Old Soil Organic Matter Has a Low Carbon:Nitrogen Ratio-A Global Meta-Analysis About ¹⁴C and Soil Stoichiometry

Soil organic matter (SOM) persistence and the SOM carbon-to-nitrogen (C:N) ratio determine long-term nitrogen (N) sequestration in terrestrial ecosystems. Yet, the question of how SOM persistence is related to the C:N ratio of SOM is still not fully resolved. Based on a global dataset on Δ¹⁴C in soils, I tested the hypothesis that SOM persistence is negatively correlated with the C:N ratio because N is enriched relative to carbon (C) as SOM ages. The results of this study demonstrate that the soil Δ¹⁴C decreased with decreasing C:N ratio, indicating that old, persistent SOM has a low C:N ratio, as hypothesized. The relationship between Δ¹⁴C and the C:N ratio had a breakpoint at a C:N ratio of approximately 13, indicating that organic matter with a high C:N ratio decomposes quickly, while SOM with a C:N ratio below 13 decomposes substantially more slowly. The Δ¹⁴C was higher in forest and shrubland soils than in grassland and cropland soils throughout the soil profile. High soil Δ¹⁴C values were associated with high soil C:N ratios in topsoil layers of forests and shrublands, suggesting that high inputs of plant detritus with high C:N ratios keep the C:N ratio elevated in the topsoil layers of these ecosystems. In conclusion, this study shows that old SOM has a low C:N ratio, indicating that organic N is preferentially retained in soils compared to organic C. The results have important implications for element cycling and soil C sequestration because they show that N is stored on average for longer periods in soils compared to C and that an increase in the amount of organic C that is sequestered in soils over centuries would strongly increase the amount of sequestered N.