Legume cover crops affect rhizosphere properties and soil phosphorus fractions but do not increase phosphorus availability in California plots

Background/Question/Methods

Phosphorus (P) is a major limiting nutrient in agriculture, where external P applications are often used to replace soil P removed by harvest and maintain soil fertility. However, economically viable reserves of mineral P are finite and a high fraction of P applied is unavailable to most crops, thus alternative approaches to maintain soil fertility while reducing P inputs must be developed. Certain plants (e.g., legumes) can mobilize soil P that is bound to soil minerals or organic matter by altering their rhizosphere: acidification, organic acids exudation, higher enzyme activity. If these plants are ploughed into the soil rather than harvested, the P they mobilize could help reduce P input requirements.

We conducted a field experiment in 2012 to determine if legume cover crops (fava bean, vetch) mobilized more rhizosphere P than cereals (oats, wheat) in California agriculture. We sampled a mixture of all these species grown as a cover crop in 0.4 plots under three management types: organic, reduced-input conventional, no input. Within each plot, three 20.5 m² microplots were sampled for rhizosphere soil and biomass of each species, and for bulk soil. Rhizosphere and bulk soils were analyzed for pH, organic acids, phosphatase enzyme activity, and P fractions.

Results/Conclusions

Fava bean acidified its rhizosphere significantly (fava bean > vetch = cereals = bulk) whereas all plants increased phosphatase activity, especially legumes (legumes > cereals > bulk). Concentrations of organic acids were higher in the rhizosphere (fava bean > vetch = cereals > bulk) where malate (rhizosphere > bulk), citrate (legumes > cereals = bulk) and malonate (fava bean > vetch > cereals = bulk) were the main organic acids found. Management did not affect the relationships among plant treatments.

Lower pH decreased calcium-bound soil P with a concomitant increase in P bound to aluminum and iron oxides. Higher enzyme activity was associated with higher organic P concentrations whereas organic acid concentrations were not associated with changes in soil P fractions. These changes did not increase the availability of readily plant-available P measured by a resin extract, except for an increase in legumes under reduced-input conventional management. Thus, despite measurable impacts on rhizosphere pH, organic acids and enzyme activity, negligible increases in readily plant-available P combined with low aboveground biomass of legumes – legumes never exceeded 20% of aboveground biomass – suggest that legumes do not increase P availability substantially under these conditions.