Potential for agricultural recycling of struvite and zeolites to improve soil microbial physiology and mitigate CO2 emissions

Recycling nutrients in agroecosystems is becoming increasingly important to promote agricultural sustainability. Struvite and nitrogen (N)-enriched zeolites produced via wastewater treatment offer the potential for nutrient recycling. However, their effects on soil properties, particularly on microbial physiology, remain largely unknown; especially regarding microbial feedback, from which losses or sequestration of essential elements may result. This study investigates the short-term (three days) physiological responses of soil microorganisms, changes in available nutrients, and the immediate effects on soil organic matter (SOM) and carbon dioxide (CO₂) emissions following the application of struvite and N-enriched zeolites derived from liquid digestate, alongside natural zeolites amendments in an acidic sandy soil. All treatments increased soil pH, which emerged as a driving factor in the dissolution of labile organic carbon (C) and the microbial production of N-, C-, and phosphorus (P)-acquiring extracellular enzymes. As soil pH increased, the stoichiometric ratio of microbial biomass C (C_mic) to microbial biomass N (N_mic), along with the enzymatic C:N ratio decreased, suggesting a superior effect on microbial N-cycling compared to C-cycling. Carbon dioxide emissions increased, particularly with the application of organic fertilizer (digestate), where the highest microbial metabolic quotient reflected increased catabolic activity due to the immediate availability of organic C. Overall, zeolitized tuffs demonstrated the potential to mitigate CO₂ emissions, likely due to CO₂ adsorption capacity.