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.