I think NO3 leaching because measurement and estimation of NO3 are easier than the N2O and NH3.

I agree with@Rajiv Kumar

Ammonium (NH4+) bonds to negatively charged surfaces of soil particles - clay in particular. The concentration of ammonium in the soil is generally quite low, because it is quickly converted to nitrate under conditions that are favorable for mineralization especially under tropical climatic conditions. Ammonia volatilization occurs when ammonia-based fertilizer are applied to the surface of the soil without mixing them into the soil. However, the concentrations of ammonia released are not high enough to cause direct environmental or human health, and most of the ammonia is re-deposited within a few hundred metres of where it was released. Unlike nitrate and nitrite, ammonia is not a human health concern in drinking water at the levels typically observed in the environment. However, it is toxic to fish at high enough concentrations. This may be the possible reasons for more studies on Nitrate leaching.

But as rightly pointed out, we should consider the N balance in total by considering all the inputs and outputs ( Inputs- Manures, fertilizers, Atmospheric deposition (wet and dry), Irrigation water and sedimentation and Biological N fixation; whereas Outputs - Crop produces, residues, Nitrate leaching, Ammonia Volatilization, Run off- N loss through soil erosion ). In other word N budgeting in any agro ecosystem will give a clear picture of the same.

Nitrate leaching from synthetic fertilisers and from livestock manure probably has the higher environmental impact as compared with ammonia and nitrogen oxides as gaseous air pollutants because it can cause direct health problems if excessively concentrated in drinking water. Nitrate leaching also causes serious ecological damage by eutrophication of aquatic ecosystems. On the other hand, Ammonia and NO2 are greenhouse gases and their real impact on global warming is the object of many research projects.

The process of losing nitrogen in the form of a gas This process takes place in two cases: 1- Volatilization: This is the process of converting ammonium NH4 + into NH3 ammonia. This process occurs more in calcareous or limestone soils with a high pH (alkaline), so do not prefer to add ammonia fertilizers so that they are not lost by volatilization. 2- Denitrification: It is the process of reducing nitrates (NO3) or converting nitrates from the water-soluble form to nitrogen gas or nitrous gas in the following forms (NO2, NO, N2O) under anaerobic conditions and with the help of compulsory anaerobic bacteria such as Pseudomonas.

Among these three ways through nitrogen is lost from the soil, leaching of nitrogen in the soils in the form of NO3 needs a lot of research to deal with it. This is because high accumulation of NO3 at the water table can result in nitrate buildup in the underground water and hence reduce its quality for domestic usage.

Full approval of Srivastava and Alexander's answers. The relative importance of N losses mainly depends on environmental conditions. NO3 leaching where and when water balance is positive. Has been much measured due to easyness of measure and acute interest of public power in rich regions by the obviousness of their effects on health??(one time exagerated!) and environment. NH3 and NOx losses are less easy to measure, have had less obvious effects on health and are harder to be linked to environmental and agricultural causes and consequences.

NO3 leaching and N2O gaseous loss researches are the most investigated theme in the world because NO3 leaching creates underground water pollution which is health hazard for human. On the other hand N2O gas emission creates global warming.

I think there are more studies on N2O emissions at present, mainly because global warming has attracted more and more attention, and nitrous oxide is a very important greenhouse gas, Especially in rice field.

When our interest is both crop nutrition as well as environmental pollution, it seems the loss of applied N via nitrate leaching is more important and it is also the most studied N loss pathway. However, in recent years, due to increased emphasis on climate change-related issues, interest in the loss of N2O from agricultural soils is increasing rapidly. As a consequence, a large number of studies on N2O emissions are appearing these days.

Yes, right. Thanks a lot for this helpful information

Bijoy Singh's discussion is absolutely right. Large number of studies on N2O emissions is conducted now a days for reducing climate change in the world. Recently some people are conducting research on NO3 leaching how to increase nitrogen use efficiency during crop production.

Despite the topic having been a focus for several decades now, the reduction of greenhouse gas (GHG) emissions has not lost its actuality. On the contrary, this issue is more prevailing than ever since we are confronted more and more with extreme climatic conditions and their severe consequences for agricultural production (Lobell and Gourdji 2012). Besides this, other ecosystem services and human health are also seriously affected (Fowler et al. 2013). The cause of this development can be, among others, attributed to the excessive release of reactive nitrogen into the environment from the agricultural sector. However, N fertilization is crucial to feed the global population (Steffen et al. 2015) and too little N input causes systems to be limited and stressed. On the other hand, too much N input will likely lead to N losses to the environment (Galloway 1998). They range from eutrophication to global acidification and the loss of stratospheric ozone (Gruber and Galloway 2008). Nitrous oxide (N2O) is one of the most important GHG contributing to climate change since it is the main source of stratospheric NOx, which is significantly involved in the stratospheric ozone depletion (Heincke and Kaupenjohann 1999). Nitrogen is lost from agricultural soils via nitrate (NO3−) leaching and the gaseous release of N2O, ammonia (NH3) and nitric oxide (NO). Especially N2O emissions have been the focus of many studies and the rising awareness induced efforts to estimate N2O emissions from agro-ecosystems with different approaches and methods.

The average N surplus on agricultural land in Austria is estimated at around 34 kg ha−1 year−1 (Umweltbundesamt 2015). Compared to other countries, this value is rather low. This development is partly due to the rising awareness in the agricultural sector to produce crops with higher nitrogen use efficiency (NUE) (Amon et al. 2014; Küstermann et al. 2010). In general, over the last 20 years Austrian agriculture reduced its GHG emissions by approx. 11% (Amon et al. 2014). However, the amount of fertilizer N application may vary largely depending on the crop grown, with values of 210 kg ha−1 year−1 and more (BMLFUW 2006).

The quantification of N2O emissions from soils is still difficult since various processes are involved in the complex interaction of N2O production, consumption and release from soils (Butterbach-Bahl et al. 2013). Moreover, microbial activity, soil physical properties, nutrient availability, water content and movement as well as land management, climate, and vegetation influence these processes (Li 2000; Schmidt et al. 2000; Haas et al. 2013) where N2O is predominantly caused by the microbial processes of nitrification and/or denitrification (Firestone and Davidson 1989). Under aerobic conditions nitrification is the main source of NO and N2O release (Li 2000) whereas denitrification is promoted by denitrifying bacteria under anaerobic conditions (Li 2000; Butterbach-Bahl et al. 2013). Soils play a predominant role in the wide range of processes involved in N cycling as they are the principle location of reactive nitrogen (Nr) transformations and main site of denitrification back to N2 (Fowler et al. 2013).

The N2O soil flux itself is determined by N2O production, consumption, transport and emission from the reacting system. In general, movement of gases in soils is based on molecular diffusion and advection (Heincke and Kaupenjohann 1999). Factors like soil porosity, temperature, moisture and clay content have to be taken into account when calculating the diffusion rate of N2O in the soil matrix (Li 2000).

The quantity of N2O accumulated in soils depends on the amount, quality and availability of fertilizer applied (Heincke and Kaupenjohann 1999). Thereby, soil permeability is also an important factor, since the gas is mainly transported via macropores whereas management and cultivation methods influence the natural soil structure, water distribution and nutrient availability as well as the microbiological processes (Heincke and Kaupenjohann 1999). Besides the water content (Li et al. 2000; Butterbach-Bahl et al. 2013) also temperature exerts influence with regard to gas diffusion, solubility of N2O (Heincke and Kaupenjohann 1999) and microbiological activity. Denitrifying and nitrifying bacteria are active over a wide temperature range (− 2 to 60 °C) and formation of N2O increases until the optimum temperature of 35–40 °C (Schmidt et al. 2000). Nevertheless, high peaks of N2O fluxes may occur during thawing as a result of N2O formation under frozen top soil layers (Wolf et al. 2012) or driven by microbial metabolism (de Bruijn and Butterbach-Bahl 2010).

However, despite many attempts it is still difficult to get exact estimates of N2O emissions due to the complexity of the N cycle itself. Like other trace gasses, N2O emissions have large temporal and spatial fluctuations (Li 2000). As stated in Haas et al. (2013) this is due to the fact that the microbial processes are strongly influenced by environmental factors and agricultural management operations.

N2O emissions are assessed by on site measurements (e.g. research station in Scheyern in South Germany, Peter et al. 2011). However, field measurements often do not measure the whole year nor entire regions with all their heterogenic environmental and management influencing factors (Butterbach-Bahl et al. 2004). Modelling approaches are therefore used in addition to measurements. Li (2000) points out the challenges of modelling trace gas emissions since these emissions evolve from different sources and are produced and consumed simultaneously in the soil where various environmental variables and reactions are involved. The advantage of calculating N2O emissions from soils with process-based biogeochemical models lies in the possibility to simulate processes and the gas flux for different terrestrial ecosystem types (Haas et al. 2013). A better understanding of the systems can be achieved thereby providing the opportunity to develop site specific strategies for improved NUE (Haas et al. 2013; Molina-Herrera et al. 2016). Butterbach-Bahl et al. (2004) and Molina-Herrera et al. (2016) compared observed and simulated N2O emissions and showed that the simulated N2O emissions were well within the span of the observed N2O emissions. DNDC was first established in 1996 by Li and since then extensively advanced and validations have increased the reliability of the model (Gilhespy et al. 2014; Molina-Herrera et al. 2016). Measurement data provide the basis to develop DNDC with the aim to extrapolate results from site to regional and even global scale (Li 2000). The process-based DNDC model is able to scale ecosystem processes from site to regional simulation domains (Haas et al. 2013) and has therefore the potential to calculate regional inventories of N losses from agricultural soils (Butterbach-Bahl et al. 2004). As stated in Haas et al. (2013) results of LandscapeDNDC were compared to measured annual N2O emissions of different ecosystems (forests, arable and grassland site) proving that LandscapeDNDC is able to simulate N2O emissions across differing ecosystems. Recent validation studies of Molina-Herrera et al. (2015, 2016) have proven the capabilities of the model to predict the N cycle in agro-ecosystems.

Since nitrate losses to the groundwater represent a potential source of environmental problems in intensive farming systems, our study also considers NO3− leaching. Nitrate is very mobile in soils and according to Küstermann et al. (2010) leaching may occur for the NO3− quantities that are not organically fixed (Klatt et al. 2017).

Within our study the overall aim was to model site specific N2O emissions and NO3− leaching losses from soils considering soil properties, climate, crop rotation and management, and to identify potential hot spots and hot moments. The following hypotheses were proposed: (1) sites in regions with less precipitation (Marchfeld, MF) release less N2O than sites in regions with higher precipitation (Grieskirchen, GK). (2) The release of N2O emissions is influenced by crop rotations and management (e.g. N-input). (3) Higher nitrate leaching occurs in lighter soils and N2O emissions are enhanced in heavy soils.