Dear Michael,

I certainly agree with you. Attached is a review article on  Agriculture in Denmark which might contribute to the discussion on this topic.

Rafik

Dear Michael,

I certainly agree with you. Attached is a review article on  Agriculture in Denmark which might contribute to the discussion on this topic.

Rafik

Yes, bio gas plants do deplete the humus content in the agricultural soil. There is very little one can do about this. The type of bio gas plant also plays a role in the depletion. The extent to which this occurs is more through plants that take way a lot of humus from the soil, say cowdung. A switch over to cow flatulence as a source of fuel and energy is a solution.

Bio gas plants are just part of the system we have created over time.  The issue is the change from a rural agrarian society to urban/rural structure.  Looking at the numbers on food product from the United Nations, it does not mater where or what culture is involved.  Approximately 1/3 of the biomass produced for food is left in the fields, 1/3 is actually consumed, and 1/3 winds up in a landfill.  It is the last 1/3 that would have been previously put back into the soil and added to the organic matter in the soil.  Today, we are using bio gas production as a means to simply reduce the waste stream.  The methane produced is a positive byproduct of the process.  Approximately 90 to 95% of the biomass input into the process comes out as digestate, which in theory could be put back into the soil.  However, the economics of the transportation precludes such replenishment of the soil.

Edward, Please excuse my limitless ignorance, in this field.  Does your analysis say that the land, where a biogas plant used to be, is rich in humus, and available for agriculture?  If not, where does the digestate go?

Just remember that we are talking about volatile solids here. Whether they decompose in a digestor or on the land, they will decompose at some point. Just think about what happens in the composting situation: you start with a big pile and end up with a small pile that is useable. Where did the rest go?

Having said that, I believe that you are partly correct. The current practice of dairy farms (and I dare say piggeries) is to wash their manure into lagoons where it is allowed to ferment generally producing methane that escapes to the atmosphere. At certain times of the year, the lagoon is emptied and the slurry is typically spread on the lands. In the lagoons there has already been a significant reduction in the volatile matter. However, the residue slurry is more manageable and generally smells a lot less - we have about 10 days a year where the prevailing wind is over our neighbours land and we know that slurry has been spread! So half/half in this case as the residue is still returned to the lands.

In commercial energy crops, the material is removed from the farm to the central biogas plant. Generally the deal is that the farmer gets the digestate back as the biogas plant has to dispose of it somehow. Again a half/half scenario.

The real win is recovering digestable material out of municipal solid waste as this never gets returned. As with all biogas plants, they have to deal with the digestate. Generally this means sending it back to farms though in some cases the solids are dried and briquetted as fuel. Overall, I would say that biogas plants are not decreasing the humus in the soil in an overall sense: some lands may not be getting the digestate back while others are with the balance being more is returned via the digestion of MSW biomaterial. 

Very interesting question raised by you Michael Heine on which debate is needed. My answer would be YES to this question.

another example of bad OM management in agriculture!

If you grow energy crops using chemical fertiliser with no recycling of digestate I think you are right. But when digesting food waste with application of digestate on agricultural soil, which is a common case in Sweden, the net effect is addition of organic carbon to soil given the alternatives incineration or landfill disposal of food waste. In relation to composting you have to compare degradation rates in order to say something. The complete answer to your question is to be found when assessing global data on anaerobic digestion including what substrates are being used, degradation rates, digestate management, soil conditions and alternative treatments. 

Although I am not an expert in this filed, my guesstimate would be that turning organic matter in biogas indirectly contributes to the depletion of organic matter but possibly in only a minor way. The most important cause of organic matter loss in soils i deep tillage, as it exposes soil organic matter to oxygen, and hence to oxydation. This was identified as one of the key drivers of the ongoing loss of fertile soils at global level when 6 groups of experts were set up by the Commission to look at soil related issues, in preparation of the EU Soil Thematic Strategy, which then gave birth to the first ever proposal for an EU soil protection Directive, that never received the necessary vetting by the EU Council and the EU Parliament.

There is a specific report on organic matter in soils prepared by the ad-hoc working group available from that page, where you can find all the other reports as well: http://ec.europa.eu/environment/soil/making_en.htm. It's about 10 yers old material but I believe it is still valid, excellent material.

Alternatively you may also address your question to Luca Montanarella, a leading, internationally renowned soil scientist working at the European Commision's Joint Research Instiute in Ispra (Italy). He is on Research Gate ad his e-mail is:[email protected].

All the best.

Dear Patrice,...

A very helpful contribution to the topic "organic matter in soils"...

All the best for the new year

Michael

The digestate from the processed biomas using biogas plant could be put back into the soil as fertilizer which would in turn enrich the soil nutrient.

Biogas plants are simply part of an integrated biosystem. Besides obtaining biogas for different applications, the effluent which is a byproduct of anaerobic digestion may be utilized as a fertilizer to enrich the soil nutrients for crop production. I therefore disagree that biogas plants are responsible for further decrease of humus contents in agricultural soils.

To answer your question in a very simple manner: Biogas plants are never responsible for decrease in humus content of soil. It is the wrong agricultural practices (flouting all rule of soil managements) depletes the humus content.

Ideally, this should be the opposite that is, biogas plant contributing to the humus content of the soil. Knowing fully well that the contents are organics these results in further decomposition in adding nutrient to the soil. Also, digestate and sludge for biogas production are known to be very useful in increasing the nutrient of the soil. Though in digestate, the presence of nitrogen and sulphur add more content to the soil which will be available for crop uptake but some maybe lost through ammonia and hydrogen sulphide from nitrogen and sulphur respectively. 

There is not a lot more to be said here. It is not AD the causes humous depletion, but the policies that support the growth of these crops where they would not normally be grown.

AD is an important technology for a number of reasons: it returns carbon and nutrients to land, it mitigates energy from agricultural and waste management processes and it generates energy. Used incorrectly however and it could be an environmental problem. It requires joined-up policy and good regulation, which are currently sadly lacking.

Bio gas production is based on the climatic (temp) factor.

If climatic change is affected soil (if excess nutrients)