As the third most important anthropogenic greenhouse gas, as well as the largest remaining anthropogenic stratospheric ozone depleting substance currently emitted, nitrous oxide (N2O) is one of the most important forms of nitrogen pollution.
How we mitigate and manage N2O emissions requires an understanding of where the sources of N2O are and how they may increase this century. One of the issues with N2O is that it is a by-product of several fundamental (and critical) reactions of the N cycle. As agricultural land has expanded and land management has advanced (including the use of legumes as N fixers), the N cycle has been altered. Another big impact has been the development of the Haber Bosch process, which plays a central role in feeding the world’s rapidly increasing population.
This growth in man-made fixed N has led to an unintended increase in global N pollution, including N2O emissions driven largely by the fact that mismatch between crop N demand and soil N supply frequently leads to N losses. It is impossible to completely eliminate global N pollution particularly from agriculture, its largest source.
This blog comes from a paper which tries to quantify the amount of N2O emitted from various sources, looks at how those emissions are calculated, what the common ways of measuring N2O are, and tries to project what will happen to the levels of N2O emissions under different management scenarios in the future. To read the full paper please click here.
Natural emissions sources
There are a range of measuring options used to assess the levels of emissions from different areas. ‘Bottom up estimation uses field based measurements and ‘top down’ estimations are based on measurements from the atmosphere and models.
There are lots of caveats and supposition to the figures as well as uncertainties, however both accounting approaches (whether you are basing measurements from the field or the atmosphere) suggest that natural emissions were and probably still are between 10-12 Tg of N2O per year.
Anthropogenic emissions (since 1850)
Difficulties in accounting strategies
Again top down and bottom up approaches can be applied to N2O emissions derived from human activities. Protocol has been developed by the IPCC for counties to estimate their N2O emissions by emissions factors (bottom up approach which calculates the amount of N2O emitted per unit of activity). Within agriculture, emissions factors are used to calculate the direct emissions from soils, amounts of fertiliser N that is leached into watercourses and volatilised as ammonia or N2O, as well as the indirect N2O emissions from downstream (which can be substantial). For example emissions from coastal, estuarine or riverine waters are estimated to be ~9% of total anthropogenic sources, although the original source of most of this N is from agricultural applications in the field.
This method (Tier 1, IPCC) has a lot of inaccuracies though and is difficult to apply across diverse systems. As with biological processes, relationships between variables are not always simple and studies have discovered that it is more likely to be non-linear than linear relationships. The non-linear relationship is likely the result of large increases in N2O emissions once N application rates are in excess of plant demands.
Advances are underway in terms of developing a more robust accounting method. Countries that have sufficient data are permitted (under IPCC Tier 2) to calculate more specific emissions and developments of validated biogeochemical models look towards Tier 3. There are new bits of kit (including laser technology) which can measure fluxes of N2O and these will all help with emission factors but it will always be a challenge due to the large spatial and temporal variation.
Providing an average for the UK doesn’t take into account the diversity of farming systems that exist, let alone the unique relationships between soil type, climate management and enterprise which makes every farm an individual entity (and probably incomparable).
Improvements in the quality of activity data for each county, including fertiliser application rates, livestock production and manure handling procedures will aid in the accuracy of estimates.
Emissions from agriculture (some headline figures)
Agriculture is the largest source of anthropogenic N2O emissions, responsible for 66% of total.
Emissions estimates include direct soil emissions from N fert and manure applications and indirect emissions from downstream or downwind waterbodies and soils after nitrate leaches away from fields and after N emitted from fields as ammonia or Nitrogen gas is deposited back.
Also included are N2O emissions resulting from crop residues, manure management, cultivation of organic soils and crop biological N fixation.
The central factor responsible for agricultural N2O emissions is a lack of synchronisation between crop N demand and soil N supply with an average 50% of N applied to soils not being taken up by the crop. This is something that needs addressing.
Other sector contributions
Industry and fossil fuel combustion (responsible for about 15% of total anthropogenic N2O emissions.
Biomass burning (~11% of total gross N2O emissions)
Waste water, aquaculture and other sources
Projections for future emissions
So as you can imagine, as we can’t reliably assess what’s going on now, it becomes incredibly difficult to predict future scenarios. Especially when there are lots of things that could change including population growth, rates of food waste, nutrient use efficiency, land use change, climate change and other variables.
Four sets of published N2O emission scenarios have been looked at to characterise the range of future anthropogenic emissions.
Scenarios looked at from research
Business as usual – if we don’t mitigate any emissions and continue ‘business as usual’ then emissions are set to increase by ~83% based on 2005 levels.
Moderate mitigation scenarios – if moderate mitigation methods were achieved an increase of 26% compared to 2005 would be possible.
Concerted mitigation – would lead to emissions reductions on average per year of 1.8Tg N2O to 2020, with a reduction of 57% by 2050.
It is important to remember however that these projections are based on 2005 figures and since then significant differences have been seen. So far (up to 2013 when the study used the data from) actual global N2O emissions have been closer to Business as usual trajectories rather than the mitigation scenario which research was expecting.
What does this all mean for agriculture?
Agriculture currently accounts for 56-81% of gross anthropogenic N2O emissions. Some N2O emissions associated with food production are inevitable, but future N2O emissions from agriculture will be determined by several factors including population, dietary habits, and agricultural management to improve N use efficiency.
Another rising issue is the growing demand for biofuels on future N2O emissions which is uncertain depending on types of plants grown, their nutrient management and the land resource needed. More research will help to full these current knowledge gaps. Accounting methodologies are not fully developed and further research is needed, however what we can do as farmers is try and look at N balances and flows in crop rotations and try and ensure synchrony between N supply and crop demand as much as possible.