Disentangling the impact of salinity and sodicity on organic matter cycling in paddy soils of Tropical Eastern Africa

• Applicant: Professor Dr. Robert Mikutta
• Subject Area: Soil Sciences
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 558017104

Soil salinization represents an increasing and severe risk for soil productivity in global lowland rice production, the essential staple food source for approximately 50% of the world’s population. However, how salt stress conditions affect soil organic matter (SOM) cycling and net organic carbon (OC) stocks in paddy soils is not conclusively understood, especially in African rice production systems. Isolated research suggests that salinity and sodicity cause different trajectories of SOM cycling but very few studies directly compared the effect of saline versus sodic conditions on the three general controls determining SOM sequestration, i.e. OM input (quantity and quality of crop residues), OM decomposition and mineralization, and SOM stabilization mechanisms. To tackle this knowledge gap, the present project aims at investigating controls and processes along the SOM process cascade under salt-affected conditions of Tropical Eastern African paddy soils. The project combines laboratory and field research in central Mozambique and relies on different soil salinity categories that comprise soils non-affected by salts and those affected by salinity and sodicity, respectively. Along these categories, we elucidate general physicochemical soil properties, SOC inputs and resulting SOC stocks, thus identifying general controls on SOC sequestration. Density fractionation combined with radiocarbon analyses will be used to characterize functional SOM fractions and their turnover. Chemolytic assessment of different biomarkers clarify the chemical composition and the microbial versus plant origin of SOM under the variable salt influence. In order to assess the SOM input and controls, a field litterbag experiment over a full cropping cycle will be established that enables determination of the rice straw decomposition dynamics across the salinity categories. Specifically, we quantify litter mass loss, the relative contribution of invertebrates and microbes to residue decomposition, and the corresponding change in litter quality using 13C-NMR spectroscopy. Finally, an incubation experiment under field-like conditions with isotopically labelled rice straw amended to salt-affected soils will provide insights into overall C fluxes, uptake of rice straw-C into functional SOM fractions, and mineral-SOM interactions under variable redox conditions. Overall, our project offers the first-time opportunity to systematically disentangle the interplay of the different controls along the SOM process cascade in paddy soil under salt-affected conditions, resulting in a conceptual model for the effects of different salt stresses on net SOM sequestration.

• DFG Programme: Research Grants
• Co-Investigators: Professor Dr. Christian Mikutta; Dr. Pauline Winkler