Now I have been criticized for burying logs and brushwood rather than burning or composting. I have been told that methane, a potent greenhouse gas, will be produced and that I will be contributing to global warming unless I stop doing this. Is there any truth in this? I will find out.
Since I was a toddler, I have been making compost. My mother used to send me down to the compost heap to put kitchen scraps onto the pile. My grandfather and father were avid composters and religiously added compost to the soil each year. As far as I can remember, they more or less built up a pile of waste vegetation and, apart from adding a little lime and soil, did little else to the heap until they dug it in.
In my teens I read a little about composting and made a little better attempt, aided by my father’s admirable wooden compost bin constructions.
 |
| The classic compost heap |
Within the 5 foot by 5 foot compost bin the top soil was dug out and a pit formed. The topsoil was kept next to the bin and added to the compost as the pile grew. Some woody prunings were put at the bottom of the bin, to aid in the drainage of the compost. Layers of weeds, prunings, leaves, kitchen peelings, vegetable tops (like potato stems and leaves) and bonfire ash were added but in no scientific order. I did not add the “right” proportions of green and brown. After about 1 foot of compost I added a layer of soil and after about 3 layers of soil I added a little lime or compost accelerator. Although the books said that I should turn the composts every so often to get air into the heap, I rarely did so. However, it made acceptable compost and each year we could have two heaps of about 5 feet tall.
In Victorian times the gardeners had no access to commercial seed and potting composts, so they had to make their own. They did this by carefully composting grass turfs, leaves and garden wastes separately. After these materials had broken down and turned into compost they blended them carefully into their own potting and seed composts. As I buy sterilized general purpose peat free compost, I do not have to be as particular about how the compost is made. It is only going to be added to the vegetable plots and not used in seed trays or plant pots.
I have, in the past, made my own seed and potting compost and I made compost as part of my job at the Glasshouse and Crops Research Institute.
The old John Innes formula for seed compost was:
Loam 2 parts
Moss peat 1 part
Coarse Sand 1 part
And for each bushel of the mixture
Chalk ¾ oz
Superphosphate 1 ½ oz
The old John Innes formula for potting compost was:
Loam 7 parts
Moss peat 3 parts
Coarse sand 2 parts
And for each bushel of this you added:
Chalk ¾ oz
John Innes Base 4 oz.
The John Innes base was:
Hoof and horn 2 parts
Superphosphate of lime 2 parts
Sulphate of potash 1 part.
The formula that I liked to use was:
Fibrous loam 4 parts
Well rotted leaf mould 2 parts
Coarse Sand 1 part
Well rotted manure or sieved compost 1 part.
Regardless of almost everything, when something decays the process is catalyzed by microorganisms and the formula is:
CxHy+(x+y/4)O2=xCO2+(y/2)H2O
Of course the process of decay is a lot more complicated than this and the contribution of archaea is not fully understood at the moment.
However, the final product of composting, everything else considered, is carbon dioxide and water. A proportion of the carbon will be diverted for a time into the bodies of bacteria, fungi, plants and animals but eventually, lastly and finally we all end up as carbon dioxide and water – with a relatively small amount of iron, calcium, phosphorus etc.
This is exactly the opposite of photosynthesis where carbon dioxide and water are combined with the help of sunlight energy to produce carbohydrates.
There are other pathways for the carbon to go depending on the microorganisms that it encounters. If anoxic conditions occur – and they very often do, then methanogenic bacteria will use organic molecules to produce methane. They are strictly obligate anaerobes, which are poisoned by the presence of oxygen levels as low as 0.18 mg/L of molecular oxygen.
2C organic+2H2O=CO2+CH4
Composting, it is suggested, when done well constitutes an aerobic environment where methanogenic bacteria will not be able to live. Let me tell you a secret, methanogenic bacteria are ubiquitous. They are everywhere and what is more; anoxic environments are more common than you expect. Obligate anaerobic bacteria can happily live in the plaque on your teeth. Micro environments in compost heaps will be anoxic. Methane is produced.
So why are we not overwhelmed by methane? Why is there so little in the atmosphere? Possibly because there are another set of bacteria called methanotrophs or methanophiles that are able to use methane as a source of both carbon and energy. What is more they can grow aerobically like Methylococcus capsulatus or anaerobically, which means that regardless of where the methane is formed (in the compost heap or deep in the ground after I have buried logs and brushwood) these bacteria can metabolize methane by either incorporating the carbon into their bodies or producing carbon dioxide and water in energy production. Some like Methylomirabilis oxyfera reduce nitrate to nitrogen with the help of other microbes and so contribute to nitrogen loss from the soil. An archaeon is implicated in the breakdown of methane by sulphate reducing bacteria, and leads to the characteristic smell of hydrogen sulphide or rotten eggs that is sometimes associated with the breakdown of organic matter. .
The difference between the relatively aerobic conditions of a compost heap (I still maintain that even the best of compost heaps are mostly anoxic) and the relatively anoxic conditions of buried vegetation is the speed at which methane can be metabolized. As methane use by methanotrophs is slower in anoxic environments there could be a buildup of methane but my contention is that I bury comparatively small amounts of green matter and that the soil methanotrophs can deal with it fairly easily. In land fill sites there is a vast amount more organic matter compared to my allotment. This has been demonstrated to overwhelm the methanotrophs and the production of methane is very evident.
So I argue that burying vegetation will produce no more methane than a compost heap would. If some of the carbon buried in whatever form is prevented from quickly decaying, possibly it could become a carbon sink. There is some evidence of carbon staying in the soil for considerable amounts of time.
Burning just bypasses all of these processes and goes straight to the greenhouse gasses of water vapour and carbon dioxide without the opportunity for carbon capture within the bodies of heterotrophs.
