How does carbon cycling help to build soil organic matter and what role does nitrogen play during the process? CPM explores the topic.
“Studies show an integrated approach of applying composts and manures with synthetic nitrogen is a good strategy for building organic matter.”
By Rob Jones
The role of nitrogen fertiliser in increasing yield and profitability is widely acknowledged. However, a predominant focus on the nutrient’s cycle and utilisation may have led to neglecting the importance of preserving soil organic matter reserves.
As researchers further their understanding of soil function and how organic matter is created, will it become easier to balance the two more effectively?
To begin, higher organic matter soils typically have better pore structure leading to increased water holding capacity and reduced nutrient leaching, while providing greater resilience. This doesn’t necessarily lead to higher crop yields, especially once soil organic carbon exceeds 2%.
Building organic matter starts with photosynthesis, explains Joel Williams, a soil health educator and consultant from Integrated Soils. “Plants take in carbon dioxide, turn it into sugar and that becomes the building blocks of their bodies. It’s how they grow biomass both above and below ground.”
When plants die, that biomass is broken down to form organic matter with the roots making a greater contribution, partly through their location in the soil. Microbes use external digestive enzymes to break down plant litter from the highly complex, high lignin-containing carbon compounds into smaller and smaller pieces, explains Joel.
“This is known as particulate organic matter – plant material such as crop residues, dead cover crops and roots in various states of decay – the fraction of carbon that’s continually decomposed and cycled.”
But this isn’t the only type of organic matter. Relatively recent research, driven by the increasing interest in soil organic carbon’s role in potentially mitigating climate change, has uncovered that as much as 50% of organic matter is derived from microbial dead bodies rather than it being virtually all decaying plant material.
When the microbial digestion process creates small enough carbon compounds to be ingested by microbes, explains Joel, the carbon becomes of microbial origin and is used to grow microbial biomass. Higher carbon-to-nitrogen (C:N) ratio litter such as from cereals, takes longer than low C:N legume cover crops, for example, to reach that point.
Plants also release as much as 30% of the carbon they make during photosynthesis through their roots as exudates. These exudates are also carbon compounds with a lower molecular weight, which can be efficiently ingested by microbial soil life directly without the inclusion of external digestive enzymes.
“When the microbes die, the microbial necromass (a large, dynamic and persistent component of soil organic carbon) has an affinity to stick to soil particles, particularly silts and clays, to form more stable organic matter. This is known as mineral-associated organic matter,” says Joel.
Clay soils form more of this type of organic matter than sandy soils, as carbon has more mineral surfaces to bind with in clay soils. However, because there’s a finite number of mineral surfaces to adhere to, this type of organic matter will eventually plateau.
In contrast, particulate organic matter doesn’t appear to saturate – such as a peat bog where plants grow and die year after year, highlights Joel. “You can build lots of this organic matter but once that environment is disturbed, it’s very prone to being lost because it’s not chemically attached to soil particles.”
In contrast, mineral association is one of two key mechanisms to stabilise carbon inputs coming into the soil, he says.
“Whereas the second is a physical process, where carbon is trapped within soil aggregates. When soil aggregates – soil particles that are clumped together partly through the glue-like substance released by microbes – any carbon inside gets physically trapped and protected. This can be either particulate organic matter or mineral-associated organic matter.”
Practically understanding how the two types are formed can help with management strategies. For example, avoiding tillage means soil aggregates aren’t broken up, helping slow down the particulate organic matter carbon cycling.
Whereas maintaining living roots as much as possible through growing cover crops will pump more exudates into the soil, which can be converted more readily into mineral-associated organic matter, continues Joel.
Adding legumes, whether in the rotation, as companion crops or in cover crop mixes, is also helpful, he points out. “Legumes get some ‘free’ nitrogen from the air to deposit organic nitrogen into the system, which is released through their nitrogen-rich root exudates and helps prevent nitrogen mining from soil organic matter when breaking down high carbon residues.”
Priming the carbon cycle, particularly by growing microbial communities, is also possible through applying carbon-based inputs such as composts and manures. “There are plenty of studies that show an integrated approach of applying these with synthetic nitrogen is a good strategy for building organic matter,” says Joel.
Carbon-based inputs also have a role in helping to optimise nitrogen fertiliser additions. Options such as humic and fulvic acids, and to a lesser extent molasses, act as a carbon sponge binding to nutrients, making nitrogen less likely to leach and helping to optimise inputs (see box).
Where molasses has the advantage over organic acids, is by providing a highly digestible, highly available carbon carbohydrate form of energy to stimulate soil biology, suggests Joel. “In the soil generally, carbon is more limiting, so when we apply some carbon it stimulates their growth.”
It also stimulates soil biology to effectively eat nitrogen fertiliser to balance the C:N ratio within their bodies to grow, he adds. “That incorporates the nutrients from that fertiliser into their cells, and it’s a way to stabilise the nutrient and help prevent it leaching, creating a slow-release fertiliser,” he concludes.
This article was taken from the latest issue of CPM. Read the article in full here.
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