When animal and plant residues decay they may either mineralize or immobilize Nitrogen depending on Carbon and Nitrogen ratio. If the C/N ratio is over 30 rather than releasing Nitrogen for crop use the net effect will be the immobilization of Nitrogen from the soil to feed the microbial decay machine. When C/N is lower than 15 the mineralization of Nitrogen may result in losses through leaching and or volatilization. The use of a 30/1 ratio is a sweet spot for making compost. When 1 volumes of green materials are mixed with  3 volumes of brown we can get a reasonable CN for composting. Stabilized composts have greatly reduced losses and environmental issues compared to raw manures. They can have much greater impact in raising the soil organic matter because of their increased half life. When proper composting is done the health and sanitary issues related to raw manures are avoided. Stabilized compost has remarkable ability to improve water relations in the soil compared to raw manure. Bottomline we should not treat compost like excrement its much much more than that.

It is advisable to distinguish between what used manure and crop residues, which are introduced into the soil. If to enhance the formation of humus, it functions effectively utilize cereal crop residues, but not legume. And if for improving soil nutrient regime, the manure will be more useful. And you must take into account the C: N ratio in organic fertilizers.

Dear Anthony,

There is a special issue published in "Agricultural Systems" which may be helpful for you. The special issue addresses biomass use trade-offs in mixed crop-livestock systems with emphasis on developing world.

• Biomass use trade-offs in cereal cropping systems

Best,

Alcade

http://www.sciencedirect.com/science/journal/0308521X/134/supp/C

Thanks Paul. I understand from your reply that animal manure (in particular herbivore faeces) should be mixed with "green" plant material (what range of C/N ratio is permissible?) and composted before being applied to the soil.

    In the work I'm planning to publish, the materials available for soil amendment were barley straw (0.5% ±0.2%N, C:N ratio 95/1) and/or the faeces of sheep fed straw and a little cottonseed cake (1.15% ±0.2%N, C:N ratio 40/1).

    In the farm situation some urine will get mixed with the faeces and spilt straw, although part of its urea-N would be converted to ammonia and lost.  With such wide C:N ratios, surely any reduction in ratio is welcome?

[I have assumed that the carbon concentration of organic matter is 45+/-1% (except for lignin, 67+/-4%).  Acid detergent lignin was around 7% of the OM fed but not analysed in faeces.]

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Thanks Sergei.  I shall value your comments on how to utilise the residues and faeces with unusually high C:N ratios.

Anthony your question is very good . This is what , we have been debating over the years and still continue to debate without any concrete vision with so many ifs and buts , considering the soil type , crop type , growing conditions etc etc....I very strongly feel that a crop residue could be used either way( leave it on the soil surface or incorporate inside the soil  )  depending upon the  soil moisture conditions and elemental composition of straw including C:N ratio , besides the nature of crop , annual or perennial in nature as the most deciding factor perhaps. You can afford to use crop residues of wider C:N ratio , even on the surface , perennial crops ; on the other hand most often than not , it gives much better results by burying the crop residues into the soil.   

Very good question. You may find a number of publications on crop residues. Utilization of crop residues depends on the socio-economic and farming system of a country. As you know in mixed livestock farming system crop residue is the main source of animal feed in the developing nations.

Use of mineral fertilizers without recycling of organic materials may result in higher yields than organic amendments alone, but this increase is not sustainable without inclusion of  soil amendments such crop residues. For instance, according to IAEA (2001), more than one half of all dry matter in the global harvest is in the form of crop residues, and in most developing countries, the amounts of nutrients in crop residues are higher than the quantities applied as fertilizers. This means that, based on the research recommendations on soil and water conservation, if at least one third of the crop residues is retained in the soil their impact on soil biophysical and chemical properties is tremendous. I think this topic is very wide that may participate experts in different disciplines.

I strongly agree with Dr. Anoop Kumar Srivastava. In general, burying the residues is better in calcareous arid lands since CO2 evolved affect changes to enhance the in-situ mineral weathering in favour of agriculture.

I'm retired from my lab, so do not have access to all of the papers in the special issue (Erenstein et al. 2015 *) referred to by Alcade Segnon (thanks!)   It is a pity that 11 of the 12 articles in that issue are behind a paywall, given their international significance.  In the particular system I'm looking at (semi-arid agropastoral) the ground is too hard most of the year, and stocking is too extensive, for manure to be easily incorporated into the soil, so some of the options discussed above may need to be modified.

Thanks, everyone, for your contributions to this thread: when I've got access to a library I'll be able to quote & discuss relevant papers.

* Erenstein, Olaf; Bruno Gérard & Pablo Tittonell (Eds) (2015) Biomass use trade-offs in cereal cropping systems: Lessons and implications from the developing world. Agricultural Systems 134:1-128  [link is attached]

As a thumb rule and in all fairness, whatever crop residues that serve as fodder for cattle/piggery/poultry/sheep/goat/wild life sheltered in zoos etc , must be fed to these living animal resources and their excretions alone should be composted with cattle non-edible plant materials such as weeds, leaf litter as a mixture to serve as organic fertilizer application to soils. Here again soil sheltered microorganisms have a key role to facilitate supply of nutrients to higher plants. All these steps constitute a biological cycle of interdependence. The C:N ratio keeps on changing with microbial activity in soils that results in stability of this ratio. In essence soil when maintained as a true living phase, understands and supplies nutrients to a growing crop at an appropriate time and duration. 

For me digestion by bovine microorganisms is not very different that digestion by SMB, both possibly give CH4 emissions when media are anaerobic. The problem is to quantify the part of residue C and N which are digested by cattle and the part returned to soil by dejection. another pb is localisation of dejections which could create more or less fertille zones as it can be seen in green circles around dejection in pastures. Then you must compare comparative evolution of crop residues by eg the TAO model which simulate C and N changes due to a given residue related to its fiber content end C:N ratio

 Dear colleagues, interesting discussion on use of crop residues. In agricultural systems crop residues are a very valuable carbon source which  are available in large quantities.There are three principal uses of  crop residues, they are valuable animal feed as mentioned by colleagues Dr Getachew,Dr.Reddy  and others,a carbon source  to cover the ground /soil surface in conservation agriculture and a source to generate bioenergy. In developing countries like India crop residues form important dietary source for animals.At least 2/3 of the residues generated are used for animal feed in India.The rest may find alternative uses.Even residue burning is a problem in Rice-wheat  cropping area where rice straw is burnt  to facilitate sowing of next  crop.Even mechanically harvested wheat straw is burnt.But now several alternatives to burning of residues are offered like collection, incorporation, and onfarm composting. The practice is banned by Government in some states.Composting of of dung, refuge,urine is well established  details as given by colleagues. Handling legume straw is no problem as it decomposes fast and is a ready supplier of nutrients.Straw needs mixing with other low C:N Ratio materials or fertilizer N to lower the C:N ratio and incorporation.. Where ever conservation agriculture is practiced ,there is it is valuable source.I believe that crop residues should not be used for bioenergy generation as the other uses are more important and environment friendly.

Soil sheltered biota as a thumb rule, should always be fed, nourished, ambience created for their comfortable living. Unfortunately, this branch of soil science has been highly neglected in preference to synthetic chemical interactions, the efficiency, budgeting, residual effects, synergism, antagonism, adsorption, desorption and what else and what not totally dominated by soil chemistry research, because of striking results obtained in the chemical fertilizer use unlike with soil sheltered micro and macro organisms which can not compete with the chemicals, in providing results. This lack of appropriate balance with in the branches of soil science for various reasons, soil biology failed to grow as compared to that for soil chemistry, soil fertility. In a way, soil physics also suffered to some extent as it is difficult to generate striking results in research to attract and sensationalize the end users. It is gratifying however, of late, environmental activists have been projecting the ill effects of indiscriminate and excess use of chemicals on human health problems, which lead to consider the need to maintain and improve biological status of soils with consequential and gradual decrease in chemical fertilizer and pesticide use. But it is a long way to go to break political-industrial-business nexus and enable soil health to be recognized to affect human health. My kudos to Dr. A. Subba Rao whom I admire for his valuable scientific contributions for the cause of soil science and crop nutrition and for his simplicity and humbleness, virtues common to celebrities in science across the world like Prof. Albert Einstein, Prof. J.C. Bose, Prof. C V Raman, Prof. B V Nath, Prof. Ch. Krishna Moorthy to mention a few.

I agree with you Dr.Srinivas Reddy that soil biological conditions and physical environment are very important to successfully practice sustainable agriculture and endorses your views. Thank for your kind words and placing my name with some great scientists ,of them some are very great.