Sink, store or source?
A soil gains carbon from dead organic matter and loses it through decomposition. Only the balance of those two rates decides which of the three it is.
Set the two rates
Litter fall, roots dying back, manure and compost added.
Faster when the soil is warm, well aerated and disturbed. Slower when it is cold or waterlogged. This one is a percentage of the store lost each year, not a fixed amount.
Where the soil is starting from, before anything changes.
Or set up a situation
5.0
2.5
More carbon is entering than leaving, so the store grows and the soil is taking carbon out of the atmosphere. It will not grow for ever: as the store gets bigger, the amount lost each year gets bigger with it, until the losses match the input and the growth stops.
This soil is a sink: taking carbon out of the atmosphere. Carbon in each year 5.0, carbon out 2.5. Starting from 50, it reaches 100 after 150 years, and settles at 100.
Try these
- Move the starting-store slider until the two numbers under the graph match: carbon in each year, and carbon out each year. The line goes flat. Now raise the input by one step. Describe what the soil is for the next few decades, and what it becomes afterwards.
- Use the two peat settings. Explain why a cold waterlogged soil ends up holding so much carbon when so little falls on it each year.
- Compare the grassland left alone with the same grassland ploughed every year. Only one slider changed. Explain how ploughing changes that rate.
- Set both the input slider and the decomposition slider to 5, then move only the starting-store slider. Neither rate has changed, but the soil moves between sink, store and source. Explain why, using the two numbers under the graph.
- A soil is a carbon source. Give two different changes a farmer could make to turn it back into a sink.
This is a model of one process, not a set of measurements. The two rates and the size of the store are in relative units so that the behaviour is visible; they are not tonnes per hectare, and the settings describe conditions rather than named real sites. Real soils also lose carbon by erosion and leaching, and gain it at different depths at different rates.
Nothing here runs away. Set the soil up as a sink and the curve bends over; set it up as a source and the curve flattens out before it hits the floor. The reason is on the interface: the input is a fixed amount each year, but the loss is a proportion of whatever is in the soil, so the bigger the store gets the more it loses.
Sink, store and source are not three soils
They are one soil at three settings, and a soil moves between them. The two grassland presets differ by a single slider: the ploughing raises the rate of decomposition, and a soil that was sitting still starts giving carbon back to the atmosphere. Add cover crops and reduced tillage and it turns around again.
A store is not a soil where nothing happens
When the line goes flat, carbon is still entering and leaving every year at exactly the rates shown underneath. This is worth being precise about in an exam answer: a store is a balance between two flows, not an absence of them.
Why the peat bog holds so much
Almost nothing falls on it. It ends up with the largest store on the model anyway, because cold waterlogged conditions almost stop decomposition, and a very small loss rate takes a very large store to balance. Then drain it, and the same soil becomes the strongest source here.