Have you ever thought about why cows digest leaves faster than stems or why a leaf breaks down faster in the soil than a stem. Well actually, cows do not digest leaves/stems, nor does the soil break down leaves/stems.
In the case of cows, it is the microbes in the rumen (first stomach) that consume what they can of what the cow has swallowed. The cow then digests the microbes as their food source
In the soil, it is microbes, such as bacteria and fungi that have the first snack on organic matter, eating what they can. Other soil life such as protozoa and nematodes then consume the microbes as their food source, and they in turn are consumed.
After entering plants, it is ongoing consumption from one living thing to the next that moves carbon through the landscape above and below ground. However, this consumption is very dependent on enough nitrogen being available.
THE CARBON:NITROGEN RATIO
Last week it was explained that the relative amounts of carbon and nitrogen in different life forms, and their waste products, is referred to as the carbon:nitrogen (C:N) ratio.
Leaves have a higher percentage of nitrogen to carbon than stems i.e. lower C:N ratio. This allows microbes to multiply faster due to the higher availability of nitrogen. The faster microbes are able to multiply, the quicker the faster moving carbon starts to move.
Given plants (and trees) are the start of the above and below ground food chain in the paddock, the C:N ratio of pasture plants sets the speed of the faster moving carbon in the paddock that producers rely on.
THE SOIL PROCESS – HOW NUTRIENTS IN ORGANIC MATTER BECOME PLANT AVAILABLE
It is the C:N ratio of organic matter that determines how quickly nitrogen is mineralised and made plant available again, as the following two diagrams demonstrate.
For materials with a low C:N ratio such as Lucerne, the breakdown will begin almost immediately, because there is plenty of nitrogen around, and this nitrogen is needed by the bugs to make proteins. The more they multiply, the more nitrogen needed.
In fact, there is usually more nitrogen in the Lucerne than is required, so some is released into the soil and can be used by plants.
However, for say oat trash with a higher C:N ratio, nitrogen is usually not released upon decomposition, and may not become available for many months. In fact nitrogen from the soil (which otherwise would be available for plant growth) is usually needed by the decomposing organisms to complete the job of breaking down the oat trash, i.e. the soil organisms remove nitrogen from the soil as they need proteins to build their bodies.
Death of soil microbes is an absolute necessity as part of nitrogen mineralisation. The C:N ratio of bacteria and fungi, the first consumers of organic matter, is lower than the soil life that consumes them. Hence, the unneeded nitrogen is released to the soil and becomes available to plants.
Looking at the two examples, 60% of the carbon consumed has been released (respired) back to the atmosphere during consumption. The rest is now in the soil microbes.
THE COW PROCESS – GETTING SHEEP AND CATTLE TO MARKET SOONER & REDUCING METHANE
Because the microbes in a cow’s rumen have the same requirements as those in the soil, leaves, which have a lower C:N ratio than stems, move faster through a cow. Likewise, green grass has a lower C:N ratio than dry grass, so moves faster.
The reason sheep and cattle are selective in what they eat, is that they intuitively know that microbes in their rumen (gut) multiply faster with a higher nitrogen diet. The faster rumen microbes multiply, the faster the cow grows.
Hydrogen is produced in the rumen as microbes consume the gut fill. Methanogens are microbes living in the rumen that remove this hydrogen by joining it to carbon to form methane (CH4), which the cow burps out.
If the rumen is emptying slower because the microbes are multiplying slower, because nitrogen levels are lower, the methane outcomes are worse. There is actually less methane produced each day, but because the cattle take a lot longer to get to the meatworks, the methane produced in total is more i.e. more per kg of production.
WAYS TO LOWER THE C:N RATIO
Resting paddocks for a short period after rain increases the percentage of leaves to stems.
This short rest after rain increases carbon flows which increase plant energy reserves and root volume, which in turn results in pastures being green for a higher percentage of the year.
Ensure animals don’t eat out the more palatable lower C:N plants.
As a general rule, higher quality pastures (lower C:N ratio), result in manure that is broken down quicker in the soil.
There is a reason why a paddock is more productive when the faster moving carbon (short term carbon) moves faster. Everything that is joined to carbon, such as nutrients and energy, becomes available to sheep and cattle sooner. Also, increasing the speed of the faster moving carbon results in nutrients becoming plant available sooner.
Increasing the speed of carbon through ruminant animals like sheep and cattle increases profits by getting them to market sooner and reduces the production of methane per kg of production.
From a “management” point of view, it is carbon flows that are important, but moreover, it is the speed of flows that is the critical thing for a rural producer.