Soil phosphorus mobilization in the rhizosphere of cover crops has little effect on phosphorus cycling in California agricultural soils

• P uptake by cereals was more important than P mobilization by legumes.
• Fava bean was the only legume to alter its rhizosphere significantly.
• Changes in pH, organic acids and enzyme activity did not improve P availability.
• We found no strong impact of P input rates on cover crop effects.
• Cereal cover crops may provide more P benefits than legumes in California.

Phosphorus (P) is a key limiting factor in many terrestrial ecosystems because most soil P is bound to soil minerals or organic matter. Increasing P cycling rates can increase P availability, including in agricultural soils that receive external P inputs. For example, cover crops may increase P cycling rates via plant uptake and P release during microbial decomposition. Cover crops and associated microbes may also change rhizosphere properties and stimulate soil P mobilization. We studied the potential of legume – fava bean (Vicia faba), vetches (Vicia dasycarpa, Vicia sativa, Vicia benghalensis) pea (Pisum sativum) – and cereal – rye (Secale cereale), wheat (Triticum aestivum), oat (Avena sativa) – cover crops to stimulate P cycling across management practices in two long-term systems trials in California. We measured cover crop biomass and nutrient content, P-mobilizing capacity (pH, organic acids, phosphatase activity) and soil P fraction changes in the rhizosphere. Cereals generally produced more biomass with similar P content compared to legumes, but higher C:P in cereal residues could favor microbial immobilization, delay residue mineralization and reduce P cycling rates. Legumes, especially fava bean, had the largest effect on rhizosphere properties by reducing pH and increasing organic acids concentrations and phosphatase activity. However, these changes in rhizosphere properties had a modest impact on soil P and did not increase soil P availability. Furthermore, we found no strong effect of management practices or soil P concentrations on soil P mobilization. Our results suggest that P mobilization in the rhizosphere of legumes is unlikely to increase P cycling rates in these soils, whereas P uptake and release in cereal biomass could have stronger effects.