Dr.Kundu and Dr,Saha good and relavent  comments. When photosynthesis/net assimilation balances respiration then there is no net emission CO2.There may some CO2 evolution during field preparation.But that may be compensated by carbon inputs made to the soil through previous crop stubles, roots,root exudates,weed  biomass incorporation during cropping period.Every year some 50% of the fresh biomass added may be decomposing during a crop  growing period.

Under intensive agriculture with high N inputs ,N2O and N2  gaseous losses may contribute to higher CO2 equivalent emissions.Under rainfed conditions unless intensive rainfall event occurs or heavy  irrigation is given and sufficint fresh organic material is avilable,the gaseous emissions of N may not be conspicuos.Indirect contribution through NH3 volatilization can also be quantified and accounted for.

Thanks sir.

Please check the following link and pdf attachments.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3783474/

In your experiment conducted under rainfed conditions and with split application of N ,the nitrogen application may  not contribute  much to global warming either through CO2 evolution or nitrous oxide emissions.However,as pointed out by Dr.Amrani,nitrogen fertilizer production,trasportation and application may generate a marginal amount of CO2 .As I understand, in agriculture, the CO2 emissions  may not contribute much to global warming but under intentive agriculture with high to very rates of N application may contribute through nitrous oxide emissions(under certain conditions).  Could you measure N2O emissions in your experiment?

I must agree with Dr Rao, N fertilization may not have a significant effect on CO2 emissions (assuming other managements like tillage and crop rotation are same). The literature has mixed response. However, under high N rate, the ecosystem respiration may increase but at the same time the net assimilation is also higher in that case, and thus gets compensated. Hence, the net C impact may not be significant when considering CO2, but CO equivalent emissions can be increased due to increased N2O emissions under high N application.

The effect of N fertilization on Greenhouse gases is 3 fold. The Carbon equivalent from a practice standpoint for a crop like maize in North America Nitrogen fertilizer would account for slightly over 50% of the Carbon equivalent based on the high energy requirement of Bosch Haber process to produce ammonia.

Of even greater importance however than direct emission which is the significant but the indirect net effect of practices on the long term soil organic matter resource.

While an agriculture system which accrues significant organic matter over time the Carbon and Nitrogen sequestration can be significant for the reduction of atmospheric greenhouse gases.   Unfortunately this positive effect is largely unutilized currently. Majority of soils have dwindled and dwindling soil organic matter resources. While natural soil organically composed of over 5% soil organic matter currently the state of most of these are near 1%. This difference is literally up in air as greenhouse gas enrichment largely. 

 The greenhouse gas role for agriculture  is first the increase in direct emissions of greenhouse gases from the inputs and secondly and more importantly to potential mobilization of soil organic matter resources generating a large potential to contribute to not only the reduction of soil organic matter critical for water relations but also acidifying the soil. 

With the problem of direct emission, indirect stimulation of reducing soil organic matter and acidification the systematic alternation of the agriculture is needed with a direction toward both the reduction of Ammonia application and the direct practices to conserve and increase soil organic matter.

Most of the worlds tillable land has lost well over half of the original soil organic matter. Implementation of agricultural systems can reverse this and is critical for long term resolution of Global climate change rooted in over enrichment of the atmosphere in Greenhouse Gases and the impoverished state of the soil on a Global level. 

If we target the simultaneous reduction in direct emissions from our production practices and focus on bottom line increases of soil organic matter the issues of global climate change and food security can be addressed but we need to change our focus from emissions from direct practice to net sequestration while reduce direct emissions. 

Before all I think should be important to specify the source of nitrogen (inorganic, urea, amino acidic Nitrogen?). I am conducting  experiments using different nitrogen and carbon sources in the presence and without plants. We are observing that using little amount of organic nitrogen we can increase organic matter content in the soil probably due to the increased activity of the soil microbes. However I think that the fertilization of soils with mineral nitrogen can increase the CO2 emission.

Dear Mr Reddy, at Iowa State University farm, application of N fertiliser in corn resulted in a reduction of both emission rate and season-long cumulative emission of carbon dioxide from soil. I suggest you to read Al-Kaisi et al (2008) J. Environ. Qual. 37(2):325-332 for the detailed results.

Dr.Kundu and Dr,Saha good and relavent  comments. When photosynthesis/net assimilation balances respiration then there is no net emission CO2.There may some CO2 evolution during field preparation.But that may be compensated by carbon inputs made to the soil through previous crop stubles, roots,root exudates,weed  biomass incorporation during cropping period.Every year some 50% of the fresh biomass added may be decomposing during a crop  growing period.

Under intensive agriculture with high N inputs ,N2O and N2  gaseous losses may contribute to higher CO2 equivalent emissions.Under rainfed conditions unless intensive rainfall event occurs or heavy  irrigation is given and sufficint fresh organic material is avilable,the gaseous emissions of N may not be conspicuos.Indirect contribution through NH3 volatilization can also be quantified and accounted for.

Dear Reddy,

This question you raised is very important.  I agree with the answers given here. There  are a number of factors that affect greenhouse gas emissions in agricultural soils. Type of soils and their management practices, cropping systems, weather condition and the rate of N fertilizer applied may have significant effects on the emissions of greenhouse gases, especially CO2 and N2O.

As others mentioned above, application nitrogen fertilizer is primarily responsible for the emission of N2O, although application rate, method, soil organic matter content and weather condition can have significant influence. For example, application of N fertilizer on soils high in organic matter because of crop residues or manure application the nitrogen may promote the microbial activities and emission of CO2 to the atmosphere. We had field experiments using different organic amendments (compost, biochar) and inorganic fertilizer on  soils  with varied soil organic matter content, the CO2 flux from the soil with high organic matter content was higher from inorganic fertilizer treated plots than the soils with low organic matter content with same treatments. So environmental, and edaphic factors as well as crop and soil management practices need to be taken into consideration.

The CO2 emission under semiarid tropical and rainfed conditions is not an issue as explicitly stated by Dr. A S Rao as to worry about. Inconsistent results obtained on response to N are really valid which should be taken note of. It is a different matter under irrigated conditions in which higher dose of basal N application would have a positive effect on CO2 production due to greater plant biomass. 

The accounting of greenhouse gas emission alone are not the issue the issue is in net sequestration or net emissions. This is generally only available by long term farming systems work which can find the accumulation or decrease of soil organic matter over time. Simply measuring the emission from soil does not give the answer as stimulated emissions can be a consequence of an environment which is capturing soil organic matter.

Thank you all respected sirs and professors for your valuable suggestions and information. Indeed it helped me a lot.

In my research I worked on effect of fertiliser N (0, 60, 120 and 180 kg N/ha supplied through urea) and tanksilt application (0 and 30 Mg/ha) as various combinations on maize crop that is grown in kharif season (June-Sept) and its residual effect on horsegram crop grown during rabi season (Oct-Feb) with respect to nutrient and water use efficiency. Simultaneously, quantification of CO2 and N2O emissions from entire cropping sequence through periodical collection of gas samples (possibly weekly).

If possible suggest me possible outcomes (generalised) of the research work conducted with respect to GHGs emissions like trend of emission, average emission per month, cumulative emissions per season etc. 

Soils: Alfisols (Sandy loam)

Climate: Semi-arid

Annual Rainfall: 690 mm