It is often claimed from proponents of conventional farming that the use of chemical fertilizers, in particular nitrogen, is a prerequisite for binding more carbon in soils (see for instance the critique of regenerative agriculture by the World Resource Institute).
This is normally founded 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.
Below, I discuss both of the arguments.
It stands beyond doubt that the supply if nitrogen fertilizers, in reasonable quantities, increases yields. It is often assumed that this also corresponds to more straw, roots and other organic matter being left in soils. This seems to be an oversimplification though. To begin with, in parallel to the increased use of fertilizers, the harvest index (the share of the above ground biomass that is the desired products, e.g. wheat kernels) of major agriculture crops has also changed a lot so that a bigger share of the carbon is allocated to the harvested crop and less to other parts of the plant. In addition, through the use of herbicides the biomass in weeds has also been substantially reduced, biomass that otherwise would contribute to soil organic matter.
The harvest index is also influenced by management and experiments show that increased availability of nitrogen as well as irrigation will increase the harvest index even more. Research in both Switzerland and Denmark demonstrate that in organic farming (by definition without the use of synthetic nitrogen fertilizers) a substantially higher share of the biomass is allocated to roots than in conventional farming.
This is even more important as recent research (e.g. Kätterer et al 2011 and Villarino et al 2021) show that roots and root exudates (carbon rich substances released from the roots to the soil, sometimes referred to as the liquid carbon pathway, or the microbial carbon pump) are much more important for building soil organic matter than plant litter and straw.
There are many review articles and meta-analyses published on the topic and they come to varying conclusions. Poeplau concludes that there is a general positive relationship between the use of nitrogen fertilizers and soil carbon in grassland, but there are also research showing the opposite, e.g Sochorova 2016. A synthesis of Bolinder et al (2020) claims that 1 kg of N can increase soil C with 1 kg compared to plots which are totally unfertilized. They conclude, however, that this is not a linear relationship, i.e. there is higher effect with low nitrogen supply than with a high supply. They also say that to compare with plots that are not fertilized at all has limited practical value as farmers, in the absence of chemical fertilizers, will try to supply nitrogen in other ways, such as with biological nitrogen fixation or use of organic amendments. They point out that many other methods than increased use of fertilizer to increase soil carbon is much more important. Another synthesis from 2021 concurs. Therein, Alexandra Tiefenbacher and colleagues show that adding N can work both ways depending on conditions. It can stimulate the decomposition of organic matter and thus reduce the carbon stock, but it can also increase primary production and thereby the supply of carbon.
Based on the research above it seems quite clear that use of N fertilizer is no shortcut to soil carbon sequestration and that its effects are uncertain, and in any case very small. If we lift our perspective this should be apparent already from the fact that most soils have been losing carbon all throughout the era of increased use of nitrogen fertilizer. On a systems level, use of nitrogen fertilizers is clearly not a pathway to increased soil organic matter.
Let’s now look at the other argument. Do you have to supply a certain amount of nitrogen to increase the carbon stocks in soils? This is based on the observation that a C:N ratio of around 10 is a benchmark for organic matter in soils, i.e. for each kg of carbon there is 100 gram of nitrogen. In a very simplistic way this is interpreted into that in order to increase soil carbon stocks with, say, 1 ton we need to supply 100 kg of nitrogen.
But while these relations indeed have some value one can’t draw the arguments thus far. The C:N ratio differs a lot between different types of organic matter and even within the organic matter itself. In plants it vary the most while the range is narrower for microorganisms. Recent research has shown that dead microorganisms make up a substantial part of the soil organic matter. But the micro life will also adapt itself to soil conditions such as the availability of nitrogen. Fungi has typically more than two times higher C:N ratio than bacteria for instance. Also within a certain group of organisms species thrive under different conditions. A high C:N ratio will stimulate bacteria which preserve nitrogen (and reduces emissions of nitrous oxide) while N- fertilization will favor the opposite.
Again, if we look through a wider lens we can see that organic soils have almost 5 times higher C:N ration than mineral soils and that there is also a marked difference between arable soils and grasslands. In addition the C:N ratio changes with soil depth. Summing up, the stoichiometric argument for nitrogen fertilizers is weak.
Both arguments are based on a static view of agriculture systems. If you start with the prevailing agriculture system, which is adapted to the regular supply of nitrogen, and just cut out nitrogen fertilizer, you will get all kinds of problems, of which reduced yields is the dominating one. But farmers will respond to changed conditions by changing management, crop and variety selection and crop-livestock integration. It is therefore not really possible, or at least not meaningful to discuss one component of an agriculture system within the framework of ceteris paribus, all else being equal.
In addition, already today, nitrogen use efficiency in the agriculture system is low and more than half of the nitrogen supply is lost in the fields. There is no research showing that increased use of nitrogen fertilizers would be a good pathway for increasing soil carbon stocks. Even with the assumed carbon storage efficiency of nitrogen fertilizer of one to one (or even slightly better) it would still be a “climate loss” to increase soil carbon through the use of nitrogen fertilizer because the whole life cycle emissions (in the range of 10 kg CO2e) from 1 kg of N surpasses the climate value of 1 kg of C (which is 3.66 kg CO2). Few, if any, of the proponents of nitrogen fertilizers as an important factor for carbon sequestration, are actually suggesting that nitrogen should be supplied in higher rates to cater for that. The effect on yields will still determine nitrogen supply.
The nitrogen argument for carbon sequestration could, therefore, 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.
Nitrogen fertilizers are not just a technicality, they are a major building block in the industrial, global and capitalist agriculture system. As such they both drive and enable the increasing metabolic rift between human society and the ecosystem that sustains it. It is hard, but definitively possible to feed the global population without them, but it will need changes in the food system and in society at large.
Teaser photo credit: A large, modern fertilizer spreader. By Rasbak – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=703196