand? It would also be interesting to see the amount of available NO3-N in these deep-layers.

Dear Mofijul,

I am not aware what was N2O sampling procedure in your study. If N2O was sampled just above the urea placement zone, then obviously it should give higher values for urea placement when compared with urea applied by broadcast method. This is because of higher nitrate available for denitrification per unit soil volume in the urea placement zone. Did you also sampled N2O from soil zones other than the urea placement zones ?   

Measuring frequency or time may be the answer of your question. Surface placement may probably result in gradual N2O emmission because a couple of cm of the water surface has higher O2 content. Therefore after fertilization continous N2O emmission occurs in the oxidative water layer at relatively small amounts. Deep placement can however cause higher N2O emmision during non irrigated period due to continous upword movement of soil solution which result in upward movement of N and oxidation in the surface. It would be better to increase measuring frequency and evaluate the cumulative emmision in both treatments.

Processes that control N2O production in soil include those that regulate nitrification and denitrification, such as available carbon, inorganic N, and oxygen as affected by soil moisture, porosity, and aggregate structure. Studies indicate that management practices that can influence emissions of N2O from agricultural soils include fertilizer N rate, type, timing and application method, crop, tillage, residue management, soil moisture content, irrigation, soil and climatic condition and. Both low and excess soil moisture affect N2O emission. N2O is predominantly emitted in agricultural soils through the microbial transformations of inorganic N, the potential to produce and emit N2O increases with the increasing availability of N.

 However, information about the influence of application methods (broadcast and deep) on N2O emission seems limited. In fact, a split application is a preferred method in terms of N-use efficiency, economic profitability, reducing N2O emission and sustainability. Overall, anthropogenic activities that lower the input of N into cropland agriculture, or reduce N availability can reduce emissions of N2O, but the effect of this reduced N input on crop yields needs to be carefully monitored in order not to compromise high crop productivity.

Interesting discussion . Here is an interesting piece of work , the abstract mentioned below throws some light in the context of on-going discussion..

Abstract :Urea deep placement (UDP) increases nitrogen use efficiency (NUE) in lowland rice fields by reducing ammonia volatilization, surface runoff and increasing nitrogen uptake. However, its effects on N losses as nitrous oxide (N2O) and nitric oxide (NO) are not yet clear. We conducted field experiments at two locations of Bangladesh — Bangladesh Agricultural University (BAU) and Bangladesh Rice Research Institute (BRRI) — to determine the effects of UDP vs broadcast urea on N2O and NO emissions from rice fields.

N2O and NO emissions were measured from three N fertilizer treatments (control [0 kg N/ha], UDP, broadcast urea) using automated gas sampling and analysis system continuously for three rice growing seasons — Aus (May–Aug), Aman (Aug–Dec) andBoro (Jan–May). Urea was applied as 2–3 split application, while for UDP treatment, urea briquettes were deep placed (7–10 cm depth) between 4 hills of rice at alternate rows to meet recommended N rates in a single application. Treatments were arranged in a randomized complete block design with three replications and N2O and NO measurements were done at every three-hour interval.

N2O emissions were sporadic and event specific. Peaks in N2O emissions were observed after broadcast application of urea, during dry period and after re-flooding of the dry soil. For the rest of the time during the rice-growing season, emissions were very low to negligible. However, across the rice-growing seasons, UDP significantly (P 

Dear Mahmood.

Thanks. You are ok. We considered both placement zone and non placement zone. Our camber area was 0.146 sq m and it covered both zone. However we got higher emission from deep placement compared to broadcasting PU in AWD condition. We measured nitrous oxide and NO emission in an automated continuous measurement system from rice field. Higher emission  under AWD system might be prolonged N supply from deep placement than PU and higher nitrification-denitrification when condition is favorable.

Dear Anoop Kumar

Thanks. I am also one of the author of the paper that you attached. This paper describes nitrous oxide and NO emission under continuous flooding condition. My query was why higher emission from deep placement than broadcasting PU under AWD condition?  

One of the prime reasons , i could see is the development of some kind of anoxia , the reduced conditions ( Could be evident from measuring redox potential ) compared to broadcast method ...

Dear Mofijul,

Surface broadcasted urea may contribute to faster and greater release of NH4 + and greater potential for NH3 volatilization losses (relative to deep-placed Urea).  As such (if NH3 losses were lower from deep-placed urea), there may be relatively more mineral N available for denitrification in the ‘deep-placed urea’ treatment than under ‘surface broadcasted’ urea treatment.  Therefore, there may be a possibility to observe more N2O emissions from the ‘deep-placed urea’ treatment than from the ‘broadcasted urea’ treatment.

A classical example for potential ‘trade-offs’ between different loss pathways for applied N.  We have seen evidence from other research, when we try to mitigate one loss pathway of applied N, increases from other loss pathways.

Dear Jayasundara

Thanks. I agree with you. for your kind information I got less NH3 volatilization from deep placement than broadcasting.

Please look at the variation in redox potential of at surface and sub-surface depths to ascertain such differences . This could be useful..

Dear Mofijul

The less amount of NH3 volatilization from UDP might explain the higher amount of N2O emission (through denitrification). Dr Susantha explained this clearly. 

The variations in redox potential also be responsible as Dr Anoop mentioned.