This document discusses nitrogen use efficiency in rice crops. It defines nitrogen use efficiency as the efficiency with which applied nitrogen is converted to grain nitrogen. It then describes several metrics for measuring nitrogen use efficiency, including agronomic efficiency, physiological efficiency, apparent recovery efficiency, and utilization efficiency. The document also discusses various management practices that can improve nitrogen use efficiency for rice grown in both lowland and upland conditions, such as optimizing fertilizer rate and timing, use of controlled release fertilizers, and adoption of nitrogen efficient rice varieties.

1. NITROGEN USE EFFICIENCY
Nitrogen use efficiency is an important index in determining how the
applied N was used by rice crop. Hence, knowledge of this plant
parameter in fundamental in improving N use efficiency And
consequently in Nitorgen management. It is defined as the efficiency
with which soil nitogen ( nitrate or ammonium form ) is converted
intograin Nitorgen. The soil Nitrogen comes from fertilizer , crop
residues, soil organic matter and manures but it is the efficiency of
conversion of fertilizer into grain that is generally of greatest concern to
the growers.
Definitions of NUE in crops and cropping systems includes Agronomic
Efficiency , Physiological Efficiency, Agro physiological Efficiency ,
Apparent Recovery Efficiency and Utilization Efficiency.

2. 1. Agronomic Efficiency – Expressed as Economic production obtained per Unit of nutrient applied.
AE = Gf – Gu / N ,
Where , Gf = Grain yield of fertlized plot
Gu = Grain yield of unfertilized plot
And N = Quantity of Nitrogen applied.
2. Physiological Efficiency – Expressed as biological yield obtained per unit Of nitrogen uptake .
PE = Bf – Bu / Nf- Nu ,
Where , Bf = Biological yield from fertilized plot
Bu = Biological yield from unfertilized plot
Nf = Nitogen uptake from fertilized plot
Nu = Nitogen uptake from unfertilized plot .
3. Apparent Recovery Efficiency - Expressed as quantity of nutrient uptake per unit of nutrient applied. Most accurately
reflects the NUE as it gives the ratio of output N to input N indicating how well the given N management strategies
performs in recovering the applied nitogen .
ARE (%) = ( Nf – Nu / Na ) × 100
Where , Nf = Nitrogen uptake ( Grain+ straw ) from fertilized plot
Nu = Nitrogen uptake ( Grain+ straw) from unfertilized plot.
Na = Quantity of Nitrogen applied.
4. Utilization Efficiency – Product of physiological and Apparent Recovery Efficiency .
UE = PE × ARE

3. Ways to increase NUE :
1. Adoption of strategies that increase crop N demand and uptake by genetic improvements .
2.Adopting management options that influence the availability of soil and fertilizer nitogen for plant uptake.
3. By achieving Balanced fertilization combined with other practices ( E.G – Improved varieties , water management, and
plant protection) . For cereals ratio of N:P:K – 4:2:1 and for legumes it is 1:2:1. It is advisable to apply the nutrients
accordingly.
4. Another method includes soil and plant analysis for nutrients and constantly monitoring growth and development and
applying amount of nutrients that correspond as precisely as possible to crop needs and growing conditions.
5. By choosing the most suitable type and rate of N fertilizer and the most appropriate application technique .
6 . By using special types of fertilizers i.e New N forms , slow and controlled – release fertilizers that avoid , or at least reduce
N losses and various forms of site- specific N management.
7. By applying fertilizer according to the prevalent climatic conditions of the area , inherent N status of field and cropping
system of the field.
8 . By applying split doses of nitogen during the growing season , NUE can be improved.
9. By applying nitogen fertilizer at right place according to the fertilizer properties. Foe E.g – Anhydrous ammonia must be
injected into the soil deep enough to seal the gas from being lost to the atmosphere.
10. By adopting Integrated Nutrient Management approach i.e Using organic manures , crop residues , Biofertilizers , Green
manures etc.

4. Management of Fertilizer N in Lowland
Conditions :
Lowland rice ecosystem is different compared to upland rice. The anaerobic soil environment created by flood-irrigation of
lowland rice creates a unique and challenging environment for the efficient management of soil and fertilizer nutrients
(Fageria et al. 2003a). Main N management practices, which can improve its uptake and use efficiency, are as follows :
1. Adequate Rate of N fertilizer: Nitrogen is a dynamic and mobile nutrient in soil plant-systems. Majorpart of the N in the
soil is present in the organic form. Mineralization of organic N depends on microbial activity, which is influenced by
environmental factors. Further, there are many sources of addition and loss pathways of N in soil-plant system, which
complicate more its balance and use by plants. When N is applied at recommended rate to crops, N use efficiency is
higher and N losses are minimum. When N is applied at higher rates than those are necessary for maximum economic
yield, N accumulates in soil profile and losses are higher .
2. Timing of Nitrogen Application : The N is lost from soil-plant system via volatilization, leaching, denitrification, or runoff
(Fageria and Baligar, 2005; Fageria et al. 2006). This suggests that there is more N available for loss at any time during crop
growing season if N is applied only once during crop growth. Hence , splitting of N fertilizer applications during crop growth
can reduce nitrate leaching and improve N use efficiency. It has been observed that Agronomic Efficiency of in lowland rice
was higher when N is applied in three split doses ( One – third at sowing , one – third at active tillering and one – third at
panicle initiation. Nitrogen harvest index and N use efficiency were also lower for part of the N applied late or at booting and
flowering compared to N applied at early or in the vegetative or initiation of reproductive growth stage .

5. 3. Source and Method of Nitrogen application – In transplanted rice systems, N fertilizer is applied to the soil surface and
mechanically incorporated before the field is flooded, applied to the soil surface and incorporated by the flood water, or
injected into the soil. Generally urea and ammonium sulphate are the comman sources of Nitrogen fertilizers used by farmers.
Nitrogen fertilizers are broadcast and mixed into soil before a crop sowing. Fertilizers, mixed into soil or injected into subsurface
are more efficient methods of N application compared to broadcast and left on the soil surface.. Placement of ammonical
fertilizers deep (15cm) in the anaerobic or reduced layer of flooded rice is an important strategy to avoid N losses by nitrate
leaching and denitrification. Use of ammonical fertilizers instead of nitrate fertilizer also prevents losses and increase efficiency.
4. USE OF N EFFICIENT GENOTYPES :Utilization of plant species or genotypes of same species efficient in absorption and
utilization of N is an important strategy in improving N use efficiency and sustainable agricultural system. Genotypes that
produce well even at low soil Nitogen content are selected. Also , the genotypes that respond well to the applied nitrogen are
selected . Thus, adoption of Efficient Nitrogen genotypes helps in reducing cost of production and improving yield.
5. Slow release Fertilizers : Slow release N Fertilizers , holds the N until plants actually require it. Thus , these can reduce N
losses and enhance NUE , decrease the labour costs and protect the environment. The application of SFRs can potentially
decrease the fertilizer use by 20-30 %. But being expensive than other conventional N Fertilizers , they aren’t very comman .
In recent decades, controlled-release N fertilizers have been developed that consist of highly soluble urea prills or granules
coated with water-insoluble materials like sulfur or polyolefin that control the rate, pattern, and duration of N release
(Shaviv 2001). Example : Neem coated urea , sulphur Coated urea , Polymer Coated urea , urea supergranules.
6. Nitrification inhibitors : Nitrification inhibitors delay the bacterial oxidation of ammonium ion by depressing the activity of
Nitrosomonas bacteria in soil for a certain period of time . The objective of nitrate inhibitors is to control the loss of nitrate by
leaching and denitrification keeping the N in ammonium form longer and increasing NUE . Comman Nitrification inhibitors are :
Nitrapyrin , Carbon disulphide , Ammonium Thiosulphate etc.
7. Urease Inhibitors : In flooded rice systems, NH4+-N originating from hydrolyzed urea accumulates in floodwater and is prone
to NH3 volatilization due to elevated pH of flood water during daylight hours and increased temperatures. In flooded rice
systems, N losses via NH3 volatilization have been recorded in the range of 20–56 % of applied N (De Datta et al. 1989; Fillery
and De Datta 1986). The use of urease inhibitors to reduce NH3 volatilization from urea hydrolysis has emerged as an effective
strategy to increase N-use efficiency of urea-based N products in rice Urease inhibitor NBPT [N-(n-butylthiophosphorictriamide]
has been reported to significantly reduce NH3 volatilization losses In lowland rice . Urease Inhibitors inhibits the hydrolytic
activity of enzyme Ureasr for 2 weeks or more depending upon soil and climatic conditions.

6. Management of N Fertilizers in Upland
conditions :
In upland rice cultivation, rice field is not flooded resulting in different conditions than Lowland cultivation. Therfore, different
management methods are used to increase NUE and these are as follows :
1. Proper rate and Time of N application : The time of application of split doses of fertilizer N varies because critical growth stages
of rice under different systems occur at different times. Nitrogen fertilizer is typically applied in at least two or more split doses in
most of the rice production systems. The first application is often termed as basal dose. Sometimes the basal application
represents the largest portion (50–75 % of the total N requirement) of N fertilizer applied in any single application during the
growing season. At some places , the basal dose is applied during early vegetative growth stage followed by the application of
irrigation. Nitrogen fertilizers should be applied at the optimum required rate because excessive N may vause toxicity and losses
which decrease NUE.
2. Source and Method of application : Options for fertilizer application include : Surface broadcast , Band application or foliar
application. The right method depends upon the characteristic of the fertilizer material being applied. Example- Broadcasting of urea
leads to volatalization losses and decrease NUE. Therefore, it a advisable to go for band placement . The right source must consider
any nutrient interactions or compatibility of crops , sensitivity of crops to the souce etc. Efficiency of some products may be reduced
due to leaching of nitrates or volatalization of ammonia under certain temperature and moisture situations. Therefore, the right
source may vary with the crop , soil properties and methods of application.
3. Slow release Fertilizers : Many times low N-use efficiency in rice is observed due to lack of synchronization between crop demand
and release of N from water-soluble sources like urea. Application of soluble N fertilizers to rice in split doses is an attempt to
increase the degree of synchronization between supply and demand of N, but there is a limit to achieve it. In recent decades,
controlled-release N fertilizers have been developedthat consist of highly soluble urea prills or granules coated with water-insoluble
materials like sulfur or polyolefin that control the rate, pattern, and duration of N release.
Examples : Neem coated urea increase the yield by 6-11%in rice , Sulphur coated urea is highly efficient soyrce of N but it is not
popular in India due to high cost of sulphur in India .

7. 4. Nitrification Inhibitors and Urease Inhibitors : Nitrification inhibitors are chemicals that when added to N fertilizers and
applied to soil, delay the transformation of NH4+ to NO2− by inhibiting or at least by slowing the action of Nitrosomonas spp.
bacteria. Many compounds that can inhibit nitrification have been identified . Some of them are – 2-chloro-6-(trichloromethyl)
pyridine (Nitrapyrin) with the trade name “N Serve”, (ii) dicyandiamide (DCD, H4C2N4), and (iii) 3,4-dimethylpyrazole phosphate
(DMPP). Even when urea is applied during nonflooded periods, applied N can be lost via NH3 volatilization in substantial
amounts. The use of urease inhibitors to reduce NH3 volatilization from urea hydrolysis has emerged as an effective strategy to
increase N-use efficiency of urea-based N products in rice. Urease inhibitor NBPT [N-(n-butyl) thiophosphorictriamide] has been
reported to significantly reduce NH3 volatilization losses due to urea application to rice .
5. Site specific Nutrient Management : Among many factors that influence N-use efficiency, one potentially important factor is
the uncertainty faced by farmers in deciding the amount of N fertilizer to be applied (Lobell 2007). This uncertainty can be
reduced by knowing the N supplying capacity of the soil. Another reason for low N-use efficiency is the inefficient splitting of N
applications and use of excess N than the required. An important element of site specific N management revolves around the
development and use of diagnostic tools and models that can assess the real N needs of crop plants . The instruments used in
Site specific Nutrient Management includes chlorophyll meter, Leaf colour charts , remote sensors Digital imageries etc.
6. Integrated Nutrient Management : It is another important approach to enhance NUE .The use of organic manures , green
manures, Biofertilizers, sewage sludge and food industry wastes helps in increasing NUE in upland conditions. Legumes being a
dual crop has the capacity to fix atmospheric nitrogen and should be included in cropping system .
7. Nano fertilizers : A nano fertilizer is a product that is made with nanoparticle (100 nm ) Or use nano technology to improve
NUE . Due to small surface area , these exhibit high CEC , High absorption efficiency which in turn will increase NUE. For Example
– Urea supergranules reduces volatalization losses , also inhibits Nitrification to some extent.

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