It should be increase as N supply is more by fixation. Moreover, the low pH also increase N2O emissions, while N-fixation will decrease pH. Thus, It should be increase.

Hi Prashanta,

Old version of the ‘IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories’ considered N fixing legumes contribute to increased soil N2O emissions by two ways: (a) The process of the biological N fixation (BNF) itself; and (b) the decomposition of N rich legume crop residues. Please see the Table 2 of the IPCC (1996) which can be found here:

http://www.ipcc-nggip.iges.or.jp/public/gp/bgp/4_5_N2O_Agricultural_Soils.pdf

However, as a result of the comprehensive review by Rochette and Janzen (2005) ‘Towards a revised coefficient for estimating N2O emissions from legumes’ (Nutr Cycl Agroecosys 73: pp. 171-179), the new version of the IPCC protocol (IPCC 2006) recommended to exclude the BNF process itself as a source of soil N2O emissions. Therefore, it is now accepted that only the decomposition of legume crop residues contributes to increased soil N2O emissions.

Some recent research have confirmed this recommendation empirically. For example, please see the paper by Zhong et al. (2011) in the following link:

http://link.springer.com/article/10.1007%2Fs00374-011-0575-z

In spite of the above conclusions, one other recent study supports the notion that BNF process itself directly contributed to increased soil N2O emissions. Please see the link below:

http://www.hindawi.com/journals/aess/2014/685168/

Therefore, a simple answer to this question may be not be conclusive.

Overall it may be possible that the contribution by BNF in cultivated legume crops to soil N2O emissions should be relatively lower than the contribution of synthetic N fertilizer to soil N2O emissions.  The paper found in the link below suggests BNF is a strategy for mitigating anthropogenic soil N2O emissions:

http://rstb.royalsocietypublishing.org/content/royptb/367/1593/1157.full.pdf

I hope that these papers are helpful in finding an answer to this question.

-SJ

Are you looking at how fertilizer treatments effect N2O production in this grassland ecosystem? What's the soil pH like? Do you have any preliminary 16s rRNA community data?

Sorry for answering a question with more questions, but like others have stated previously, there's no discernible link between nitrogen fixation and N2O emissions, but there have been a number of other studies on different conditions and their effects on N2O emissions.

I'd also strongly recommend reading Mike Jetten's review of the Microbial N-Cycle, it's slightly outdated, but the concepts are all the same.

Hello Ryan

Yes, I will be looking at N2O production under high and low N-fertilizer condition. I will have 3 pH conditions as treatments and the pH value of control plot is around 6.5. I don't have such preliminary data as you mentioned. 

Thank you everyone for the great answers, specially Susantha for the comprehensive and detailed one. All the responses are very helpful for me towards understanding the issue.