Soil Organic Carbon Regulates Nitrogen Mineralization and Uptake from Citrus Sewage Sludge in a Wheat Cropping System

The need for more sustainable agriculture less dependent on mineral fertilizers has intensified the interest in the reuse of agro-industrial by-products as alternative nutrient sources. This study investigates the agronomic potential of citrus sewage sludge (CSS), derived from citrus wastewater treatment, as a nitrogen (N) source for wheat cultivation. An experiment was conducted using two Mediterranean soils with contrasting physicochemical properties, comparing a non-fertilized control (CTR), inorganic N fertilization (NH4NO3) (CTR + N), and CSS; fertilizers were applied once at 30 mg of N per plant. Differences in soil organic carbon availability and C/N ratio, together with carbonate-related properties, influenced N dynamics in the soil–plant system. In the soil with higher oxidizable organic C and a more favorable C/N ratio (S1), CSS increased soil ammonium concentrations by about 70% compared with the control and by nearly 50% compared with the soil characterized by lower organic C availability (S2). In S2, the lower concentrations of both NH4+ and NO3− indicate reduced microbial mineralization and nitrification, consistent with its lower availability of readily degradable organic carbon. Moreover, wheat grown with CSS exhibited a total biomass about 40% higher than that of the CTR. The Mineral Fertilizer Replacement Value (MFRV) reached 73% in S1 and 46% in S2, confirming the potential of CSS as a sustainable N source, particularly in soils where organic C availability supports microbial activity and N transformations. Future strategies should focus on improving CSS use through specific soil management practices.