| 12 Jun 2026
Old organic matter in temperate forest soils is very nitrogen-rich
Abstract. Soil organic matter (SOM) is important for Earth’s climate regulation and primary production. Here we tested the hypothesis that old SOM fractions are rich in organic nitrogen given the high affinity of organic nitrogen compounds to interact with mineral surfaces. For this purpose, we studied temperate broadleaf and coniferous forest soils located in Sweden. We used ramped thermal fractionation to obtain persistent SOM fractions, and we evaluated the persistence using 14C. The results show that old SOM fractions in the temperate forest soils are very nitrogen-rich, indicating that organic nitrogen compounds persist for a long time in soil. A larger proportion of the total organic nitrogen than of the total organic carbon was found in the old SOM fractions, suggesting that organic nitrogen compounds decompose on average more slowly than nitrogen-free organic compounds. The size of the old SOM fractions was correlated with the soil clay content but did not differ significantly between broadleaf and coniferous forests, suggesting that association with minerals rather than forest type or organic matter quality affect the persistence of SOM. This study has important implications for element cycling as it demonstrates that nitrogen in temperate forests persists for a long period in SOM.
I reviewed this short, but very interesting manuscript by Spohn et al., who used thermal oxidation and radiocarbon dating to show that old organic matter in Swedish forest soils is strongly enriched in nitrogen. The approach taken is generally interesting, but the authors would need to proof, that the N-enrichment is not an artefact of the thermal oxidation. This is most likely very difficult. The so-called Maillard reaction, which is well known in the pyrogenic carbon community and also food chemistry, is the condensation of sugars with amino acids under high temperatures (see e.g.: Nitrogen Enrichment during Soil Organic Matter Burning and Molecular Evidence of Maillard Reactions | Environmental Science & Technology). Although thermal oxidation might be a way to isolate biogeochemically meaningful organic matter fractions, they do not remain unchanged. The KCL extraction done by the authors to quantify the amount of mineral nitrogen in the samples after thermal fractionation is not enough, because here its about the formation of aromatic nitrogen. Unfortunately, the methodological choice here is most likely not suitable to test the hypothesis of the authors. As a consequence, the manuscript needs to be rejected at that stage. Â A simple physical fractionation might have been the better choice to separate younger and older OM.
Apart from this fundamental comment, I’d like to mention that although short papers are nice to read, I found this one a bit too superficial in some parts. Surely, the authors mentioned a couple of times, that N-rich organic compounds might have a higher affinity to be sorped and stabilized, but readers would probably like to get some more background and deeper understanding on that. Also, the step of organic matter transformation (e.g. by microbes) is somewhat missing. We know that a substantial part of mineral associated OM consists of microbial necromass, which is per se quite enriched in N. Although the authors have no data on this, there could be more in-depth discussion on the different stabilization pathways. I also miss a bit of an implications section in the discussion (what does that mean for the ecosystem, for the different biogeochemical cycles, how far can you generalize these results, and who should care about them). Also, a proper conclusions section is missing.
Specific comments:
Line 12: It could also indicate, that organic matter becomes enriched in its transformation processes and that more processed organic matter persists longer.
Introduction: Maybe at least mention the ‘microbial carbon pump’ and how this relates to N
Line 53: Could you please elaborate a bit more on your exclusion criteria of Podzols? To avoid strong eluviation, you could have selected via OM content.
Line 58: Why larger than 55 years?
Line 74: Why so restricted to the 0-10 cm layer?
Line 75: So you took the soils from the archive? Could be mentioned.
Line 78: Why 35°C? This seems rather low.
Line 1745ff: This whole section seams to belong to the section above (on thermal fractionation). Maybe move upwards?
Figure 2: I don’t really like the x-axis names. Soil, 325°C, 400°C. Please consider changing.Â