By Jakub Olewski

On 23-25 June 2009, I had the pleasure of attending the Soil Organic Matters conference at Rothamsted Research Centre in Harpenden, Hertfordshire, UK. The three-day event had two main aims: (i) to acknowledge the great contribution to soil science made by Professor David Jenkinson FRS, and (ii) to bring together scientists from around the world working in the areas in which Prof. Jenkinson laid the foundations in order to see where current research is heading. The conference was supported by the Agriculture and Horticulture Research Forum (AHRF) and the British Society of Soil Science (BSSS). It would not be possible to summarise all the conference here, but I would like to present my impressions from the presentations I liked the most and found most relevant to vegan organic farming principles and practice.

Note: in the article I use the terms ‘soil organic matter’ (SOM) and ‘soil organic carbon’ (SOC). The term SOC is used mostly to emphasise that SOM is mainly made of carbon (C) which, when mineralised, ends up as carbon dioxide (CO2) or methane, this being of great interest as they are greenhouse gases. Sometimes SOM and SOC are used interchangeably because the complex SOM molecules are, on average, composed of 58% C, and vice versa, SOC is C that builds the SOM molecules – one cannot exist without the other.

Climate change issues

Jakub with a trial clover harvest. Photo: Emilka Plocka

Given the current critical situation of our planet, one of the main themes of the conference was climate change. Prof. Jenkinson started the presentations. In his introductory comment he highlighted the linkages between atmospheric CO2 and C stored in soils and plant biomass. He stressed the fact that although anthropogenic emissions are relatively minor compared to amounts of C circulating between atmosphere and land and the ocean, a large proportion of them is not absorbed and cumulates. The turnover of soil C also becomes faster as the climate warms – in the given time more C is entering the soil, but also more is released – what is worrying is that large C stocks in permafrost (up to 1000 gigatons) might become unstable and quickly mineralised and released in the form of CO2 and methane. Prof. Jenkinson also pointed out that warming would inevitably lead to tree loss in the tropics and greening in the north. It is unlikely that release of C from standing biomass stocks in the tropics could be offset by expansion of the forest further north; especially because ‘greening of the Arctic’ actually accelerates mineralisation of SOC.

The greening of the Arctic was further explored by Professor David Hopkins (SCRI, Dundee) who showed some examples of ‘priming in action’ by plants in tundra that can cause release of older C from organic soils. It is hypothesised that this could be due to factors such as: rise in microbial activity stimulated by roots, drying and aeration of deeper soil layers. Plant soil interactions were a pervasive theme and were also discussed by Dr Andreas Heinemeyer (University of York) who presented ‘overflow tap’ theory which links Net Primary Productivity (NPP) to forest soil carbon dynamics. One of the most important findings presented was that when there is surplus tree carbon (from photosynthesis), for example at optimum NPP or elevated CO2, additional C produced by photosynthesis is transported through roots to the mycorrhizal network where it is released. Because carbon compounds released by roots are easily decomposable they cause the so-called ‘priming effect’. The priming effect is a stimulation of decomposition of otherwise stable organic matter by the addition of small amounts of easily decomposable material. Dr Heinemeyer emphasised the need to include root-derived (mycorrhizal) carbon input into soil respiration models to be able to more accurately predict C fluxes and their interactions. Dr Heinemeyer explained that a better understanding of carbon cycle feedback is crucial to the improvement of future climate change predictions.

Behaviour of land C sink in the changing climate is the largest uncertainty (up to 250 parts per million by volume of atmospheric CO2) in climate change models as Professor Cox (University of Exeter) and Professor Pete Smith (University of Aberdeen) pointed out. Prof. Cox showed that one of the reasons for this are tipping points which have a strong theoretical and experimental basis, but are very difficult to predict due to complex feedback mechanisms. It is often claimed that increased temperatures and higher CO2 levels could result in greater NPP and increased deposition of carbon. However it is near certain that the so-called ‘CO2 fertilisation effect’ will not cause an increase in carbon store. Computer simulation models, such as ‘RothC’, predict runaway collapse of C stocks after a short period of gradual increase. The reason for this is accelerated decomposition of SOC as the temperatures rise unmatched by an equivalent increase in SOC creation. Prof. Cox named the phenomenon – the ‘Jenkinson effect’. Another worrying uncertainty in how land C sink will behave with rising temperatures is the stability of peatlands. This is because decomposition of SOM is known to have a heating effect, so increased decomposition due to rising temperatures could lead to a positive feedback phenomenon called ‘compost bomb’ leading to even more accelerated decomposition and release of further C into the atmosphere.

Where nitrogen fits in

Taking care of the clover. Photo: Emilka Plocka

The last day of presentations concentrated on soil nitrogen (N). The importance of N, with respect to climate change, is no less than that of C because the cycles of both elements are linked. The first presentation, ‘Mankind’s love-hate relationship with N-fertiliser’, was by Professor Chris van Kessel (University of California). Humanity nowadays has become ‘addicted’ to synthetic fertiliser-N and it seems virtually impossible that the world could do without it. Yet the efficiency of use of this nutrient is inherently poor. Globally, only 50% of applied N on average is recovered in the crop and the rest is lost to the environment. Despite some of N being stored in SOM, the main pathways of its loss are harmful to the environment as N is still in reactive form (leaching of nitrates and dissolved organic N, ammonia and nitrous oxide (N2O) volatilisation). One of the most important concerns nowadays regarding N is N2O production by agriculture, which accounts for 69% of anthropogenic emissions of this greenhouse gas to the atmosphere. Although the emissions from newly produced synthetic fertiliser-N are estimated to be only 1% of the total N2O emissions, if secondary sources such as animal manure produced from feed, and leached N are accounted for, this proportion rises to between 3 and 5%. The analysis of a large number of studies cited in the presentation showed that N2O emissions were highest per unit of product for both the high and low rates of N input. Hence, the important conclusion made by Prof. van Kessel was that although there was no single practice or strategy that could have led to reduced N losses, the optimisation of food production should have been based on the losses per unit of yield to better reflect the N economy.

Going veggie: better use of nitrogen!

The most important statement of this conference for me was in Prof. van Kessel’s presentation when he said that one of the ways to improve the efficiency of N use would be a move towards a vegetarian diet. Of course I was very happy to hear that, especially because it was backed by very strong evidence; he did that by just quoting three numbers: 3, 14 and 21. What ‘3, 14 and 21’ means is: we need 3g of N in the available form supplied to soil to receive 1g of N in wheat protein (equiv. of 6.5g of protein), to receive the same amount of protein in milk and meat we need almost 5 and 7 times more N respectively! What this means is not only that more energy and other resources are needed to produce fertiliser, but also, more importantly, it means that the losses of N are much, much greater. Most of N lost from food production is still in available (reactive) form and it is harmful to the environment in many ways before it is transformed back to the most abundant, neutral N2 (which comprises more than 70% of the air we breathe, but is directly inaccessible for most living organisms). Reactive N is lost in the form of: nitrates and dissolved organic nitrogen, which pollute water bodies; nitrous oxide that contributes significantly to global warming; and last, but not least, ammonia which plays a large role in the acidification of ecosystems. After looking into another paper cited by Prof. van Kessel , I did a rough calculation of how the losses of reactive N would be reduced if the world followed a vegan diet and the approximate value was 50% reduction (keeping the same level of protein consumption). A few other estimates also show that a reduction of such magnitude would be very likely. For example, if the population of the USA switched to a Mediterranean diet (i.e. consumed about 7 times less meat!) they could reduce their needs for synthetic N fertiliser by about 65%. Transfers of N can be estimated on various scales; a comparison of efficient nitrogen use between vegan-organic farming and horticulture, and other systems such as organic, that still uses animals, and conventional farming would certainly be very interesting. But this is something for another article…

Positive change on climate change

Coming back to climate change, the good news among scary predictions is that soil management in many countries around the world now uses an approach that looks at the whole lifecycle of the given produce, which is likely to provide sound solutions applicable at regional and country scales. Changes are happening and sometimes they are smaller than we would wish, but they are heading in the right direction. Indeed agriculture has a great potential for climate change mitigation – one of the most cost-efficient measures can be found in this sector – soil science plays a key role in the process of unfolding it, as underlined in Prof. Pete Smith’s presentation. In this regard, we need to see more scientists advocating a shift to a plant-based diet because this is the only way that humanity can stop ‘eating the Earth’. Decreased demand for meat would in turn help a more rapid development of stockfree organic agriculture.

In SOM research it is very important to look from the perspective of years and decades by means of long-term experiments. Professor Martin Gerzabek (BOKU) presented results of investigation into SOM dynamics – mineralisation and stabilisation – under different cropping systems. Longterm data from Fuchsenbigl in Austria were used to calibrate the ‘Roth-C’ model (computer simulation model for SOM). These results were then compared with other long-term experiments from Rothamsted, UK and Ultuna, Sweden, in order to investigate how straw removal influences SOM dynamics of different soils under different management and climate. The results showed that straw removal leads to a longterm decline of SOM, but the trends vary for different sites and different plants. For example, oilseed rape increased SOC stocks even when all the straw was removed due to high root biomass. The findings of Prof. Gerzabek were accompanied by a presentation from Dr Anne Bhogal (ADAS) who also presented long-term experimental results. Dr Bhogal’s results indicated that repeated additions of various organic C materials led to numerous improvements of soil quality and fertility. I do not have to say here that adequate inputs of organic materials – for example chipped branch wood and various green manures – are at the core of vegan-organic farming methods.

The hot topic of the conference was biochar. There is a growing interest in using biochar as a climate change mitigation tool (e.g. see article by Craig Sams in last GGI, issue no. 23). Dr Saran Paul Sohi presented preliminary results of a joint study between CEH Edinburgh and Rothamsted Research. The presentation highlighted the complexity of biochar-based mitigation options and the attempts of researchers to answer basic questions regarding biochar by means of a life-cycle approach combined with ongoing experimental work. Once again the need for a systemic approach to the promotion of the environmental benefits of new soil management strategies was emphasised.

The last word…

Between the presentations, participants of the conference vigorously discussed what kind of soil science research is currently needed. There was some agreement in this respect, and the conclusion was that we need long-term, well-documented and sampled field studies as well as mechanistic understanding of the basic processes governing SOM turnover, such as microbiological laboratory studies. We need to look at various ‘strings attached’ to new ‘silver bullets’ to mitigate climate change using a life cycle and interdisciplinary approach, but on a manageable scale. Investing much effort and resources to put all soil scientists, microbiologists, chemists, physicists and so on in one particular site and letting them measure everything that is there might not be the best approach to solving our problems; focused questions need to be asked first. The conference gathered together more than a 100 researchers from vibrantly various shades of soil science from around the world. It was a great event that effectively expressed how vital and timely soil science is. I hope that I was able to give you a glimpse of that.

The author wishes to thank the British Society of Soil Science for covering the conference and travel costs.

Editor’s note: Jakub is doing a PhD studentship at the Scottish Agricultural College and University of Aberdeen; he is studying ways of using legumes to improve nitrogen use of crops – very relevant to VON’s work!

This article appeared in Growing Green International magazine Num 24 (Winter 2009), p12.