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29.07.15 Inventories and scenarios of nitrous oxide emissions

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)

Picture source: California Department of Food and Agriculture, Crop N uptake and partitioning 

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.

Source: Davidson, E. and Kanter, D. Inventories and scenarios of nitrous oxide emissions,  Environ. Res. Lett. 9 (2014) 105012

27.07.15 Global best practice guidelines for reducing greenhouse gas emissions from livestock

The Livestock Research Group (LRG) of the Global Research Alliance on Agricultural Greenhouse Gases (GRA) and Sustainable Agriculture Initiative (SAI) Platform have joined forces to compile information about greenhouse gas (GHG) mitigation options currently available, and a roadmap of emerging options based on current research, to help make progress on meeting global food demand while reducing the food industry’s contribution to global climate change.

The document summarises current best practices ready for implementation at the farm level, as well as emerging options at various stages of research to reduce the greenhouse gas emissions intensity of livestock production across a range of farm systems.

The document covers intervention options for animal feed and nutrition, genetics and breeding, rumen modification, animal health, manure, and grassland management. It provides a readily accessible guide to current best practices that can help reduce emissions intensity of livestock production, but also outlines current areas of active research that offer opportunities for industry to engage with the science sector to help expand the range of options and their effectiveness in different farm systems.

The document was commissioned by the New Zealand Agricultural Greenhouse Gas Research Centre (NZAGRC) on behalf of the LRG co-chairs and the SAI Platform.

Download the guidance here.

Source: Global Research Alliance on Agricultural Greenhouse gasses

24.07.15 Compost and climate change: a novel mitigation strategy?

Native soils are thought to take up more of the greenhouse gas methane than land used for farming. This study shows that, while agriculture can exert an adverse impact on soil methane uptake, the application of soil conditioners like compost may compensate for loss of the methane sink function. The researchers propose new land management strategies based on this finding.

Agriculture has become the most dominant land use in Europe. Traditional landscapes have been transformed into modern, intensive agricultural land, notably owing to the EU’s Common Agricultural Policy. This entails the increased use of soil conditioners, biobased residues added to soil to improve its quality and fertility.

While the addition of these residues may make the land better for growing, it can also decrease the methane consumed by agroecosystems. This is of concern from a climate change perspective as methane is a potent greenhouse gas with a global warming potential more than 30 times that of carbon dioxide. However, agricultural land does have the potential to take up methane, as well as emit it. Methanotrophic bacteria, which use methane as a source of carbon and energy, are found in wetland agricultural soils like rice paddies as well as dry (aerated) soils. While methanotrophs within rice paddies have been studied extensively, those in well-aerated soils have received little attention, as they are assumed to have a low capacity for methane.

This study is the first to properly test this assumption. The researchers measured methane uptake in two aerated soil types — sandy loam and clay — taken from two typical agricultural fields in the Netherlands. The researchers applied organic conditioners to the soil, then measured the effect on methane uptake. The conditioners tested were sewage sludge, aquatic plant material, compost, wood material and compressed beet leaves, added at amounts typical of intensive agricultural practice.

After being added to soil samples, the mixtures were incubated in a chamber for approximately two months. The researchers measured methane and carbon dioxide flows, as well as the rate at which methane was oxidised.

Their analysis revealed a surprising finding: the addition of the soil conditioners contributed to increased methane uptake. The researchers suggest the conditioners had this effect by increasing the nutrients available in the soil by introducing new methanotrophs, both of which can stimulate methane oxidation (although the latter by a lesser extent).

The researchers determined methane uptake rates at a range of methane concentrations using the untreated agricultural soils. The agricultural soils showed the ability to oxidise methane over a wide range of concentrations, from atmospheric levels to very high concentrations, but after treatment, methane consumption increased up to threefold higher than in the untreated soil.

Consistent in both soils, amendment with compost had the greatest effect, and was able to offset approximately 16% of net emitted carbon dioxide. Applying compost to agricultural soils could thus reduce the impact of carbon dioxide and methane emission — both of which are greenhouse gases.

The transformation of land for intensive agriculture is known to reduce methane uptake relative to natural landscapes. This study makes recommendations for management strategies to compensate for this. The authors suggest simple changes, such as the repeated application of compost, could reduce the impact of greenhouse gas emissions. It is important to note that this research was conducted in the laboratory. The researchers therefore recommend field-based studies, as well as investigations of the impact of the intervention on nitrous oxide emission, another major greenhouse gas.

Source: Science for Environmental Policy, 23rd July 2015



16.07.15 Theme of the month: Nitrous Oxide

So we are having a bit of a change this month and focussing for the next few weeks on nitrous oxide and the issues that arise from agriculture and land use that concern emissions.

The deal with Nitrous Oxide

Nitrous Oxide is over 300 times more harmful than carbon dioxide, so reducing the emissions of this gas is particularly important.

About 66% of man made nitrous oxide emissions come from agriculture. Within agriculture, nitrous oxide is emitted from the nitrogen in fertiliser, manure and slurries, and crop residues. The next most important sources are burning fossil fuels for energy and transport, making up 15% of emissions and forest fires and biomass cooking at 11%.

Warming impact

Although there is a far lower concentration of nitrous oxide in the atmosphere than carbon dioxide, it’s a important greenhouse gas for two reasons, its very efficient at absorbing energy and it stays in the atmosphere for a long time.

Agricultural sources of nitrous oxide

Nitrous Oxide (N2O) is produced naturally in soils through the microbial processes of denitrification. These natural emissions of N2O can be increased by a variety of agricultural practices and activities, including the use of synthetic and organic fertilisers, production of nitrogen-fixing crops, cultivation of high organic content soils, and the application of livestock manures and slurries to growing crops. All of these practices directly add additional nitrogen to soils, which can then be converted to N2O. Indirect additions of nitrogen to soils can also result in N2O emissions. Surface run-off and leaching of applied nitrogen into ground water and surface waters can also result in indirect additions of nitrogen to the soil. Nitrous oxide is also produced through the denitrification of the organic nitrogen in livestock manure and urine. The production of N2O from livestock manure is likely to depend on the composition of the manure and urine, the type of bacteria involved in the process, and the amount of oxygen and liquid in the manure system.

What can we do about it?

We can’t get away from the fact that nitrous oxide naturally occurs as part of the nitrogen cycle. However we can influence the amount of indirect emission by adapting the way that we manage our soils, manures, fertilisers and rotations. These are the things that we will be focussing on this month (and hopefully provide you with lots of useful insights for you to consider                                                                                        whilst sitting on your tractor for harvest!).

Sources: FCCT Toolkit, IPCC report, Davidson and Kanter, (2014) Inventories and scenarios of nitrous oxide; Environmental Research Letters

16.07.15 IYS2015 The Soil Atlas

The Soil Atlas 2015 presents facts and figures about earth, land and fields; its broad ranging significance and its current state in Germany, Europe and the world.

Price explosions and land speculation, increasing soil loss as a result of erosion and sealing, the effects of globalised agro-industry on production and food availability across the globe, the problems associated with the land distribution all impact on the management and risks for soils that we need to grow productive crops and feed the growing population.  These are all looked at in this new soil atlas.

The Soil Atlas provides insights into the current state of the soils on which we depend and highlights the threats posed to them in numerous illustrations and texts.

The Soil Atlas 2015 aims to inform and improve the ability of consumers to make informed decisions and sketches out pathways to a responsible agriculture and soil policy.

Download the atlas here. For more information on other soil resources for the International Year of Soils click here.

For more information on practical ways to build and maintain soils please visit the soils pages of the FCCT Toolkit.

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