Do pseudo-sands‘ internal structures determine biogeochemistry of tropicals‘ critical zone?

Deeply weathered soils are iconic components of the tropical critical zone. Being short of exchangeable nutrients, physical features beyond sheer depth determine more than elsewhere the provision of life-sustaining resources. Biogeochemical models can hardly capture tropical fluxes, due to them being mainly developed and validated for temperate zones, focusing more on topsoil processes. To overcome this systematic bias, pedotransfer functions are regionally adapted for modeling soil water movements and then used for tropical biogeochemical modeling. However, soil texture based on single grain size distribution neglects the impact of actual soil structures in the field and leads to pronounced discrepancies between field measurements and model predictions for tropical soils. A prominent example of this mismatch is overestimated N₂O emissions.

Now, scientific efforts are being made to correct this systematic bias in predicting soil functioning. A well-known characteristic of tropical soils, potentially responsible for the systematic error, are water-stable aggregates called pseudo-sands. In the field, they are perceived as sand, but in the lab measured as clay and silt. The simple assumption that pseudo-sands act just like sands in the field seems to work satisfactorily for certain hydrological predictions. We pursue the hypothesis that, biogeochemically, pseudo-sands do not simply act like sands. We provide evidences why pseudo-sands cannot be simply treated neither like “regular sand” nor like the sum of its units. The long-term goal is to develop tropical biogeochemical model versions related to the properties of pseudo-sands that will lead to improved models of the critical zone of the tropics.