COS 64-2
Legume cover crops affect rhizosphere properties and soil phosphorus fractions but do not increase phosphorus availability in California plots

Wednesday, August 7, 2013: 8:20 AM
M100HC, Minneapolis Convention Center
Gabriel Maltais-Landry, Biology, Stanford, Stanford, CA
Kate M. Scow, Land, Air, and Water Resources, University of California-Davis, Davis, CA
Peter M. Vitousek, Department of Biology, Stanford University, Stanford, CA
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.