Monday, 14 February 2011

Composting woody material Montezuma method and Hugelkulture

I think that a lot of misconceptions are being perpetuated by people that see composting more as a crusade rather than just a method of recycling.  There are some reasons why good well made compost was valued in the past. It was used in various mixes to make seed and potting compost.  Fine grained well rotted material was needed to produce this.  Woody material was avoided because it did not rot down very quickly and tended to reduce the level of nutrient in the compost.  The main decomposers of woody material are the fungi and they send out foraging hyphae that glean nitrogen and other nutrients to make the fungi’s structure. The extent of nitrogen depletion in soils with buried woody material seems to be questionable and the results of experiments are not very clear.  It would seem that sometimes there is relatively little nitrogen loss. 

We must also not loose sight of the fact that invertebrates also have an important role in the decomposition of woody material and these organisms would add to nitrogen to the soil when they die.   

A few years ago I downsized the amount of land I cultivated and this meant that I resented any space that was not being utilized for crop growing.  I made the decision to bury all my waste plant material rather than compost it so that I could dismantle the compost bins and use the area for planting. 

So I have, for many years, been burying woody plant material in order to recycle it and not have to burn it.  I call it the Montezuma method because the South American native civilizations used it to make their gardens on lakes, on mountains and in most inhospitable and infertile areas.  Chinampas were developed using woody material to produce growing platforms that sludge could be put onto to raise the level of the Chinampas above the water of  lakes.   These gardens were very productive and some are still in existence today.   Ancient civilizations throughout South America used similar brush wood techniques to produce terracing on steep sided mountains.  Not only did the brush wood rot down to produce very friable soil but it also helped in drainage and moisture retention. 

A composting method that is similar to the South American method is the Hugelkultur.  It is an old form of composting that was developed in Eastern Europe.  Brushwood either fresh or beginning to rot was gathered into a pile or put into a pit, covered with other compost material like tree leaves, and  finally a layer of soil was used to cover the pile.  The permaculturists suggest that the soil could be planted immediately with a variety of quick growing vegetables.

I bury my logs and brushwood much deeper for several reasons.  If fungi are rotting down the wood, I would like them to glean any of the nitrogen that has leached from the top soil because then I might have some opportunity to recycle it into the top soil when I dig deeply again. The brushwood’s ability to absorb moisture from the soil and then allow it to return to the soil in a more controlled way means that deep rooted plants like runner beans will have a source of water throughout the growing season.    It also means that cultivation of the top soil is relatively easy because digging over an area that is full of brushwood would be difficult. 

Only incomplete information about the factors that influence the decomposing activity of fungi and bacteria has been gleaned for wood, however moisture content, oxygen concentration, acidity, temperature and nutrient concentrations – especially nitrogen - seem to all play their role.  

The degradation of these woody carbon sources initially does not require the production of lignin degrading enzymes.  However, in the later stages of decomposition the ligno-cellulose matrix in wood is attacked by slow growing saprotrophic (an organism that feeds by absorbing dead or decaying organic matter) lingo-cellulolytic fungi.  These fungi are able to decompose lignin using extra cellular enzymes allowing them access to previously inaccessible cellulose and hemicelluloses. 

When nitrogen, in an available form, is added to a pile of woody material there is usually an elevated respiration or a loss in mass.  This might be down to relief of nitrogen as a limiting factor for opportunistic fungi and bacteria. 

Nevertheless, other research found that there was no effect when nitrogen was added giving unclear data about how nitrogen affects wood decay.  The variable responses to the addition of nitrogen may well be due to the different populations of bacteria and fungi.  As bacteria are more efficient in using nitrogen, it is suggested that they will have a greater response to the addition of nitrogen.  High levels of nitrogen lead to increases in the growth of microorganisms probably because less energy is expended in foraging. 

We must remember and take into account that woody material is nitrogen poor and fungi have developed strategies to deal with this limited amount of nitrogen.  Unfortunately for us, Tone, one of these methods is to translocate soil nitrogen via hyphae in the soil to the wood leading to soil nitrogen poverty for other plants.

Bacterial growth is greater on small sized particles of woody material probably because they have relatively large surface areas, which make close contact with nitrogen in the soil.  The larger the wood fragment on the top of the soil the more likely it is to favour fungi, possibly because the wood might be more difficult for the bacteria to reach.

There was more fungal biomass in large woody fragments than in small ones.  The rate of decomposition was always greater in larger pieces of wood than in smaller pieces.  More bacteria were found in smaller wood pieces possibly because they could not make their way into large solid pieces of wood.  It could be suggested that fungi grew better in large pieces of wood because there was less competition from bacteria.  Bacteria produce a great number of antibiotic compounds that prevent fungi from exploiting a food source, while fungi also produce an armoury of chemicals that hinder bacteria. 

It seems that buried wood develops more fungal growth and cellulase and hemicellulase enzyme activity than surface wood.  Early decomposition is begun by opportunistic cellulolytic fungi with a definite movement of nitrogen from the soil to the wood.

Addition of nitrogen to the wood gave an increase in the rate of decay and fungal growth.  Nevertheless, over longer time periods the difference between bacteria given nitrogen and those that had none became much less distinct.  When nitrogen was added to small woody fragments there was an initial drop in the bacteria population, which wore off over time.  This suggests that fungi are favoured  when nitrogen is added to woody material buried in the soil.

The higher rate of decay in nitrogen rich soils may be due to more efficient microbial decomposition or different communities of fungi that are more efficient at decomposition but also have a higher nitrogen demand. 

The effects of lignin decomposing fungi are seen later in the process and this decay is relatively much slower.  Lignin is difficult to degrade and prevents access to other cell wall components.  Lignin is a complex polymer of phenyl propane elements, which have cross links to each other with a variety of different chemical bonds.  The initial break down of lignin is undertaken by white rot fungi which produce extracellular lignin and manganese peroxidases.  This process is aerobic and in anaerobic conditions lignin can persist for a long time.

Adding a small amount of nitrogen to woody material often increases the rate of decomposition.   Further additions of nitrogen did not increase decomposition rate any further. 

There is confusing data about the immobilization of nitrogen due to the addition of woody fragments into the soil.  Some research has shown no nitrogen immobilization and growth retardation while others have shown marked immobilization and greater effects.

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