How does carbon cycling help to build soil organic matter and what role does nitrogen play during the process? CPM explores the topic.
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 soilstypically 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.”
Particulate organic matter
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 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.
Benefits of molasses
Liquid carbon-based fertilisers based on sugar cane molasses, such as L-CBF Boost from QLF Agronomy, help to prime the carbon cycle in the soil, according to the firm’s US-based vicepresident of agronomy, Tim Chitwood. The molasses acts as a carbon source that provides energy to microbes and helps them to cycle nutrients, he continues. “We’re mimicking what Mother Nature wants to do, and that’s cycle carbon.”
When combined with other management practices such as a more diverse rotation, growing cover crops and reduced tillage, using L-CBF Boost amplifies the effect of making soils more biologically inhabitable, he says. There are three main times to use a molasses-based product, suggests QLF Agronomy’s UK national sales director, David Maxwell. “First, applying it with a starter fertiliser increases microbial loading and helps with rooting and resilience.
It’ll also help to increase root biomass, allowing the crop to unlock and make more from any residual or newly applied fertiliser,” he explains. “If you’re direct drilling, leaving a larger root network will also add to your organic matter over time.” L-CBF Boost, however, is most commonly used in the early spring with fertiliser applications. “That’s about improving nutrient use efficiency by spiking the bacteria to consume applied nitrogen and move it from the leaky nitrogen cycle to the more stable carbon cycle,” adds David. The product can also be used with foliar nitrogen applications, which can potentially help to reduce the total nitrogen applied. “Foliar applications are 3-4 times more efficient, and by using them, you reduce the amount of salts (from the urea) you’re applying to the soil, producing more of an environment that’s better for microbial activity,” he concludes
Strategic Focus
Building soil organic matter in his mostly clay/loam soils is a key part of Will Oliver’s strategy for Osbaston House Farm. “We’re always thinking about soil organic matter,” he says. The 600ha farm makes good use of organic manures from a 200,000-bird broiler unit with litter and digestate applied in spring on wheat, and sewage sludge before grain maize. Grain maize trash also adds organic matter to soils within the rotation with reduced tillage and direct drilling used when appropriate to avoid soil disturbance. “We’ve done a lot of mole ploughing this year with the aim of improving drainage. Hopefully that’ll help to improve soil health in the long term.” Cover cropping and catch cropping are utilised in a rotation that features winter wheat and winter beans, as well as the grain maize.
Will is also making use of L-CBF Boost, having first trialled it on a few tramlines in a maize crop a few seasons ago. Applied in a mix with the pre-emergence herbicide, pendimethalin, he could see to the line where L-CBF Boost had been used. “I think it’s helping to feed the soil biology and hopefully get more out of the organic manures we’re using,” he explains. After also seeing a small yield increase in the grain maize, he trialled it in wheat, again applied with a pre-emergence herbicide. “We didn’t see much difference at emergence, but there was a yield benefit of around 0.2 t/ha in our tramline trials.” While he believes it’s a little early to say whether L-CBF Boost has directly contributed, the combined effect of his management changes is leading to improved organic matter levels in his soils. “We want more resilient soils so they can cope with these freak weather events we seem to be getting,” concludes Will.
