Nitrogen drawdown is the loss of nitrogen to plants in the presence of high amounts of carbon, resulting in slower growth and in extreme cases, deficiencies. Drawdown occurs when large amounts of high-carbon materials such as sawdust or wood chip are added to soil, and when the carbon:nitrogen ratio of that material is higher than around 25-30:1.

But why does drawdown happen, and how can the effects be reduced?

Let’s step back and examine soil carbon firstly. To a chemist, an organic compound is any which contains carbon, and an inorganic compound is any which does not. Organic chemistry is in fact a very important industrial field involved in the manufacture of plastics, fertilisers, fuels, pharmaceuticals, and many other products of modern life. ‘Organic compounds’ and ‘carbon compounds’ are pretty much synonymous terms in the chemistry world.

Having said that, chemists make exceptions for simple molecules such as carbon dioxide (CO₂) and the carbonates (those containing the CO₃^2- carbonate ion), and consider those inorganic.

Soil chemists adopt this approach by dividing soil carbon into organic carbon (almost a tautology!) and inorganic carbon.

Soil organic carbon (SOC) comes from once-living materials containing proteins, fats, carbohydrates (sugars, starch, cellulose) and lignin. Soil inorganic carbon (SIC) is primarily carbonate minerals (with the CO₃^2- carbonate ion) from weathered rocks. We won’t be discussing SIC in this post.

Matter high in organic carbon tends to be old, brown(ish) and dry — think dried leaves, dried grass, straw, paper, cardboard, sawdust, sticks, and wood chips.
Matter high in organic nitrogen tends to be fresh, green and moist — think fresh leaves, fresh grass, green weeds, green plant clippings, and food scraps.
Materials such as fresh coffee grounds and fresh manures are brown in colour but are fresh and not dry, and are actually higher in nitrogen rather than carbon.

Large amounts of carbon-rich material in the presence of water and oxygen will stimulate decomposition of that material by microbes, as SOC is a food source for them. Through a chemical process called oxidation, these decomposer microbes oxidise the carbon to carbon dioxide to release energy they then use for growth and reproduction. But growth and reproduction requires the synthesis of DNA and the building of amino acids and proteins, all of which require nitrogen. Nitrogen thus becomes a limiting factor in decomposition.

Let’s revisit the nitrogen cycle. Nitrogen fixers convert nitrogen gas into ammonia, ammonia is then converted into ammonium, and the nitrifiers convert ammonium into nitrates.

Plants obtain their nitrogen needs from ammonium and nitrate, but ammonia and nitrate are used by microbes for their nitrogen needs.

As nitrogen is the limiting element in decomposition, a too high carbon content relative to the nitrogen in the matter being decomposed will lead to microbes seeking additional nitrogen elsewhere so they can continue to access that carbon. This they find in the surrounding soil’s nitrogen reserves of ammonia and nitrate.

Thus when matter introduced to soil has a lot of organic carbon and not much nitrogen, soil microbes draw down the soil’s own nitrogen reserves of ammonia and nitrate to continue feeding on the carbon which sustains their population growth. No nitrogen reserves are left for plants, which leads to slower growth at best and a nitrogen deficiency at worst.

High organic carbon materials such as straw and wood chips are often used as mulch as they are slow to break down, but the reason they are slow to break down is because of their high carbon to nitrogen ratio. This page lists a range of ‘browns’: straw has a C:N ratio of 75:1 and wood chip has a ratio of 400:1 for example.

Adding these directly onto the surface of soil will cause nitrogen drawdown at that mulch-soil interface. This may not be a problem in this narrow zone compared to the soil profile overall. But it could be a problem when taking into account other factors, such as sandy soils and heavy rainfall, as nitrate is highly soluble and highly mobile, and very readily leached from soils. If most of the nitrate in the soil has already moved beyond the root zone, and microbes are drawing on what little may remain at the surface, there is even less available for plants to access.

Incorporating organic carbon-rich materials such as finely chopped straw or sawdust directly into soil will have more severe effects, as this material has been introduced into a larger and deeper volume. Microbes further down the soil profile can now access this rich food source, causing them to draw on the nearby nitrogen reserves.

Mulching is still a highly desirable practice as it helps retain water, suppresses weeds, cools the soil surface, and insulates the soil in general by reducing the temperature fluctuations caused by a regularly heated and cooled surface. So how best to reduce, or even stop, nitrogen drawdown?

Provide a barrier between the mulch and soil with something that has already been decomposed!

Any well-aged or composted manure or well-decomposed compost can be applied as a barrier, as the organic carbon content in these has already been converted to a more stable form that isn’t as biodegradable by microbes — humus. Simply apply this well-rotted material to a thickness that covers the soil completely, say 2-5 cm, and then apply the carbon-rich, slow-to-break down mulch over that.

Well-decomposed compost, while less biodegradable, is still organic matter, and an important nutrient source and reservoir. Additional benefits come with an insulatory mulch top layer, which slows the rate of water infiltration into the soil, and creates a cool, moist and stable environment within the compost layer.

This stable layer encourages the growth of microbes, arthropods and earthworms within it, which release still more nutrients from it. The arthropods and earthworms especially will further draw the humus into the soil, which improves its structure and water-retaining ability. Together with the slower rate of water infiltration, and the increased water-retaining properties of organic matter generally, leaching of these nutrients is reduced, making these more available to plants.

The soil will continue to improve more and more with time if left undisturbed. Rather than disturb the mulch (which is bound to be decomposing underneath anyway) to reapply new compost each fertilising period, simply add the compost on top of the mulch, and apply new mulch to that. The original mulch will break down more rapidly when sandwiched this way, and only help improve the soil structure still more!