Nitrogen Fertilizer and Soil Organic Matter: What Does the Evidence Say?

Authors: Jordon Wade and Andrew McGuire

Does synthetic nitrogen fertilizer burn up soil organic matter? Whether you are focused on soil health, soil sequestration, or soil carbon credits, this is an important question. The persistent claim is that synthetic N fertilizer can “burn” soil carbon by supercharging the soil microbes. This claim mainly arises from a 2007 research article from researchers at the University of Illinois (Khan et al., 2007, see comments on this paper below) and has recently resurfaced in another article (Jesmin et al., 2021) and the resulting (flawed) media coverage. However, these studies are far from conclusive – so what does the broader scientific literature say? And what have we learned in the last few decades on the relationship between synthetic N and soil organic matter?

A bit of background

The term “soil organic matter” (SOM) usually refers to a large range of compounds of varying origins and complexity that come from dead plants or soil organisms. About half of that soil organic matter is carbon, or soil organic C (SOC) (Figure 1), which is the focus of soil carbon sequestration. The rest is mainly comprised of other compounds. Some of these compounds provide plants nutrients (e.g., nitrogen, phosphorus, and sulfur), while others do not (e.g., hydrogen and oxygen). The terms “soil organic matter” and “soil carbon” are often used interchangeably in everyday conversations, but the distinction is important to keep clear when discussing specific soil processes.

What we know

One thing is very clear in the scientific literature: soil microbes are usually N limited. On average, soil organic matter, the food/energy source for microbes, has a C:N ratio of approximately 10:1, but microbes have a C:N ratio of approximately 8:1.

This means microbes need more nitrogen than is found in organic matter. If we have 100 lbs of soil organic carbon, there will be ~10 lbs of soil organic N. However, 100 lbs of soil microbial biomass will need ~12.5 lbs of organic N. (Figure 1). To get this, microbes will scavenge the soil for N that isn’t readily available in the soil organic matter. Enter N fertilizer.

Synthetic N is readily available to microbes, so when fertilizer is added, it causes the size of the microbial biomass to rapidly increase (Geisseler and Scow, 2014). In the short-term, this increases the microbial activity (aka CO₂ production or “respiration”) – but what about the long term?

Well, we now know that when those microbes die, they can become a persistent form of soil organic matter (the mineral-associated soil C). One recent meta-analysis, using 428 observations from 52 studies, showed that synthetic N additions increase both this persistent form of SOC (mineral-associated organic matter, MAOM) and the more microbially-available form of SOC (particulate organic matter, POM), as well as an overall increase in soil organic matter (Rocci et al., 2021). Another similar study using 803 comparisons from 98 published studies showed a similar result: N additions increased both forms of SOC (Tang et al., 2023). Importantly, both of these meta-analyses found this effect regardless of what type of system they were looking at, be it cropland, grassland, or forest. This is not a new finding, as many studies have shown that increases in soil carbon also require other nutrients, such as P, K, and molybdenum (van Groenigen et al., 2006; Van Groenigen et al., 2017).

One possible reason for the overall increase in SOM is simply that more nutrients (aka fertilizer) lead to more plant growth and therefore more residue input. However, it’s important to distinguish between simply increasing the fast-cycling carbon and retaining the slow-cycling carbon (or ideally, both!). One recent meta-analysis looked at this using isotopes, which help us determine both the age and the source of the soil carbon. They found that N fertilization can do both: it increases the amount of incoming “new” carbon in residues while also slowing the loss of the “old” carbon (at least at higher N rates) (Huang et al., 2020) (Figure 2).

In summary, we have multiple syntheses of field experiments (soils under long-term cropping systems) showing that synthetic N:

1. Increases microbial biomass,
2. Increases both the readily-decomposable and less-readily decomposable pools of soil carbon (the latter of which form from the dead microbes), and
3. Increases “new” carbon inputs while also slowing the loss of “old” soil carbon.

Altogether, this is pretty strong evidence that synthetic N doesn’t cause organic matter to be “burned off”.

But why is this happening? Understanding the mechanism will go a long way towards explaining the results we have seen in the field.

Why would synthetic N help slow soil organic matter losses?

To investigate why synthetic N can slow (or even reverse) soil carbon loss, we need to look more closely at lab experiments. Lab experiments are often shorter-term than field experiments, but they let us look closely at specific root causes of observations made in field studies. One of the easiest ways to approximate field processes is to look at enzyme activity with the addition of fertilizer N.

Microbes produce enzymes that target specific bonds in residues in order to access their energy. Which enzymes are being produced can give us insights into which molecules are or aren’t being targeted by the soil microbes. One recent meta-analysis showed that N fertilization increased enzymes that we generally think of as degrading newly-added residue (hydrolytic activity) and decreased those we think of as degrading “native” or older soil organic matter (oxidase activity) (Jian et al., 2016). This agrees with the results of many field experiments we discussed previously: N additions make for more residue input while slowing down any breakdown of old soil C. However, these classifications of enzyme classes are very broad – can we get more specific on the why of synthetic N and the soil carbon story?

Another study sought to do just that by integrating both laboratory enzyme measures and in-field measurements of soil carbon from 40 studies around the world (Chen et al., 2018). They found that N additions decrease the activity of lignin-degrading enzymes and increase the activity of cellulose-degrading enzymes. Importantly, they linked the lab and field by showing that the increases in soil carbon storage from N additions were linked to the differences in lignin-degrading enzyme activity. They also (once again) showed that N additions increased the older soil carbon pools. Sensing a trend here?

Putting it together (aka the tl;dr)

So, back to our question, does synthetic nitrogen fertilizer burn up soil organic matter? In short: the available evidence suggests no, it does not. Instead, we see many field experiments, both in agricultural and in less-intensively managed systems (e.g., grasslands and forests) where N increases soil carbon. Those increases are because of increased residue inputs into the soil, as well as better retention of the older soil carbon. This seems to be happening because of the effects on specific soil enzymes.

It’s worthwhile to pause and stress how rare this is: we see evidence from both agricultural and non-agricultural systems, spanning both field and lab experiments, converging on the same conclusion. These are not isolated studies, but are meta-analyses (a quantitative study-of-studies) that include work from around the world. This evidence is quite robust and has stood up to extensive scrutiny from researchers the world over.

Synthetic N fertilizers have a complicated history. On one hand, synthetic N fertilizer has allowed the world’s population to double (Ritchie et al., 2022), while on the other hand there are a myriad number of environmental and public health risks that arise from excessive N applications (Keeler et al., 2016; Houlton et al., 2019). These consequences can be quite serious and merit the close attention that they receive from agricultural stakeholders. Without a doubt, there is an urgent need for better management of synthetic fertilizer N. However, it’s a potent tool in the toolbelt for maintaining (or even building!) our soil carbon, so hopefully we can give it the thoughtfulness and consideration that it deserves.

Jordon Wade is the Soil Health Assessment Lead for Syngenta Group, a global agricultural technology company. The opinions here reflect a data-driven approach to soil health, which is supported by his peer-reviewed research findings pre-dating his role at Syngenta.

References

Chen, J., Y. Luo, K.J. Van Groenigen, B.A. Hungate, J. Cao, et al. 2018. A keystone microbial enzyme for nitrogen control of soil carbon storage. Science advances 4(8): eaaq1689.

Geisseler, D., and K.M. Scow. 2014. Long-term effects of mineral fertilizers on soil microorganisms – A review. Soil Biology and Biochemistry 75: 54–63. doi: 10.1016/j.soilbio.2014.03.023.

van Groenigen, K.-J., J. Six, B.A. Hungate, M.-A. de Graaff, N. Van Breemen, et al. 2006. Element interactions limit soil carbon storage. Proceedings of the National Academy of Sciences 103(17): 6571–6574.

Houlton, B.Z., M. Almaraz, V. Aneja, A.T. Austin, E. Bai, et al. 2019. A World of Cobenefits: Solving the Global Nitrogen Challenge. Earth’s Future 7(8): 865–872. doi: 10.1029/2019EF001222.

Huang, X., C. Terrer, F.A. Dijkstra, B.A. Hungate, W. Zhang, et al. 2020. New soil carbon sequestration with nitrogen enrichment: a meta-analysis. Plant and Soil 454: 299–310.

Jesmin, T., D.T. Mitchell, and R.L. Mulvaney. 2021. Short-term effect of nitrogen fertilization on carbon mineralization during corn residue decomposition in soil. Nitrogen 2(4): 444–460.

Jian, S., J. Li, J.I. Chen, G. Wang, M.A. Mayes, et al. 2016. Soil extracellular enzyme activities, soil carbon and nitrogen storage under nitrogen fertilization: A meta-analysis. Soil Biology and Biochemistry 101: 32–43.

Keeler, B.L., J.D. Gourevitch, S. Polasky, F. Isbell, C.W. Tessum, et al. 2016. The social costs of nitrogen. Science Advances 2(10): e1600219. doi: 10.1126/sciadv.1600219.

Khan, S.A., R.L. Mulvaney, T.R. Ellsworth, and C.W. Boast. 2007. The myth of nitrogen fertilization for soil carbon sequestration. Journal of Environmental Quality 36(6): 1821–1832.

Ritchie, H., M. Roser, and P. Rosado. 2022. Fertilizers. Our World in Data. https://ourworldindata.org/fertilizers (accessed 1 September 2023).

Rocci, K.S., J.M. Lavallee, C.E. Stewart, and M.F. Cotrufo. 2021. Soil organic carbon response to global environmental change depends on its distribution between mineral-associated and particulate organic matter: A meta-analysis. Science of the Total Environment 793: 148569.

Tang, B., K.S. Rocci, A. Lehmann, and M.C. Rillig. 2023. Nitrogen increases soil organic carbon accrual and alters its functionality. Global Change Biology 29(7): 1971–1983. doi: 10.1111/gcb.16588.

Van Groenigen, J.W., C. Van Kessel, B.A. Hungate, O. Oenema, D.S. Powlson, et al. 2017. Sequestering soil organic carbon: a nitrogen dilemma. ACS Publications.

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Comments on “The myth of nitrogen fertilization for soil carbon sequestration.” by Khan et al. (2007)

By Andrew McGuire

The title sounds so certain and triumphant. The “myth” has been revealed, and nitrogen (N) fertilizer discredited. The Khan et al. (2007) paper and a later paper from the same group of authors (Mulvaney et al., 2009) received a lot of attention both within the scientific literature—cited over 800 times—and in the popular press. However, as we detailed in our post above, there was no myth, and N fertilizer can actually build organic matter. Here I present a short analysis of why these papers were mistaken in their conclusions.

It should be noted that in the journal these two papers were published in, the papers received multiple published responses (Reid, 2008; Powlson et al., 2010a, 2010b; David et al., 2010), a fact that should suggest further scrutiny is required since it is rare to get even one published response. However, neither the Khan et al. paper nor the responses are open access, so most people are not able to read them in full.

In short, Khan et al. and Mulvaney et al., using data from the long-term Morrow plots in Illinois, conclude that N fertilizer depletes soil carbon (and associated nitrogen). Whereas the ten authors behind the four published responses say what was observed was actually more likely due to the change from the use of manure to the use of synthetic N than the use of synthetic N itself.

Manure has long been known to increase soil organic carbon (SOC) levels when applied at moderate to high rates because it is a source of organic carbon. The SOC levels at the Morrow plots had been increased by over 60 years of receiving manure applications. Then, when N fertilizer replaced the manure, SOC levels declined. Khan et al. and Mulvaney et al. attributed this decline to the use of N fertilizer rather than to the stopping of the use of manure. The decline does show that N fertilizer cannot maintain SOC levels previously increased by manure, but as the response authors point out, this does not mean that N fertilizer is the reason for the decline. The real driver of the decline in SOC was the elimination of manure C additions.

Khan et al. go on to cite other previous research to support their claim that N fertilizer causes soil organic C and N levels to decline, concluding that “Among field studies involving synthetic N fertilization and reporting baseline data, the usual finding has been a decrease over time in SOC storage.” I checked nineteen of their citations (the cited studies done in North America, two I could not access) and while these studies did observe decreases in soil organic matter levels in treatments using N fertilizer compared to baseline data, just as was observed with the Morrow plots, a different story reveals itself if you look at the unfertilized controls.

We have known for more than a century that the conversion of native vegetation to annual cropland results in a long-term decline in soil organic matter. Therefore, when assessing the impact of N fertilizer on soil organic matter, it is necessary to compare N fertilized plots to unfertilized control plots and not just the baseline SOM measurement made at the beginning of the study. This decline of SOM was the background setting for the N fertilizer studies that Khan et al. cited (Figure 1).

What Khan et al. didn’t mention and didn’t include when discussing the previous work of others on the topic was the results of the no-fertilizer treatments in each study. These no-fertilizer treatments resulted in a larger SOM decline than the treatments with N fertilizer in 16 of 19 studies (listed in References). A more accurate interpretation of these studies would be that while N fertilizer has not reversed the decline of SOM caused by land-use changes, it has slowed it. Further, the research we cited in our blog post above shows that once the land-use related decline is over, the use of N fertilizer can maintain or increase SOM compared to unfertilized treatments.  

All the published responses to Khan et al. and Mulvaney et al. (Reid, 2008; Powlson et al., 2010a, 2010b; David et al., 2010) mention this misleading and selective use of citation. However, publishers do not often link such responses to the original papers, so most people never see them. Further, published responses are often locked behind paywalls, adding another obstacle in the way of readers trying to get a sense of the discourse. What most people end up seeing is only an overly confident paper title and abstract, which does not allow for the needed fact-checking. The result has been a myth about the detrimental effects of N fertilizer on soils that is not easily nor quickly corrected.

References for Comments

David, M.B., G.F. McIsaac, and R.G. Darmody. 2010. Additional Comments on “Synthetic Nitrogen Fertilizers Deplete Soil Nitrogen: A Global Dilemma for Sustainable Cereal Production,” by R.L. Mulvaney, S.A. Khan, and T.R. Ellsworth in the Journal of Environmental Quality 2009 38:2295–2314. Journal of Environmental Quality 39(4): 1526–1527. doi: 10.2134/jeq2010.0003le.

Khan, S.A., R.L. Mulvaney, T.R. Ellsworth, and C.W. Boast. 2007. The myth of nitrogen fertilization for soil carbon sequestration. Journal of Environmental Quality 36(6): 1821–1832.

Mulvaney, R.L., S.A. Khan, and T.R. Ellsworth. 2009. Synthetic Nitrogen Fertilizers Deplete Soil Nitrogen: A Global Dilemma for Sustainable Cereal Production. Journal of Environmental Quality 38(6): 2295–2314. doi: 10.2134/jeq2008.0527.

Powlson, D.S., D.S. Jenkinson, A.E. Johnston, P.R. Poulton, M.J. Glendining, et al. 2010a. Comments on “Synthetic Nitrogen Fertilizers Deplete Soil Nitrogen: A Global Dilemma for Sustainable Cereal Production,” by R.L. Mulvaney, S.A. Khan, and T.R. Ellsworth in the Journal of Environmental Quality 2009 38:2295–2314. Journal of Environmental Quality 39(2): 749–752. doi: 10.2134/jeq2010.0001le.

Powlson, D.S., D.S. Jenkinson, A.E. Johnston, P.R. Poulton, M.J. Glendining, et al. 2010b. Reply to Additional Comments on “Synthetic Nitrogen Fertilizers Deplete Soil Nitrogen: A Global Dilemma for Sustainable Cereal Production,” by R.L. Mulvaney, S.A. Khan, and T.R. Ellsworth in the Journal of Environmental Quality 2009 38:2295–2314. Journal of Environmental Quality 39(4): 1528–1529. doi: 10.2134/jeq2010.0004le.

Reid, D.K. 2008. Comment on “The myth of nitrogen fertilization for soil carbon sequestration”, by S.A. Khan et al. in the Journal of Environmental Quality 36:1821-1832. J Environ Qual 37(3): 739; author reply 739-740. doi: 10.2134/jeq2008.0001le.

Reviewed citations from Khan et al. (2007), Table 3.

Bloom, P.R., W.M. Schuh, G.L. Malzer, W.W. Nelson, and S.D. Evans. 1982. Effect of N Fertilizer and Corn Residue Management on Organic Matter in Minnesota Mollisols1. Agronomy Journal 74(1): 161–163. doi: 10.2134/agronj1982.00021962007400010046x.

Brye, K.R., S.T. Gower, J.M. Norman, and L.G. Bundy. 2002. Carbon Budgets for a Prairie and Agroecosystems: Effects of Land Use and Interannual Variability. Ecological Applications 12(4): 962–979. doi: 10.1890/1051-0761(2002)012[0962:CBFAPA]2.0.CO;2.

Buyanovsky, G.A., and G.H. Wagner. 1998. Carbon cycling in cultivated land and its global significance. Global Change Biology 4(2): 131–141. doi: 10.1046/j.1365-2486.1998.00130.x.

Campbell, C.A., and R.P. Zentner. 1993. Soil Organic Matter as Influenced by Crop Rotations and Fertilization. Soil Science Soc of Amer J 57(4): 1034–1040. doi: 10.2136/sssaj1993.03615995005700040026x.

Campbell, C.A., and R.P. Zentner. 2019. Crop Production and Soil Organic Matter in Long-Term Crop Rotations in the Semi-Arid Northern Great Plains of Canada HHH. Soil Organic Matter in Temperate AgroecosystemsLong Term Experiments in North America: 317.

Clapp, C.E., R.R. Allmaras, M.F. Layese, D.R. Linden, and R.H. Dowdy. 2000. Soil organic carbon and 13C abundance as related to tillage, crop residue, and nitrogen fertilization under continuous corn management in Minnesota. Soil and Tillage Research 55(3): 127–142. doi: 10.1016/S0167-1987(00)00110-0.

Cope Jr., J.T., D.G. Sturkie, and A.E. Hiltbold. 1958. Effects of Manure, Vetch, and Commercial Nitrogen on Crop Yields and Carbon and Nitrogen Contents of a Fine Sandy Loam Over a 30-Year Period. Soil Science Society of America Journal 22(6): 524–527. doi: 10.2136/sssaj1958.03615995002200060013x.

Dodge, D.A., and H.E. Jones. 1948. The Effect of Long-Time Fertility Treatments on the Nitrogen and Carbon Content of a Prairie Soil1. Agronomy Journal 40(9): 778–785. doi: 10.2134/agronj1948.00021962004000090003x.

Doran, J.W., E.T. Elliott, and K. Paustian. 1998. Soil microbial activity, nitrogen cycling, and long-term changes in organic carbon pools as related to fallow tillage management. Soil and Tillage Research 49(1): 3–18. doi: 10.1016/S0167-1987(98)00150-0.

Izaurralde, R.C., W.B. McGill, J.A. Robertson, N.G. Juma, and J.J. Thurston. 2001. Carbon Balance of the Breton Classical Plots over Half a Century. Soil Science Soc of Amer J 65(2): 431–441. doi: 10.2136/sssaj2001.652431x.

Lesoing, G.W., and J.W. Doran. 2019. Crop Rotation, Manure, and Agricultural Chemical Effects on Dryland Crop Yield and SOM over 16 Years in Eastern Nebraska. Soil Organic Matter in Temperate AgroecosystemsLong Term Experiments in North America. CRC Press. p. 197–204

Olson, K.R., J.M. Lang, and S.A. Ebelhar. 2005. Soil organic carbon changes after 12 years of no-tillage and tillage of Grantsburg soils in southern Illinois. Soil and Tillage Research 81(2): 217–225. doi: 10.1016/j.still.2004.09.009.

Pikul, J.L., T.E. Schumacher, and M. Vigil. 2001. Nitrogen use and carbon sequestered by corn rotations in the northern corn belt, US. The Scientific World Journal 1: 707–713.

Rasmussen, P.E., and W.J. Parton. 1994. Long-Term Effects of Residue Management in Wheat-Fallow: I. Inputs, Yield, and Soil Organic Matter. Soil Science Society of America Journal 58(2): 523–530. doi: 10.2136/sssaj1994.03615995005800020039x.

Robinson, C.A., R.M. Cruse, and M. Ghaffarzadeh. 1996. Cropping System and Nitrogen Effects on Mollisol Organic Carbon. Soil Science Society of America Journal 60(1): 264–269. doi: 10.2136/sssaj1996.03615995006000010040x.

Vanotti, M.B., L.G. Bundy, and A.E. Peterson. 1997. Nitrogen fertilizer and legume-cereal rotation effects on soil productivity and organic matter dynamics in Wisconsin. Soil Organic Matter in Temperate Ecosystems: Long-Term Experiments in North America; Paul, EA, Paustian, K., Elliott, ET, Cole, CV, Eds: 105–119.

Varvel, G.E. 2006. Soil Organic Carbon Changes in Diversified Rotations of the Western Corn Belt. Soil Science Society of America Journal 70(2): 426–433. doi: 10.2136/sssaj2005.0100.

Vitosh, M.L., R.E. Lucas, and G.H. Silva. 2019. Long-Term Effects of Fertilizer and Manure on Corn Yield, Soil Carbon, and Other Soil Chemical Properties in Michigan. Soil Organic Matter in Temperate AgroecosystemsLong Term Experiments in North America. CRC Press. p. 129–139

Wander, M.M., S.J. Traina, B.R. Stinner, and S.E. Peters. 1994. Organic and Conventional Management Effects on Biologically Active Soil Organic Matter Pools. Soil Science Society of America Journal 58(4): 1130–1139. doi: 10.2136/sssaj1994.03615995005800040018x.

Wilts, A.R., D.C. Reicosky, R.R. Allmaras, and C.E. Clapp. 2004. Long-Term Corn Residue Effects: Harvest Alternatives, Soil Carbon Turnover, and Root-Derived Carbon. Soil Science Society of America Journal 68(4): 1342–1351. doi: 10.2136/sssaj2004.1342.