From the law of the minimum to soil health
How nitrogen fertilizers changed the food system. Part 3.
A more than hundred year old focus on easily available nutrients has led farming astray. Instead, nutrient availability is to a large extent an emergent property of healthy soils.
In my previous article I wrote about how synthetic fertilizers in combination with plant breeding and other technologies has changed the crops and the cropping systems. Here I will try to expand on how it also changes the soil.
The (too) simple law of the minimum
The development of soil science in the 19th century were carried out in simplified greenhouse systems with sandy soil in pots. By changing one variable, e.g. the supply of one nutrient, and all other things being equal, it can neatly be demonstrated that there is a direct link between availability of nutrients and plant productivity. This led to the formulating of Liebig’s Law of the minimum stating that growth is dictated by the scarcest resource. This, in turn, laid the foundation for the dominant view that generous amounts of dissolved N must be available to plants to satisfy high N demand during short periods of vigorous plant growth.
Hydroponic cultivation in a research station in Sweden 1879, a board in the premises of the The Royal Swedish Academy of Agriculture and Forestry, Photo: Ann-Helen Meyer von Bremen.
Many statements about nitrogen fertilizers are founded on this narrow and reductionist scientific basis where one have compared the use of N-fertilizers with a control where no fertilizers are applied. In most cases those experiments are based on monoculture growing of corn or wheat or some extremely simplistic crop rotation.
The Green revolution - the greatest and most successful greenwashing ever.
Plant breeding has had increased yields as main purpose and it has been based on ample supply of fertilizers. This leads to plants with a higher harvest index (more kernel and less straw and root) which in turn means that the plant is less able to look for the nutrients it needs, and that much less of the plant is recycled into the soil. The plant is also less motivated to exchange energy (carbohydrates) for nutrients with the steaming microbiome of the soil. Less active and shorter roots also means that the plant is more susceptible to drought. Because of a less healthy soil, the resistance to pest is weakened. In this way plant breeding, fertilizers, pesticides and irrigation go hand in hand. This agro-industrial complex is often referred to as the Green revolution. One of the earliest and most excessive examples of greenwashing.But real life farmers will hardly ever grow the same crop year after year without fertilizers. In the absence of fertilizers they will apply better crop rotations and use biological nitrogen fixation, the process by which bacteria – free living or in symbiosis with leguminous plants – convert atmospheric nitrogen into reactive nitrogen. And real life soils are not life-less substrates for plant roots to grow in. Considering how dynamic – and messy – a living soil is, it is understandable that most soil science has been based on reductionist views and admittedly the methods have also delivered some remarkable results. But in the end, the limitations of the simplistic explanations are more and more visible. Even in conventional farming the interest for “soil health” has increased a lot the last decade.
Fertility is an emergent property of soils – and so is the need of N-fertilizers
The notion of inorganic nitrogen pools being the fundament of plant growth persisted through the mid-1990s and was just questioned by a few dissidents, most of them associated with the organic movement. But thing have changed lately and “Today, the availability of nitrogen can be seen as an emergent property of the plant-microbe-soil system” as stated in a very good overview article, The nitrogen gap in soil health concepts and fertility measurements, by Stuart Grandy and colleagues published in Soil Biology and Biochemistry (2022).
Emergent properties are characteristics of systems that the individual parts of a system do not possess. It is thus not possible to study the individual components and understand how a system functions. The system can be a living creature such as a human, an organ (the brain) an ecosystem or life itself. The soil has a number of emergent properties which can’t be reduced to simple components, the soil is alive. It is no coincidence that one of the canonical books of the organic movement had the title The Living Soil (by Eve Balfour).
Keeping with the term emergence, I would say that the need for chemical fertilizers is also an emergent property of soils. In a fertilizer intensive system, most natural processes regulating nitrogen between plants, microbes and minerals, e.g. biological nitrogen fixation and myccorhization are suppressed. Plants also adapt their physiology in response to nitrogen availability. With ample N supply, plants invest less in roots and more in leaves and shoots. Losses of nitrogen are big, which in turn necessitates regular addition of mineral nitrogen. As there are few pathways for N, other disturbances, e.g. drought, will disrupt N delivery to plants (Fontaine et al 2023,Grandy et al 2022).
It should be noted that the view of soil as a living organism is not new or unique for the organic movement or for the modern soil health narrative. There are many proverbs throughout the world where soil is referred to as a source of life and wealth. In ancient Hebrew, adamat, the word for soil, comes from the same root as Adam, the first man. In Latin, humus, one of the main soil constituents, shares the same root with homo, for man. In that sense, the reduction of soil to an arena for chemistry and physics is a modernistic parenthesis.
The nitrogen triangle in soil
The more you dig (!) into the underground life the more you see and the more you realize that we still don’t know more than a fraction of what is going on. In general, a healthy soil will have many avenues by which nitrogen is made available to plants.
Fine roots can grow into a porous soil where oxygen stimulates microbial activity and release of nitrogen. Root exudates increase microbial activity and stimulate the production of organic acids and enzymes that mobilize nitrogen in mineral associated organic matter (MAOM)*. Protists, nematodes, and other predators feed on microbes, releasing nitrogen in a “microbial loop.” Mycorrhizae can penetrate soil aggregates to access pockets of N. As expressed by Grandy at el (2022): “The multiple pathways for plants to access N provide more resilient N supply under variable conditions: if one “pipe” is shut off, many others may still provide N.” Lately, it has also been shown that plants can use nitrogen in organic forms, e.g. as amino acids. How important that is, is not clear (Näsholm et al 2009).
During a growing season, several hundreds of kilograms of nitrogen is moving between the different agents in the soil. The plants and the microbes “cooperate” and “compete” in many different ways (I write both in quotation marks because those human words are to my mind misused to describe processes in nature, well they are misused also in describing human-human interactions). Plants compete, mostly efficiently, with microbes that will immobilize nitrogen and they have assistance from mycorrhizal fungi. The energy for the soil life comes from the photosynthesis as litter from plants, but even more importantly, from the rhizodeposition, i.e. the excretion of dissolved inorganic and organic substances from living roots. Plants can modify the microbial community by various signal substances and hormones in the exudates and through this they can also regulate the supply of nutrients. Of course, also bacteria and fungi have their own agenda; Mycorrhiza can also stimulate or inhibit the activities of other free-living organisms. (Fontaine et al 2023,Grandy et al 2022, Roque-Malo 2020)
When plants grow most vigorously they need a lot of nitrogen, a maize crop may need 3 kg of N per hectare and day. In a healthy soil this can still be delivered via the mineralization of soil organic matter by microorganisms. As Grandy et al phrase it: “Plants are not just passive players in the N cycle but actively shape intricate three-way interactions with microbes and minerals “.
Nitrogen and carbon are companions
There is a wealth of scientific literature on the impact of nitrogen fertilizers on organic matter (aka carbon). The proposition that application of nitrogen fertilizers is good for binding more carbon in soils is normally built on two different arguments: 1) Through the use of nitrogen fertilizers, total photosynthesis is increasing, thus more carbon is bound by plants. Some of this carbon is taken away from the land in the form of higher yields, but there will also be more straw, roots and residues left in the field; 2) There is a rather narrow relationship (stoichiometry) between carbon and nutrients like nitrogen and phosphorus in soil organic matter and, therefore, one have to supply concomitant quantities of those nutrients to increase carbon content.
When one look deeper into these claims, one can see that the evidences are weak and not universally valid. Most of them are based on the kind of reductionist research described in the beginning of this article. In addition, most research on soil carbon has been directed to top layers of organic matter, while there is also huge carbon storages in the subsoil and not all carbon is stored as organic matter. One estimate is that soil carbonates (inorganic C mainly as CaCO3) account for more than 2300 Gt carbon in the top 2 m, which is almost equal to organic C stocks. These inorganic C stocks are continuously lost as CO2 by neutralization of N-fertilization-induced soil acidification (Zamanian et al 2021). If we lift our view from field trials to systems we see that most soils have been losing carbon all throughout the era of increased use of nitrogen fertilizer (Lal et al 2015). On a system level, use of nitrogen fertilizers is therefore clearly not a pathway to increased soil organic matter. The nitrogen argument for carbon sequestration could be seen as a distraction or possibly a method of diverting focus away from the substantial greenhouse gas emissions associated with their production, transportation and use. I wrote a longer article on the topic some years ago (Rundgren 2021).
Having said that, there is a strong link between nitrogen and carbon and it is not only nitrogen availability that is an emergent property of soils, also organic matter and its various fractions are emergent properties and the same biological processes (e.g. the “soil microbial pump” (Liang 2020)) that drive nitrogen are also important for carbon accumulation in soils.
In the next article in the series I will discuss the effects on human health.
* Mineral-associated organic matter (MAOM) has been considered being fairly resistant to decomposition compared with organic matter existing in bigger organic particles. However, recent experimental and theoretical evidence shows that root exudates may mobilize MAOM, thereby providing plants and microbes access to a large and N-rich pool (Grandy et al 2022, Jilling et al 2021).
Previous N-articles:
References
Fontaine, S. et al, Plant–soil synchrony in nutrient cycles: Learning from ecosystems to design sustainable agrosystems , Global Change Biology Volume30, Issue1.
Grandy et al the nitrogen gap in soil health concepts and fertility measurements, Soil Biology and Biochemistry, Volume 175, 2022, 108856
Jilling, A. et al, 2021. Priming mechanisms providing plants and microbes access to mineral-associated organic matter. Soil Biology and Biochemistry 158, 108265.
Lal, R., Wakene Negassa, Klaus Lorenz, Carbon sequestration in soil, Current Opinion in Environmental Sustainability, Volume 15, 2015, Pages 79-86
Liang, C. Soil microbial carbon pump: Mechanism and appraisal. Soil Ecol. Lett. 2, 241–254 (2020). https://doi.org/10.1007/s42832-020-0052-4
Näsholm, T., Kielland, K. and Ganeteg, U. (2009), Uptake of organic nitrogen by plants. New Phytologist, 182: 31-48.
Roque-Malo, S., Woo, D. K., Kumar, P. (2020). Modeling the role of root exudation in critical zone nutrient dynamics. Water Resources Research, 56, e2019WR026606
Rundgren, G. Nitrogen fertilizer is not a climate solution, Garden Earth, 8 Aug 2021.
Zamanian, Kazem, Jianbin Zhou, Yakov Kuzyakov, Soil carbonates: The unaccounted, irrecoverable carbon source, Geoderma, Volume 384, 2021, 114817.