Soil Texture Modulates Phosphorus Dynamics, Enzyme Activity and root Responses to Phosphorus, zinc and Biofertilizer Inputs in Citrus Rhizosphere

Purpose: Phosphorus (P) fertilization is essential for citrus establishment in tropical soils, where soil texture strongly influences P availability and rhizosphere nutrient dynamics. This study evaluated the effects of localized P application, alone or combined with zinc (Zn) or a bioinput, on rhizosphere P dynamics, soil enzymatic activity, plant nutritional status, and root development during the initial establishment stage of ‘Valencia’ sweet orange grafted onto ‘Swingle’ citrumelo grown in contrasting soils. Methods: A greenhouse experiment was conducted using rhizotrons filled with clayey and sandy soils under a completely randomized 5 × 2 factorial design. Five fertilization strategies (Control, Monoammonium phosphate (MAP), MAP + Zn, MAP + Bio, and Organic) were evaluated. Soil chemical properties, P fractions, acid phosphatase, β-glucosidase and arylsulfatase activities, foliar P and Zn concentrations, and root morphological traits were determined. Results: MAP increased available P by 327% in the clayey soil, where moderately labile P represented up to 93% of total rhizosphere P. In contrast, the sandy soil was dominated by the non-labile P fraction, accounting for 67–90% of total P. Enzyme activities were three to five times greater in the clayey soil, indicating enhanced biological nutrient cycling. MAP + Zn and Organic fertilizer increased soil Zn availability and foliar Zn concentration, whereas MAP + Zn promoted the highest foliar P concentration. The P supply also stimulated root system fasciculation, increasing total root area by up to 62% and primary root diameter by up to 312%, whereas P deficiency promoted deeper root growth. Conclusions: Soil texture regulated P partitioning in the citrus rhizosphere and determined the biological, nutritional, and root responses to phosphate fertilization. These findings improve the understanding of P cycling in contrasting tropical soils and demonstrate that soil texture modulates rhizosphere P dynamics, nutrient acquisition, and root development during the initial establishment stage of citrus.