conclusion

To summarise the posts over the last few weeks, I have looked at where and why domestication first happened and the debates surrounding the timing of its origins.  The timing of domestication's first effects on the climate has been an important point and is an on going debate.  The blog then proceeded to discussing how agriculture and its expansion has affected the environment and how cultivation contributes to climate change.  I then discussed some of the possible strategies to combat further increases in greenhouse gas emissions and the feasibility of their adoption. 

Through researching these posts I have discovered how interlinked climate and domestication really are.  Climate played a key part in why domestication developed and also agriculture is playing a main role in altering climate though emissions.  Climate is always affecting agriculture and the changes associated with anthropogenic climate change will be vital in determining food security in the future.   Agriculture will always be a key issue as people will always need food and as the population continues to grow so its will demand.  It has also come to light that looking at past practices in agriculture could help us in the future, returning perhaps to more sustainable methods used in the past will aid environmental problems that are arising such as soil degradation and erosion.  

Thank you for taking an interest and reading these posts, I hope they have bettered your knowledge of the relationship between domestication and climate. 

summary of mitigation

In the last few posts I have discussed various mitigation strategies that have been put forward aiming to reduce greenhouse gas emissions from agricultural sources.  

A key theme I noticed throughout looking at these articles was economic factors.  In many places around the world, especially in developing nations, it will be very difficult to implement a number of these strategies due to the economic cost.  Consequently, it would be beneficial to attempt as many as possible in countries that can afford to have the schemes, as this would reduce a proportion of emissions.  However, populations in the developing world are growing quickly and are projected to hold the majority of future growth.  This will mean a greater demand for food.  Many developing countries provide cultivated products that other places in the world rely on, such as rice.  Therefore it will be necessary to reduce emissions from these countries, but it will likely require subsiding and the provision of economic incentives for them to be adopted.  

Personally, the strategies I would consider best to implement would be those incorporating more organic methods.  These not only help with the reduction of fossil fuel burning and greenhouse gas emissions but they also improve food security around the world by improving soil quality to its former states.  To name a few of these practices would be,  no-till, crop rotations and cover crops.  I believe that draining rice paddy fields in the non growing is also a feasible and beneficial strategy, as it does nothing to the yield and lowers methane emissions.  Also better use of fertilisers is very important as this will reduce nitrous oxide emissions, reduce fossil fuel burning and avoid soil degradation.  Planting trees, or returning to native habitat on marginal croplands is also very beneficial, as it reduces emissions with very little impact.  All these strategies are less economically intense than some making them easier to implement.  

It is important when considering mitigation strategies that the yield is not changed.  This will make strategies more unpopular as they will be an economic cost to farmers and it will be uncertain if enough food can be produced for the population at the current price.  it is also important that the quality of produce is not affected by strategies as it will put people off buying it.  

Using mitigation strategies is necessary to reduce greenhouse emissions from agriculture as they do contribute to a large proportion of anthropogenic atmospheric inputs.  However it will be a lengthy process requiring management, ample research and high financial investment.  

is organic the way forward?

In recent years organic farming has become increasingly popular.  Not only is it perceived as being healthier, it is also believed that it can mitigate climate change.  Organic farming, as its name suggests is natural farming, where food is produced using techniques such as crop rotation, green manure, compost and biological pest control.  This is different to conventional farming which uses manufactured or synthetic fertilisers and pesticides, plant growth regulators livestock antibiotics, food additives and genetically modified organisms.  It is common belief that organic farming could mitigate climate change as it seems more natural.  This post is going to discuss ways organic farming can reduce greenhouse gas emissions and whether it is a feasible mitigation strategy.  

This article by the ITC (2007) suggests reasons why organic farming is a possible mitigation strategy, here is a summary of some of the main arguments put forward:  

The article stipulates how important synthetic nitrogen fertilisers are in conventional agriculture as high nitrogen concentrations are required to meet the required yield.  It states that in 2005 global nitrogen fertiliser consumption was 90.86 million tones, and it takes approximately 90 million tonnes of fossil fuel to produce this nitrogen fertiliser, this is about 1% of global fossil energy consumption.  By changing to organic farming it would not be necessary to produce this fertiliser, reducing carbon dioxide emissions.  

Organic agriculture is self sufficient in nitrogen, mixed organic farms recycle manures form livestock and crop residues into compost, these are then used as fertiliser.  Leguminous crops also provide additional nitrogen in sufficient quantities.  Emissions of nitrous oxide are directly related to the concentration of available nitrogen in the soil, therefore as I mentioned in a previous post if adequate nitrogen is in the soil for the crop then less is lost.  As mineral fertilisers are not used and there are reduced livestock units per hectare there is less nitrogen in the soil resulting in a smaller amount of loss as nitrous oxide.  

A reduction in nitrous oxide emissions can also be achieved though using diverse crop rotations, this improves soil structure and quality, reducing erosion which leads to loss of nutrients.  Soils managed organically are much more aerated and have significantly lower mobile nitrogen concentrations. 

Similarly, as soil quality is better where it is organically farmed, less carbon is lost.  Conventional farming encourages carbon loss due to soil erosion.  Carbon is stored in organic soils due to the use of green and animal manures, crop rotations with inter cropping, cover cropping and composting techniques.  Organic farms generally use conservation tillage or no-till, I explained how this benefits the soil in a previous post.  

this picture shows organic soil (on the left) to have better drainage and water holding capacity, it is less waterlogged than conventional (on the right) this reduces soil erosion

When looking at an opposing angle, put forward by Cassman et al (2003) surrounding nitrogen levels it is possible to see a different perspective on organic farming.  This article states that although it is believed that organic farming offers environmental benefits, it is just as difficult to prevent nitrogen loss from the soil from organic fertilisers as it is synthetic.  There have been many studies on leaching from organic soils, with varied results, some show it to be higher than synthetic and others lower.  Yield reductions are also associated with organic farming, and organic farming produces more expensive products whilst requiring more government subsidies to remain economically viable.  

Consequently this has created the idea of including some practices from organic farming in conventional systems.  This is suggested by Pimentel et al (2005).  There are many benefits of organic farming such as improved soil quality which could make conventional agriculture more sustainable.  Lal (2004) suggests that using practices from organic farming, such as no-till farming, cover crops, nutrient management, argo forestry, can improve carbon content of soils.  This in the long run can improve food security, which is becoming a more pressing issue as more soils are degraded. 

It would be effective to include some areas of organic farming in conventional farming, to make it more sustainable.  It does not seem logical to use organic farming as a mitigation strategy as it is not possible to determine whether producing food this way would yield enough for the global population.  Producing food organically also requires higher economic investment and produces more expensive products.  As Cassman et al state, organic farming could be feasible in industrialised countries but it would not be able to secure the food supply in the developing world where it is necessary to maintain low food prices.  

Eutrophication

As discussed in the previous post, the inefficient use of fertilisers in agriculture has lead to an excess of nutrients such as Nitrogen and Phosphorous in the soil, which is then leached or released as a gas to the atmosphere.  I have discussed the impacts of nitrous oxide, this post will analyse the impact of those nutrients leached.  

Eutrophication occurs where nutrients, namely phosphorous and nitrogen, found in agricultural fertilisers are washed (leached) by rainfall into a freshwater or coastal systems.  Here I am going to focus on freshwater systems.  Leaching causes increased productivity  lakes, causing large algal blooms to grow.  These dominate the lake preventing other plants from getting the nutrients and light they need, reducing the biodiversity of the lake and the water quality.  When these large algal blooms die they sink to the bottom where they decompose, causing deoxygenation of the water, affecting fish and other organism.  It becomes difficult for these organisms to live due to the deoxygenation of the lake and they eventually die.  Consequently lakes end up a green colour and are species limited, dominated by algae, these lakes referred to as being in a turbid state (see picture below).  These lakes are very common in agricultural areas.  

Below is a simple but effective animation of the process of eutrophication. 

There have been several strategies to prevent the situation getting worse, and to restore these freshwater systems.  As I mentioned in my previous post, better, more efficient use of fertilisers in agriculture is key as this reduces losses.  It is important to attempt to stop the problem at its source.  Over recent years buffer areas have been introduced.  Haycock and Burt (1993) showed this to be a successful measure at reducing the level of nutrients reaching freshwater systems.  They undertook a study on sections of the River Leach, making a grid of bore holes on the buffer area, from which they could take water samples to measure nutrient concentrations.  They found that there were sharp losses in nitrates with increasing distance into the buffer. This strategy has been successfully adopted.  Re-meandering rivers and streams is also a possible approach, as this means the water flows more slowly allowing more time for deposition before it reaches lakes.  These ideas intend to reduce the level of nutrients entering the lakes. 

However. restoring turbid lakes is much more difficult.  Even when the excess of nutrients being added to the lake has been stopped the lake still remains eutrophic due to nutrients embedded internally in the lake.  Therefore it is possible to dredge lakes of all there sediment, however this is a very expensive strategy and causes loss of the lake habitat.  Another possible restorative measure is biomanipulation.  This is where the lake ecosystem is manipulated, for example by removing fish, to see if the original ecosystem can be restored.  Sondergaard et al (2007) evaluated data from more than 70 restoration projects conducted in shallow, eutrophic lakes in Denmark and the Netherlands.  They found the most common biomanipulation measure to be removal of zooplanktivorous fish.  They showed some success, over half the lakes had decreased levels of phosphorous, nitrogen and chlorophyll a.  This shows potential for biomanipulation to work, however it is a very complex process.  The article states on the long term only a few lakes recovered, most returned to a turbid state after ten years or so.  Consequently, biomanipulation could work but it needs to be maintained and repeated over long time frames otherwise the fish recolonise and the turbid state returns. 

For more information on eutriphication, this article by Smith et al (1999) explains it well, explaining both coastal and freshwater eutriphication giving examples of restoration success stories.  

Reference: 
Haycock, N.E. & Burt, T.P. (1993) The sensitivity of rivers to nitrate leaching: The effectiveness of near-stream land as a nutrient reduction zone. In: D.S.G. Thomas & R.J. Allison. Landscape Sensitivity. John Wiley & Sons, 261-272.

improving the use of nitrogen in agriculture

Over recent years the use of fertilisers containing nitrogen has increased with demand for food.  Using these fertilisers adds nitrogen to the soil allowing a greater yield of crop to be grown.  They are used in both arable and livestock farming, spread or sprayed not fields for cattle to graze on.  In a previous post I discussed the problems relating to increased use of these fertilisers, namely nitrous oxide emissions, an important greenhouse gas.  These nitrous oxide emissions are resultant of both mineral fertilisers containing nitrogen and organic fertilisers such as animal manure.  A major issue arisen recently is that these fertilisers are not efficiently used.  In this post, articles suggesting various mitigation strategies to improve the efficiency of fertiliser use and consequently reduce nitrous oxide emissions will be discussed.  

Articles by Monteny et al (2005) and Pautisan et al (2004) discuss various strategies for reducing emissions describing and explaining how they work, as summarised bellow:

• Type of fertiliser - This is important as there are many different types of fertiliser and some such as nitrogen based fertilisers result in greater emissions than others, for example ammonium.  
• The use of slow release fertilisers - These have been formulated to attempt to coincide nitrogen release with plant growth.  Here the fertiliser has been coated meaning the release of nitrates is much slower and much more controlled.  This reduces nitrous oxide emissions as only one application of fertiliser is necessary and there is a much smaller pool of nitrogen in the soil, restricting loses.  
• Addition of nitrification inhibitor - this can be added to fertiliser, examples being Nitrapyrin and Dicyandiamide.  These inhibitors aim to delay the transformation of Nitrogen into nitrous oxide helping to match the timing of the supply with crop demand.  
• Land drainage - It is commonly thought that there is a relationship between nitrous oxide emissions and water filled pore space.  When water filled pore space is above 70%, it results in significant nitrous oxide emissions.  Therefore improving the soil’s physical conditions, for example reducing soil wetness through draining, will reduce emissions.  Wet, compact soil conditions lead to anaerobic conditions, enhancing denitrification.  
• Conducting soil Nitrogen tests - This is where soil is tested to discover how much fertiliser is actually required to achieve the desired crop yield.  It is a common problem that soil is over fertilised and too much nitrogen is added to the soil, meaning that the plants only use their required amount and the excess is lost, either as nitrous oxide or through leaching.  Therefore by measuring how much is in the soil and knowing how much is needed for the crop it is possible to add sufficient fertiliser to grow the crop rather than adding an excess amount.  Mcswiney and Robertson (2005) undertook a study in South West Michigan USA over three years to show that nitrous oxide emissions increase mainly in response to additions of nitrogen to crops that exceeded their needs.  They added different amounts of various fertilisers to nine fields and measure nitrous oxide fluxes, available nitrogen in the soil and grain yields.  They found that nitrous oxide fluxes could be reduced by using less fertiliser, whilst having no effect on crop yield.   
• Improving the timing of fertiliser addition - Often fertiliser is not added when the plants are at their full capacity to absorb it, consequently leading to losses through leaching and as nitrous oxide.  Chambers et al (2000) state that in order to reduce losses fertiliser should not be applied between autumn and early winter.  
• Cover crops - these can also reduce nitrous oxide emissions from soil as they can catch any residual nitrogen left in the soil instead of it being left bare, where it would otherwise be emitted as nitrous oxide or leached.  

An article by Zhu and Chen (2002) looks at the success that some of these strategies have had in China.  Through undertaking filed micro plot experiments with nitrogen based fertiliser they were able to demonstrate that nitrogen recovery in rice plants when fertiliser is added to the crop in its early growing stage, is in the range of 22-52%.  Whereas this can be increased to 55-69% when added at the vigorous growth stage, reducing nitrogen loss.  Another possible strategy discussed in this article is deep placement.  This is where fertiliser is placed deep into the soil, rather than just sprayed on the surface.  This again was successful in reducing nitrogen loses, in Fengqiu, Henan Province, nitrogen loss through ammonia volorisation was between 20-48% when urea was surface broadcast.  This was reduced to 11-18% when placed deep into the soil.  

There seem to be many positives regarding these strategies to reduce nitrous oxide emissions, many are easy to implement, are cost effective and are shown to be successful.  Most are related to improving the efficiency in using fertilisers, improving timing and reducing wastage though excess application.  Using control released fertilisers or nitrification inhibitors have the slight disadvantage in being more expensive, but in the long run less fertiliser is used and consequently the cost of labour to implement them is reduced which is attractive to farmers.  However a disadvantage is apparent that nitrous oxide fluxes are reliant on environment, experimentation is required on the local scale to develop an optimal nitrogen management scheme, which requires longterm investment and research.   

livestock cause climate change?

Farm animals account for a large amount of greenhouse gases emitted into the atmosphere from agriculture.  On average a cow releases between 70 and 120kg of methane per year!  Greenhouse gases are released from various areas of livestock breeding.  Carbon dioxide is released from the burning of fossil fuels for energy, such as electricity.  Monteny et al (2006) state an important source of methane is related to the way cows and other similar animals digest their food, whereas in chickens, pigs and other animals that digest their food differently, manure is the most important source of methane. The main sources of Nitrous oxide are in fertilisers, land applied animal manures and in the urine of grazing animals.  

Many mitigation strategies aim to reduce methane emissions from livestock as this is where the greatest proportions of emissions lie.  This is probably also because methane is one of the easier issues to mitigate as it is thought to be related to breed and diet of the animal.  This post will discuss the various ways in which methane can be mitigated through diet.  

Beauchemin et al (2007) undertook a study on cows, aiming to reduce methane emissions by altering their diet through adding lipids.  They found that cattle fed sunflower seeds produced less methane per day than cattle fed other lipid sources as the feed was less digestible.  Lipids reduce methane emissions by decreasing the amount of organic matter fermented in the rumen (part of the cow’s digestive system), as the lipids replaced barley grain.  Overall all lipid sources were equally effective in suppressing methane emissions (about 15%) when differences in intake and fibre digestion were accounted for.  However, adding lipids may increase the cost of feeding livestock, and therefore be less appealing to commercial farmers.  Beauchemin et al suggest sunflower oil as the best out of the three they used, as it increased rate of gain of the cattle as well as lowering methane emissions, hence being more attractive to farmers.  

Machmuller et al (2000) undertook a similar study to Beauchemin et al, but using sheep.  Sheep digest in a similar way to cows, so here they measured the change release of methane after adding different ingredients to their diets, against a control.  They found reductions in methane emissions from all, coconut oil 26%, rapeseed 19%, sunflower seed 27% and linseed 10%.  Again they found sunflower seed to reduce digestibility.  A possible problem with adding lipids and other supplements to animals diets is the reaction of buyers, people may be put off buying meat from animals that had these additives in their diet.  

Another possible way to alter livestock diet to reduce methane output is to change forage to concentrate ratios.  This is shown by a study by Lovett et al (2003), who investigated animal performance and methane emissions using 36 cows over an 11 week period.  They had various forage to concentrate ratios, some supplemented with coconut oils.  Reducing the forage to concentrate ratios resulted in significantly increased rates of weight gain, whereas the coconut oil had no impact on  weight gain of the cow.  Both the change to lower forage to concentrate ratios and the addition of coconut oil reduced daily methane emissions.  

These papers present reasonable strategies to mitigate methane emissions, an article by O’Mara et al (2008) in Livestock and Global Climate Change also suggests that both the addition of lipids and changing forage/concentrate ratio of livestock diets can reduce daily methane emissions.  However they say that more data and information need to be collected on diet alteration strategies .  It is necessary to ensure that these strategies are functional in different environments, can be easily adopted, and only have a small economic cost otherwise they will not appear appealing to farmers.   

Tillage??

This post is going to look at another crop management strategy, aiming to increase soil carbon content, tillage.  Tillage is the preparation of soil before crops are planted, it can be done by hand or machine and involves processes such as digging and overturning.  Altering tillage methods can now be used as a mitigation strategy for climate change across the world due to advances in farm machinery and farming methods.  

Tillage causes disturbance of the soil which stimulates losses of carbon through enhanced decomposition and erosion.  Therefore it has been put forward that changing conventional tillage methods to reduced/conservational tillage or no-till can result in carbon gain in the soil and a reduction in carbon dioxide emissions as a result of a lessened use of farm equipment.  The picture below describes the variation between the different types of tillage.  

A study done by West and Marland (2002) in the USA suggest that reduced tillage practices could contribute to making US agriculture ‘carbon neutral’ over the next forty years.  In this study, they compared conventional tillage methods with no-till.  They estimated the energy use and carbon emissions from all aspects of crop growing, such as including fuels, fertilisers and farm machinery.  They estimated no-till emitted less carbon dioxide from agricultural operations than conventional tillage and that no-till increased carbon sequestration due to less soil disturbance.  The enhanced carbon sequestration is limited over time as once it reaches a peak the soil will be unable to absorb anymore carbon, however they argue that if no-till practices are continued then the soil carbon content will be maintained, and the reduction in carbon dioxide emitted to the atmosphere as a result of lessened fossil fuel use will continue indefinitely.  Cerri et al (2004) undertook a similar study in Brazil and found that converting to no-till methods is increase carbon sequestration in the soil, currently accumulating 9 Mt C per year.  

Changing to reduced tillage or no-till seems a successful way of lowering carbon emissions, but there is some uncertainty whether converting to these methods is beneficial in all locations.  Hermle et al (2007) studied tillage in North East Switzerland.  They analysed different tillage methods in the region, from no-till to ploughing and found there was little difference in the amount of carbon sequestrated.  The reason given for this is the climatic conditions, being moist cold-temperate solid.  

Another problem that has arisen as a result of suggested tillage mitigation strategies is the impact on nitrous oxide emissions.  This is discussed by Li et al (2005), it is thought that switching from conventional to no-till can increase nitrous oxide emissions by 2.5±0.5 kg N ha^-1 yr.^-1 for humid environments and 0.8±1.0 kg N ha^-1 yr.^-1 for dry environments which offsets some of the carbon sequestration gains as nitrous oxide is also important greenhouse gas.  Some have found these nitrous oxide emissions to subside after a few years after the conversion but these results are variable.  Li et al do state that despite the nitrous oxide emissions, no-till cropping is beneficial as it reduces fossil fuel use, it reduces soil erosion, enhances soil fertility and also water holding capacity.  

Using reduced tillage or no-till does seem on the whole to be a successful mitigation strategy to improve carbon sequestration.  In many studies across the world it has shown to be effective in increase soil carbon content.  Using reduced tillage or no-till is also beneficial as it reduces carbon dioxide emissions from the practice of farming, such as machinery use.  These methods also improve the quality and fertility of the soil as less erosion occurs and less nutrients are lost, therefore despite uncertainty about nitrous oxide emissions, if the soil holds its nutrients better then perhaps fewer fertilisers can be used on the soil reducing nitrous oxide emissions in the long run.    

Crop management strategies

Agricultural soils are a major cause of the increasing amounts of carbon we are seeing in the atmosphere.  This increase in atmospheric carbon levels has resulted from change in land use, land has been cleared on a huge scale to provide space for agriculture.  Therefore there has been a huge change to the species in the ecosystems and they have become less diverse leading to a reduction in carbon sequestration.  This post will discuss papers suggesting mitigation strategies aiming to increase and maintain carbon sequestration rates through varying crop management strategies.  

A paper by Freibauer et al (2004) which discusses economically viable potentials for increased carbon sequestration in soils in Europe.  They recommend the promotion of organic inputs on arable lands, introduction of perennials such as trees on arable land set aside for conservation and the promotion of organic farming. Similarly a paper by Lal (2003) suggests that effective ways of mitigating carbon loses are to use natural fertilisers such as manure and again plant perennials in marginal lands.  Lal also suggests that it is beneficial to diversify mono cultures and plant winter cover crops.  

Through planting perennials such as trees and other plants in marginal areas of crop lands it leads to a diversification of the land resulting in a higher carbon sequestration rate, as different species sequestrate different amounts of carbon, for example trees absorb much more than smaller plants due to their larger size.  Using natural fertilisers such as manure, are more beneficial than artificial fertilisers as not only do they maintain soil carbon levels more efficiently, they also improve the quality of the soil, leading to less degradation.  Planting winter cover crops can increase carbon sequestration because having plants present on fields for the majority year increases carbon intake into the soil rather than leaving fields bare where little carbon sequestration can take place.  

A paper by Vaccari et al (2011) suggested an alternative strategy, increasing soil carbon storage by using Biochar.  Biochar is a carbon rich product obtained through carbonisation of biomass and can be used for carbon sequestration.  Biochar is very resistant to decomposition, and there is some evidence that Biochar stores atmospheric carbon from centennial to millennial timescales.  They found most of these studies had been undertaken in tropical locations and there was a lack in temperate regions.  They undertook their study on durum wheat in Mediterranean climate conditions, and their results showed the viability of Biochar application to crops, showing positive effects of up to 30% on biomass production and yield, and it was successful for two consecutive seasons.  

In papers by Zhengchao et al (to be published 2013) and Alvarez (2005) it was shown that the use of fertilisers can increase soil carbon storage.  This occurs due to an increase in crop yield as a result of fertilisers, as there are more plants, more carbon is sequestrated. However, a major problem of using fertilisers to increase crop yield and consequently increase carbon storage in the soil, is that fertilisers result in higher nitrous oxide emissions.  Nitrous oxide emissions are becoming more prominent and I will discuss various mitigation strategies to reduce these in a following post.  Therefore using this strategy, the reduction of carbon dioxide in the atmosphere is offset by an increase in nitrous oxide.  

After reading papers on this topic, using Biochar seems a possible method, however, it is likely a wider range of research is necessary and that it would be more expensive for farmers to implement.  Whereas planting perennials in marginal areas, using natural fertilisers and planting winter cover crops seem to be the most efficient crop management strategies to improve soil carbon content.  

Rice Management Strategies

In this post I am going to go back to rice paddies and discuss possible mitigation strategies aiming to reduce future methane emissions from these fields.  This is briefly outlined in the IPPC Report.  

In a paper by Lindau (1994), fertilisers containing nitrogen were added to rice fields before they were flooded in Louisiana, USA.  Then, during the growing season methane fluxes were measured at regular intervals.  As a result, it was found that methane emissions reduced, where ammonium sulphate was added emissions reduced by 55% and where potassium nitrate was added they reduced by 59%.  However these results are very variable as discussed in an article by Banger et al (2012) who state that adding nitrogen fertilisers to rice paddies have complex impacts on methane emissions.  They found that out of 155 data pairs in rice soils, 98 of these had increased methane emissions.  The downside to this strategy is that it does have variable results and it can also lead to increases in nitrous oxide emissions.  

Cai et al (2000) undertook a study at 8 sites in China finding that methane emissions varied greatly between sites.  They found that in the non rice season, waterlogged and flooded fields continued to emit methane, but they did find that these emissions were lower at sites at mid and higher slope locations compared to those at the base of slopes.  This is thought to be as a result of better drainage.  A paper by Gou and Zhou (2007) links to this, they describe various rice management strategies, one being field drainage in the off-rice season.  By draining the fields the anaerobic environment is lost which reduces methane emissions.  They also suggest that intermittent irrigation of rice paddy fields can be effective in reducing emissions.  However, a downside is that draining and flooding the fields is a very water intensive process.  

In the paper by Gou and Zhou (2007) they also suggest that rice variety affects methane emissions and often hybrid varieties lead to lower methane emissions than common ones. They suggest this as a possible mitigation strategy to lower methane emissions.  They also discuss fertiliser management, where chemical fertilisers are replaced with organic.  it has been suggested that replacing chemical fertilisers with peat moss can reduce methane emissions.  

These strategies offer potential to reduce methane emissions from rice fields.  The demand for rice is growing with the population.  Draining is probably the most successful rice management strategy as it does not alter the crop yield and will consequently be the most appealing to farmers.  However the downside of this is that it does require vast amounts of water which may not be available in some regions and could also be very expensive.  Adding nitrogen fertilisers has been shown to be successful in some areas but it is variable on location and fertiliser type.  There is also the negative aspect that it can increase nitrous oxide emissions.  Similarly changing the rice variety maybe difficult to implement as some hybrid species may have difficulty germinating in some locations and they may not provide the same yields as common rice, which could be an economic cost to farmers.  

Increasing carbon sinks: a success story?

In this post I am going to focus on mitigation strategies aiming to reduce atmospheric concentrations of carbon dioxide through land use change.  In a previous post I explained how agricultural expansion has caused land use change namely through deforestation, on a huge scale, which in turn leads to increased concentrations of carbon dioxide in the atmosphere. Here I am going to discuss a few mitigation strategies that aim to revert these effects by increasing carbon sinks.  

The first strategy, which the IPCC suggests to be one of the most effective methods of reducing emissions is to allow or encourage cropland to revert to another land cover that is similar to the native vegetation of an area.  This will increase carbon storage (as cropland does not store much carbon), for example converting arable land to grassland results in the accrual of soil carbon because of lower soil disturbance and reduced carbon removal in harvested plants. Lal 2004 suggests that restoring land use to as it was before clearance, especially on marginal cropland (as shown below) and using recommended management practices, will have significant effect on reducing the rate of enrichment of atmospheric carbon dioxide.  Another benefit is that an alternative land cover to cropland have lower nitrous oxide emissions due to few nitrogen inputs as fertiliser.  

It is also possible to convert drained croplands back to wetlands, this can result in the rapid accumulation of soil carbon, removing it from the atmosphere.  However a downside to this is methane emissions can increase.  

Another strategy put forward to lessen emissions from land use change is to prevent deforestation and protect forests keeping them intact.  Soares -Filho et al (2006), wrote a paper specifically looking at conservation in the Amazon Basin. They state that by 2050, following current trends of agricultural expansion in that area, 40% of Amazon forests will be destroyed.  Not only will this significantly reduce biodiversity, but also they estimate this will release 32 ± 8 Pg of carbon in to the atmosphere.  Consequently they suggest that a network of protected areas is necessary to prevent the destruction.  However implementing policies of protected areas to stop deforestation, is difficult.  As Sathaye et al (2006), suggest there are many economic incentives behind deforestation, and consequently location of these protected areas is very important as economic alternatives need to be found to implement these polices. 

The final strategy I looked at, was afforestation, where forests are replanted.  Articles show this to have mixed success rates.  In some areas it can yield considerable soil carbon accumulation rates for example Post and Kwon (2000) have found afforestation to be successful at absorbing carbon from the atmosphere in the northern hemisphere over the few decades.  On the other hand, Tate et al (2005), found the opposite in New Zealand, that after afforestation the soil absorbed less carbon, than it did before.  In Richards and Stokes (2004) paper, which reviews recent afforestation studies, found hat afforestation has good potential at reducing atmospheric carbon dioxide but it is a very complex process, where location and tree specie were very important in determining the success of carbon accumulation.  However a major downside to afforestation is the economic cost of it, it requires high investment and will be several decades before revenue can be generated.

From looking at mitigation strategies that have been implemented, trying to protect or increase the size of carbon sinks, it seems clear that different policies work better in different places.  I think it important these policies are implemented, however some economic incentive will probably have to be found to encourage governments to put them in place, especially those whose main income comes from agriculture and logging.      

what are mitigation strategies?

My next few posts are going to move back to current issues surrounding agriculture, and discuss possible mitigation strategies to help reduce the impact of climate change in the future.  

Mitigation strategies aim to reduce the effects of global warming, either through decreasing concentrations of greenhouse gases, wither by reducing sources of emissions or by increasing sinks.  

Mitigation strategies have developed significantly in recent years since the climate change consensus.  This is where scientists now agree that recent changes such as the increase in global temperatures, are a result of anthropogenic forcing and humans are having a significant enough affect on the world to alter the climate.  

Climate modelling is important in creating mitigation strategies.  This is where different scenarios are projected to predict the effect on various parameters, for example temperature and precipitation patterns.  Climatologists use various scenarios, anywhere between those projecting what would occur if we carry on emitting greenhouse gases as we are with no reductions, to stopping emissions completely.  Modelling makes it possible to project the level concentrations of greenhouse gases need to be reduced to lessen the effect on climate change which is important in determining viable mitigation strategies.  

I am going to discuss mitigation strategies relating to agricultural emissions.  The IPCC gives good background information into what I am going to look at.  In particular I will focus on those surrounding deforestation, rice cultivation, livestock and nitrogen fixing in my following posts.  

when did domestication begin?

In this post I am going to discuss a more historical aspect to domestication after reading a couple of papers debating when domestication first occurred.  The oldest evidence for agriculture, a few rye grains, has been found in Syria, which is in the 'Fertile Crescent' where agriculture is thought to have begun (as shown on map below).

During the last ice age humans existed as part of sparse populations, belonging to hunter gatherer societies.  As the climate became milder towards the end of the ice age, they built  permanent houses and made tools, this was an important step towards more modern settlements.  However, an abrupt cooling event lasting 1300 years occurred, called the Younger Dryas happened between 12,900 and 11,600 years ago.  Pollen records from within the Fertile Crescent show that a cooling of the climate was felt in this area.    

Both Balter 2010 and Pringle 1998 discuss the arguments surrounding the beginnings of domestication.  One hypothesis, as believed by Bar-Yosef is that the cold period brought on by the Younger Dryas caused domestication, to provide a more stable food source so humans had a sufficient amount to eat.  There is some evidence supporting this, rye grains have been found in Abu Hureyra settlement in Syria dating back to 13000 years ago around the beginning of the Younger Dryas.     

The other hypothesis, opposing this is that there was actually a return to a more mobile lifestyle during the Younger Dryas, meaning a return to hunter gatherer styles of society.  They argue that it was not until warming began after the Younger Dryas that domestication occurred, and the evidence found is not strong enough to definitely suggest crop cultivation.  Wilcox and Rosen are supporters of this hypothesis, Wilcox dismisses the evidence found in Abu Hureyra as no other evidence has been found in other locations.  Rosen on the other hand believes that it is more likely that humans domesticated once the Holocene had begun and warmer temperatures had returned as populations will have grown putting pressure on resources leading to domestication.  

Personally, after reading the two articles and looking at the varying viewpoints I agree with the latter hypothesis.  This is because I believe a return to colder climates would have caused human populations to return to hunting as they did throughout the last ice age rather than domesticate.  Also, populations are likely to have been smaller during the cold period, meaning there would be less population pressure to domesticate.  

Nitrogen fertilisers and climate change

Over recent years, primarily since the ‘Green Revolution’ in the 1960s, nitrogen has been increasingly used in agriculture, now more nitrogen is produced artificially for fertilisers than is produced naturally by the earth.  

Nitrogen naturally forms in soils in tropical and temperate regions of the earth and in the oceans.  However recently it has been used as a fertiliser, is spread over fields (see picture below) as it increases crop yields as more nutrients, namely nitrogen is available in the soil encouraging greater plant growth.  This has consequently led to an increase in nitrous oxide concentrations in the atmosphere, as more nitrogen in the soil means a greater rate of microbe activity, which creates and releases nitrous oxide. This is important, because nitrous oxide has a warming affect on the planet, being a greenhouse gas, but it also according to Crutzen and Ehhalt (1977)  has a depleting affect on the ozone in the stratosphere, which means more UV rays can penetrate through warming the planet.  It is thought that nitrous oxide accounts for 6% of total anthropogenic radiative forcing (Davidson 2009).  Nitrous oxide production only occurs under specific conditions and results from the combination of aerobic and anaerobic processes.  Nitrification is the process of ammonium transforming to nitrate, an aerobic process and denitrification, the formation of nitrogen gas from nitrate reduction, an anaerobic process.  This nitrogen gas is then oxidised to form nitrous oxide (Monteny et al 2005)

Nitrogen fertilisers are used to increase crop yields so it is possible to produce more food from the same amount of space, making food production more efficient.  This consequently led to an increase in population, and is one of the reasons why we have seen the population expand so rapidly over recent years.  But in turn the increasing population needs food so results in higher crop yields being needed.  This means that fertilisers are used at an increased rate having negative effects on the planet.  Not only does it result in increased concentrations of nitrous oxide in the atmosphere but also the use of fertilisers causes loss of soil nutrients, soil acidification and erosion reducing soil quality in the long run.  

The work of Boering et al. is discussed in this article, who looked at samples of Antarctic ice dating between 1940 and 2005, to reconstruct nitrous oxide concentrations in the atmosphere between these times.  They found that it is possible to differentiate between natural and agricultural nitrous oxide, due to its isotopic composition.  This is useful as it allows us to determine when concentrations of nitrous oxide rose significantly as a result of agriculture and see how successful mitigation strategies are at reducing it.  

The use of nitrogen as a fertiliser is important due to the negative affect it has on climate change, as it is the predominant cause of increase in nitrous oxide concentrations. For more information on the effects of nitrous oxide on the atmosphere, I found this link useful. 
The diagram below shows how nitrogen fertilisers are incorporated into the nitrogen cycle.  

rice paddies and methane emissions

Another way domestication has had a significant impact on concentrations of greenhouse gases, is the effect of rice paddies on methane concentrations.  It is thought that rice paddies could contribute to 20% of current methane emissions.  

Rice paddies date back to the beginning of agriculture and archeological evidence shows the first paddy to be in Korea.  Since this time rice cultivation using paddy fields has developed all over the world, in Europe, the USA and across much of Southeast Asia. China is now the greatest producer, accounting for 36% of the worlds rice.  Methane is produced by anaerobic bacteria in the flooded paddy fields. (see picture below)

From 1940-1980 methane emissions have increased by 49% and during this time, the global rice harvest has increased by 41%.  This is as a result of higher yielding crops, expansion of crop areas and increasing use of fertilizers, but this increase in production of rice has meant increase in methane concentrations.  

According to Aselmann and Crutzen (1989) rice paddies cover 1.3x106 km2  of the Earth's surface.  They undertook a study determining methane emissions from both rice paddies and wetlands.  They estimated rice paddy methane emissions to be between 60-140 Teragrams per year.  They found emissions to be highly seasonal, with greatest emission levels to be in the summer in both Hemispheres.           

A paper by Cao et al. (1996), used modelling to predict methane contributions from rice paddies across the world.  They found this difficult as not only does it vary greatly between regions, but they also found great seasonal variation between emissions as well.  They estimated emissions to be 50-60Tg yr.^-1.  Another study by Liu and Wu (2004) uses models again to estimate methane emissions from Taiwanese paddy fields. Here they found temperature to be the most important factor affecting the amount of methane emitted, so the higher the temperature the higher the concentration of methane.  They also found there were seasonal variations in methane emissions.  

Bachelet and Neue (1993), estimated methane emissions from Asia, as this is where 90% of rice is produced.  Here they evaluated different approaches of estimating methane emissions, again they found a major weakness to be that some methods used a constant rate of emissions for all months, which affects the results, as there are seasonal variations.  From their comparisons they concluded that in the past it seemed emissions for this area had been over estimated.  They gave an estimate of methane emissions from Asian rice fields to be 63 Tg yr.^-1, which is different from Cao et al.  

Consequently, this shows that current methane emissions from rice paddies are uncertain, as suggested by Aselmann and Crutzen, due to the complexity of measuring/modelling as a result of spatial and temporal differences.  However, it is important that reliable predictions of methane emissions are calculated as methane has a a warming effect  21 times greater than carbon dioxide.  

Methane emissions are set to increase as rice production increases to supply a growing population.  However Nueu (2007) suggests that we do have sufficient understanding of rice production and its effect on methane emissions to put in place policies to reduce impacts. These are important and I will discuss mitigation strategies surrounding rice cultivation in a later post.  As Nueu says, many people rely on rice as a staple in their diet and at the rate that population is set to increase, at least 880 million tonnes of rice will need to be produced by 2025 to keep up with growth.  Unless changes are made this will lead to huge increase in atmospheric methane concentrations.  

Deforestation

Before agriculture, forest covered 57 million km² of the earth (Malhi et al 2002), however, as a result of domestication much of this forest cover has been lost.  As domestication became more wide spread and intensive, it was necessary to clear land to create space for grazing and crops.  

Deforestation has an impact on global climate through increasing the release of carbon dioxide into the atmosphere.  Clearing often occurs through burning the forests which its self releases high concentrations of carbon dioxide into the atmosphere, but also the vegetation and the soil store large amounts of carbon which is subsequently released into the atmosphere when forests are burnt or logged.  Crops or grasses in grazing areas do not store as much carbon as large forested areas, meaning there is a higher concentration of carbon in the atmosphere.  

Burning of forest from charcoal records can be seen around the beginning of domestication 8000 years ago, and forest clearances seems to accompany the spread of agriculture.  It seems that most temperate forests, for example in Europe and China, have been cleared progressively since the beginning of domestication, a very small amount survived to the industrial era (Malhi et al, 2002).  However, until 1700 only about 7% of global forests had been lost (Goldewijk, 2001).  This has significantly increased in recent years and deforestation has contributed to 45% of the increase in atmospheric carbon dioxide since 1850 (Malhi et al, 2002).  This is as a result of more widespread deforestation and deforestation of more densely forested tropical areas which are thought to be ‘carbon sinks’.  

Carbon sinks are forests which are thought to be fertilised by the recent increase in atmospheric carbon dioxide, leading to increased growth rates meaning the forests store more carbon so there is less in the atmosphere. The Amazon rainforest is an example of this and it is thought to be actually slowing the anthropogenic effect, however if these forests are deforested they then become a huge source of carbon (Laurance 1998).  They are very resilient if they remain intact, however many, like the Amazon are being deforested for agricultural purposes, for example by 2001 837000 km² was cleared for cattle ranching and soya bean production (Malhi et al 2008).  Clearing leads to fragmentation of the environment, limiting regrowth.  Much of the tropical forests in southeast Asia have been lost, the map below shows loss in Borneo, and projections for the future.  

It is likely that if large areas of these tropical forests are deforested, they will become large sources of carbon which will increase atmospheric concentration.  Therefore policies are continually being put in place to limit deforestation and protect forests.  There are many campaigns, such as Green Peace, their video below is useful for seeing the effects of deforestation in the Amazon rainforest.