We have been removing organic matter in the form of crop products for millennia. Although some carbon was returned via animal manure, this has declined greatly for the past century. This issue is not just about biofuel crops - for many other crops both the grain and stalks are harvested for food and straw/fibre, e.g. wheat. However roots are rarely harvested and these return significant amounts of carbon to the soil. This is why we do not always need to add organic carbon as well as fertilisers to crop systems. Soil carbon is also replenished by other rhizosphere components such as fungi, including mycorrhizae. We still understand very little about the rhizosphere but it is clearly a crucial component of soil fertility and crop performance.

I suppose the answer might be second-generation biofuels (using non-food feedstocks or waste substrates), as also suggested by the EU SET Plan and Renewables Directive...

Life and living on earth need to be in balance. You cannot hype on political issue/agenda. Science is much more complicated and that you have to face and make politians be aware of. You have as a scientist that responsiblity. Yopu have to be a good scientist with a crtical, sound and healthy helicopter view.

I'm agree, biofuels that can also be produced with waste of agricultural biomass,through thermoconvertion such as pyrolysis which is a promising process to produce bio-oil, bio-char and biogass

We have been removing organic matter in the form of crop products for millennia. Although some carbon was returned via animal manure, this has declined greatly for the past century. This issue is not just about biofuel crops - for many other crops both the grain and stalks are harvested for food and straw/fibre, e.g. wheat. However roots are rarely harvested and these return significant amounts of carbon to the soil. This is why we do not always need to add organic carbon as well as fertilisers to crop systems. Soil carbon is also replenished by other rhizosphere components such as fungi, including mycorrhizae. We still understand very little about the rhizosphere but it is clearly a crucial component of soil fertility and crop performance.

In two words: "not monoculture"! I would agree: soil fertility is the key component. With cover-cropping, coppicing of medium sized nitrogen fixers in a "chop-n-drop" method, and other soil amends (like compost and mulch), soil fertility and site productivity can be increased, allowing us to even remove existing biomass for other purposes. Certain waste products of the desired processes (you mentioned biofuels) can also be returned to the system. Even in large-scale (hundreds or thousands of acres/hectares) this is becoming more common, but not common-place enough.

Dear Thomas,

The question is on the table, indeed.

Usage of agricultural by-products like stalks, straw, wood waste to produce heat, energy or new feedstocks for chemistry like so called bio-oils or green oils is an integral part of considerations about the economics of biorefineries. As ethics play a minor part in this game (unless imposed by politics or public opinion) price will dictate the direction - which is and has been, by the way, the case with fossil resources as well, for example the historical move of chemistry away from coal to crude oil, gas, shale gas (?), and renewable resources.

Best regards

Rainer

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Biochar produced from different types of biomass yields biofuel and biochar primarily. The biochar is now being added to soil to increase plant growth (e.g. corn), minimize fertilizer use, and sequester carbon (mitigation of climate change). I believe this is a sustainable model that needs to be fully developed and applied.

Dear Valentine,

Did you calculate cost for collection, transport, drying, energy and other cost for pyrolysis just to make what is now called "biochar" ? I doubt very much that this is more cost-effective and sustainable than what farmers have done since historical times: plowing under the straw or even to burn it on the field. You certainly need an added value (for the green oil and even better, for the charcoal) to make this approach sustainable.

Best regards

Rainer

Check this paper:

Corn Ethanol Production, Food Exports, and Indirect Land Use Change

T. J. Wallington,* J. E. Anderson, S. A. Mueller, E. Kolinski Morris, S. L. Winkler, and J. M. Ginder, Systems Analytics and Environmental Sciences Department, Ford Motor Company, Mail Drop RIC-2122, Dearborn, Michigan, 48121-2053, United States

O. J. Nielsen, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark

As was mentioned before, use none food/feed feed stock - agricultural waste, algae (once we find an economical way to harvest them, among others hurdles).

On a longer run - improve photosynthsis so plants (and algae, cyanobacteria) fix more CO2 to organic carbon. Now all we need is a realy smart way to do it...

Yoram

A very interesting question and discussion indeed...

Organic material is the prime product that managed ecosystems deliver not only for food, feed, fibre but also increasingly for bio-energy purposes. Theoretically some of the byproducts from bio-energy production or bio-material recycling can be used to ameliorate the soil. Common by-products are digestate, biochar and compost. The problems related to transport and quality of the by-products remain largely unsolved. For the same reasons not all crop residues can easily be used as feedstock for bio-energy such that quantities may be optimistically overestimated. Land management practices such as crop rotations including cover/catch crops and the use of mulches may alleviate the problem of declining soil organic matter and in addition create a win-win situation for both farmers and bio-energy producers (or bio-material producers for that matter). Of particular interest is the contribution of the soil food web to organic matter dynamics! ... and the discussion is far from finalised here as not all contributing factors to soil organic matter are properly understood.

I have tested the use of subsurface injection of gaseous emission of internal combustion engines in order to use the subsoil as green filter for these pollutants.....one indication of these tests was the increase of carbon content of soil and less need to fertilizers...

You are right to think this way, but then the technique and feedstock used for the production of bio-fuel determine whether or not to worry about soil fertility. For instance, pyrolysis whether fast or slow will give bio-oil, bio-char and bio-gas. The bio-oil can be used to power engines, though sometimes not without upgrading, while the bio-char can both be used as sources of power and soil amender or improver as it is is rich in essential plant nutrients.

Another thing is that "whole or edible" agricultural crops are not necessarily used to produce bio--fuel. Rather wastes from them are usually encouraged to be used so as not to compete with food consumption and the fact that something like root is left on the field still make available some organic carbon to the soil.

What kind of organic carbon is needed for maintaining soils quality? If we add sugars, alcohols or other labile organic carbon, this carbon will be consumed very fast by aerobic bacteria at the soil top layer and no organic matter will be measured in soil after some few days. But this is the kind of organic matter that a biogas plant requires.

Contrarily, the refractory, or slowly biodegradable, organic matter in an anaerobic digester will remain in the digestate, being the organic matter that should be applied to soils

In my opinion, the reconciliation can be found in this way: to recover the fast biodegradable matter as energy (biogas) and to forward the slowly biodegradable from digestate to soil or to further aerobic composting process (and finally to soil).

It may be interesting to prepare carbon budget in such systems to audit inflow/outflow of C.

From my point of view: "just ruling prices and making laws for soil exploitation if energy is involved". We should have learnt something about economy and energy from past experiences. It is a primary need and if coming directly from soil exploitation should be kept outside economical speculation. As a colleague of mine said: We can live without OIL but not without SOIL

This sounds like another debate on fuel vs food. Though in my own opinion soil carbon does not neccessarily have to be supplied by food crops only ;existing weeds cleared before planting and turned over during ploughing also contribute to soil carbon which doesn't serve biofuel purposes.Hence, long-term maintenance of soil fertility through soil carbon would still continue as long as there are other plants residues not qualfied as biodigester "feedstock".

Moreover there are some agricultural practices that should be checked and if possible stopped which allows for loss of existing soil carbon such as double or triple ploughing activities in loose soils.I

THOMAS,

It might be useful to consider the total asset base as a mitigator in solving your problem. Last year my team mates and I were asked to consider a similar problem as part of our post-graduate studies at Portsmouth (UK) Business School.

We took a typical strategy approach, i.e. assets through a process produce output. In this case the locus was Sahel (lower Saharan) Africa. Thus we had assets of solar energy, time and almost limitless land, yet limited water. .Our solution recommended underground reservoirs using photo-bioreactor process utilising sunlight and Calcium Carbonate in the local soils to feed algae hence green carbon to start soil conditioning and other agro-economic processes. Power, space and time might resolve the gradual soil condition / multiple plowing conundrum as it might biofuel demands. Likewise cost against return demands.

Carbon from crop residues is elusive as it easily and quickly return to the atmosphere with the oxidation of the organic matter. What definitively contributes to the C sequestration in the soil are the deep roots, those that go beyond the arable profile. Deep roots also helps canalizing water deep into the soil thus helping water infiltration (diminishing soil erosion), that's say, sequestering water and nutrientes, ultimately resulting in long-term increase of soil fertility. See references below:

Kell, D.2011. Breeding crop plants with deep root: their role in sustainable carbono, nutriente and water sequestration. Annals of Botany 108(3):407-418.

Glover, JD et al., 2007. Future farming: A return to roots? Scientific American 297: 82-89.

All these answers are appropriate. When considering corn stover for biofuel, one must also consider what is the biomass yield and what is the level of residue needed to prevent erosion and maintain soil health. Some systems produce more residue than is needed for these functions, while in warmer climates, possibly no stover should be removed. Based on soil properties, climate, and management, some soils may be "C saturated" and further large additions of crop residues do not significantly increase SOC. Carbon from roots is also an important aspect, as already described. In some systems, return of residues may have little effect on SOC sequestration, as these materials are quickly decomposed. However, roots tend to be less rapidly mineralized and may lead to more long-term refractory C. We conducted a study with biomass sorghum for biofuel generation for six years, where we have observed significant increase in SOC to a meter depth. Treatments with biomass return show no more SOC than those where all biomass is harvested, indicating that roots are the major contributor to SOC sequestration in this system. Dr. Matsuoka above also indicated that deeper roots are also generally slower to decompose, partly because of a less favorable environment for decomposition with depth, thereby also contributing to longer-term sequestration.

How can we reconcile the competing demands for organic carbon in agriculture, i.e. C for biofuels (e.g. from maize) vs. C for soil fertility?

I fully agree with F.M.Hons above. This is a site specific problem and highly dependent on the biomass yield of the particular crop and the level of residue required to prevent soil erosion and avoid losses of soil organic carbon (SOC). In Mediterranean environments of Central Chile we find that direct drilling is essential to avoid SOC losses and that an organic mulch of around 4 Mg / ha of crop residue on top of the soil brings soil erosion down to minimum values. In the case of the corn crop our mean grain yield is around 12 Mg / ha (irrigated summer crop) with a harvest index close to 0.5 what implies that after grain harvest you have in the order of 12 Mg / ha above ground corn stover biomass . It follows tha you could safely harvest 8 Mg /ha for bioenergy or other purposes.

Following these simple agronomic rules we have turned a negative SOC balance (-2.5 Mg C /ha/year) under conventional tillage to a positive one (SOC gain of around 0.5 Mg C /ha /year) in periods of 10 years or more and at the same time we have increased soil fertility and health (judging from the increase of soil biological activity and nutrient availability).

Our SOC papers are available on Researchgate.

Denis Murphy quite rightly points out important issues re the unavoidable removal of carbon and that roots contribute a great deal of carbon. Another way to improve soil carbon balance is to convert some organic material to biochar, then return it to the soil. Biochar is persistent for time orders of magnitude greater than normal soil carbon.

Continuing on Xavier Flotats answer above, how about using shredded old newspapers as organic carbon amendment? There are abundant supply, recycling them is a pain, and they will degrade slowly. Transport and ink eco-toxicity might be an issue though.

Just a thought...

Yoram

My answer is exactly the same of Xavier. We have direct experience that long chain refractory carbon compounds are present in high percentage (50-70%) in the solid fraction of the digestate from biogas plants. These compounds are the best promoters of SOC. For this reason I use to say that we must build digester plants were we have waste biomass and soil desertification. Furthermore I totally disagree with the practice of producing energetic crops for feeding digesters.

In developing countries apart from competition for. Biofuels there Is competition from live stock for fodder. Some options that would be good are harvesting the crop at higher heights so that some anchored residues are left on the ground and the above harvested biomass can be used for other purposes like fodder or feed stock.

The other option is to grow high biomass crops in between the crop rows so that this also would increase biomass to the soil.

The third strategy would be to grow for agroforestry systems or utilise the bunds for planting of trees and use the biomass as mulch.

Another option that can be adopted to increase Soil c is by recycling the residue to soil after using it as feed stock. For eg. Bio char after extraction of ethanol from corn,

I'd recommend reading this quite interesting article:

http://www.ars.usda.gov/is/AR/archive/feb14/soil0214.htm

Rattan Lal has an interesting article discussing the competition for carbon, concerning aspects of using the agronomic residues as source for biochar

Lal, R.. 2008. "Black and buried carbon impacts on soil quality and ecosystem services". Soil & Tillage Res. 9: 1-3.

Anaerobic digestion (syn. biomethanation or biogas production) is an excellent alternative to this food/feed - energy contest when considering soil organic carbon and nutrients (both contributing to soil fertility). The biomass from energy plants is converted into an energy carrier, biogas (CH4 and CO2); the undigested material (digestate) contains NPK in a highly bioavailable form and organic carbon under a very stable form (non microbiologically degradable biomass), ideal to contribute to soil humus. Limitation to the use of such digestate in an optimized manner is (1) legislative in nature (EC Nitrate Directive), and due to (2) the high proportion of P compare to N in the row digestate. Digesate treatment has the potential to answer both limitations.

The planet, at roughly 1/3 of its surface being land, of which only 11% is tillage with another 26% capable of being used for pasture. Not all the remaining "space", yet a great deal of the spac is capable of producing carbon for the addition to tillable soils if required. Cover crops also should be considered. The critical loss of soil fertility and or the need to increase crop yields on the 11% of tillable ground is a concern due to growing global populations and decreased efficiency of production of proteins through increased consumption of meat and dairy proteins. For these reason we need to investigate methods to ensure excellent carbon levels, the base of fertility and the food source for 50% of the biomass in the fields (fungi) are imporoved. Yet, to take a myopic view of a single element does not address the need for other elements in the soil to maintain fertility, yield and a burgeoning growth of microbes and larger beasties of the soil. The nitrogen hungry carbon requires sufficient quantities of N to kick-start the carbon breakdown process. So to simply discuss C additions to the soil results in a ripple of adjustments required to maintain balance.

Taking 21 million gallons of watewater, treating it in a two phase process (first obligated aerobic bacteria followed by an anaerobic pond with an algae cap) allows us to achieve regulatory BOD requirements for irrigation on crop fields. Numbers of experiments have been performed with various crops to determine optimum uptake and transportation rates. (Note: most global studies regarding irrigation investigate the minimum amounts of water to be applied to a crop to sustain profitable yields. Just the opposite is true for the maximum applications and soil moisture levels required to evacuate the treated wastewater.) Using optimum soil moisture levels though various stages of crop growth to ensure maximum transpiration has had the benefit of producing massive amounts of carbon. Corn irrigated with wastewater, with N an P already supplied via the treated effluent, we have yields of corn that surpass by 40% any of the yields garnered by corn producers in our area. The carbon production in plant and cob size increase in the same order of magnitude.

The benefits are multifaceted: 1. Valuable nutrients are not hauled to the landfill and lost; 2) The water is returned to the aquifer via transpiration to which the moisture precipitates out of the atmosphere within 200 km; instead of being dumped into the local sewer system at added treatment costs 3.) Carbon levels increase in the soil due to increased crop residue; 4.) Crop yields are greatly enhances making crop production very profitable; 5.) Ethically the corporation is perceived by customers and the public as being socially and environmentally responsible.

The singular approach to carbon levels is to not address the synergistic relationship that exists differently in each bio-region. As scientists we must continue to view the interrelatedness of the components of the environment and still have carbon to burn.

I would like to raise some aspects partly mentioned and add another thought. Firstly, there is no silver bullet solution. We have to combine various pieces of the puzzle and find local solutions. Reducing soil disturbance by increasing the proportion of perennial crops is known to contribute to soil organic matter accumulation. There is some research on perennial cereals. These might be an alternative source for biofuel production, like other known plants (e.g. miscanthus) not yet established due to economic (logistic/technical) reasons. Of course there are constraints to it. Additionally, clever crop rotations with integration of multiple cropping and cover crops/catch crops also allow for an increase in the annual productivity per area and thus especially below ground carbon. Plant breeding might in future contribute by providing new varieties with larger rooting systems. We can also use some of the old varieties able to procude more biomass per area. These might be especially suitable for biomass production used energetically. However, we will probably in the near future have to make stronger and multiple use of biomass using it in cascades to produce food, fodder, fuel, fibre and fine chemicals. Returning valuable domestic organic matter to the fields is an ultimate aim for a more sustainable soil organic matter management. Last but not least, we have to better understand how we can positively influence the soil microbial community towards accumulating organic matter in the soil, while at the same time maintaining other ecosystem services (e.g. mineralisation, nutrient mobilisation). A diverse, probably fungi dominated microbial community with a high microbial biomass having a lower metabolic activity might be a future scenario to aim at for sequestering more carbon in the soil in a sustainable cropping system. Nevertheless, as long as we use more energy (even though more efficient) and burn more carbonaceous compounds, we will never achieve both goals.

The benefits that confer organic matter loads to soil productivity are no longer a debatable issue and it is also true as highlighted by colleagues earlier that the quality of the organic loading impact on soil fertility positively as well as negatively strongly inherent to the soil organic matter stock and the quality of material being applied, disposal of use paper for instance might not be that desirable due to very high C:N ratio.

Having mentioned above, I think the question raised by Dr Thomas is very pertinent. With the realization of biofactory benefit in countries with sole natural resource being agricultural the competition for biomass is already soaring with harvesting indices shifting from a few percentage to 100%. Crop leftover in field is often a question of economy rather than sustainability at source.

Reference has been made about recycling biofactory waste (effluents and solid) to crop land, this approach is very likely limited to a few acres within the biofactory area since the transportation cost incurred as well as application cost are serious limiting factors that unfortunately will again prove uneconomical.

Based on above, I think there is need to establish sustainability or threshold levels of soil fertility parameters (ratios should in my opinion be avoided they often undermined impending deficiencies) of crop lands that can dictate more accurately crop harvest indices that will guarantee the maintenance and recovery of land fertility in the long term and including biological indicators that will define for instance the need for crop rotation.

Optimising photosynthesis function in plants with nutrition management can correlate to improved carbohydrate production in plant and contribute to improved plant performance and Soil Organic Carbon. To the extent that a plant over its growing life has access to balanced nutrition, and robust root development, conducive to soil biota function, so to can its carbohydrate production vary and potential of rhizosphere deposition including Mycorrhizal fungi function vary , impacting on Glomalin production which has important contribution to SOC. Agronomy to facilitate large root development and rhizosphere function can include prudent use of Humic & Fulvic inputs with fertilisers and minerals, with selected microbial inoculum to achieve improved cation and other mineral ratios, and work toward achieving optimum ratio of active fungi : active bacteria ( equally in soils for broad acre cropping).

Carbohydrate production variation trend in plants can be simply monitored with use of a refractometer providing brix readings-by farmers and agronomists. Experience correlates a trend of improving plant nutrition status, brix levels, plant performance, and SOC.

Effectively it amounts to farmers utilising Sun heat energy more efficiently with soil, water, & minerals to photosynthesize more Atmospheric CO2 into plants , with improved proportionally, carbohydrate level deposited in root rhizosphere, which is processed by soil biota to improving SOC.

World decline in SOC levels can be reversed with focus on optimising photosynthesis function in plants.

The answer is that we require to maintain SOC and at the same time need to reduce pressure on natural resources like oil, water etc.. for sustainable agriculture and feed the growing population. we have to pave an intermediate way. so the solution is that we can go for incorporation practices for crop residue in alternate years. Then we can go for C for biofuels also. Moreover we can add FYM, incorporate short duration nitrogen fixing crop like moong grown in fallow period or liquid slluries from biofuel plants, biochar to soil in the alternate or no-incorporation years. I hope this will work.