Preferential sorption of N by clays: an essential control on soil organic carbon retention?

Soil organic carbon (SOC) is the most viable and stable pool for C sequestration in terrestrial ecosystems, through judicious management practices for agricultural and forestry production, or reclamation of degraded areas. Soils vary much in their capacity of SOC retention, but in similar environments, finer-textured soils typically hold more SOC than coarser ones, since SOC is mostly stabilized by sorption by clays. However, many questions remain on how these processes occur and their implications for SOC stabilization or destabilization. Nevertheless, much information can be given by soil C:N ratios, since they vary much less than SOC or N concentrations across soils and land use systems, although interpretation of such data is much more complex than for fresh or composted organic residues. In Brazil, C:N ratios of clay fractions are typically lower (~10) than in bulk soils or coarser fractions, regardless of depth (Zinn et al., SSSAJ 71, 2007). More recently, a global meta-analysis confirmed this trend, especially in arid zones, where clay C:N is often <9 (Amorim et al. Geoderma 425, 2022), who argued this is due to sorptive mechanisms, absent in coarse fractions. More recently, Amorim et al. (Catena 230, 2023) showed clay enrichment factors were always >1 and much higher for N than SOC in contrasting Brazilian soils, regardless of depth. These trends strongly suggest N preferential sorption compared to SOC, and thus, N is crucial for SOC sequestration and stabilization. Further research on how soil texture and clay mineralogy affect this phenomenon is proposed, including functional characterization.