Altering fertilizer phosphorus recovery through rhizosphere acidification of black locust (Robinia pseudoacacia L.)

Andrew Robert Gillespie, Purdue University

Abstract

The ability of legumes to acidify their rhizosphere and increase uptake of fertilizer phosphorus was tested for a forest tree species, black locust (Robinia pseudoacacia L.). A four-part study was undertaken to evaluate the magnitude of (1) rhizosphere acidification through measurements of locust acid efflux and pH buffering in six forest soils, (2) pH effects on soil P buffering of four fertilizer sources of varying solubility, subsequent bioavailability, and locust P uptake rates as a function of increased solution concentrations, (3) enhanced P uptake by locust acidifying its rhizosphere in field trials with six forest soils, and (4) enhanced P uptake by black walnut (Juglans nigra L.) when interplanted with locust and alfalfa (Medicago sativa L.) acidifying their rhizosphere and with walnut and orchardgrass (Dactylis glomerata L.) not exhibiting rhizosphere acidification. Mechanistic mathematical models were used to predict acid-P diffusive interactions and resulting locust P uptake. Results from part one were used in an acid diffusion model to predict pH gradients adjacent to locust roots. These gradients were combined with results of part two to predict locust uptake of solubilized fertilizer P using a nutrient diffusion model. Model predictions were tested in part three with field trials showing (1) close correlation between predicted and measured locust P contents for all soil x fertilizer combinations and (2) the importance of legume rhizosphere acidification in enhancing P fertilizer recovery/uptake in a range of forest soils. Additionally, part four revealed the ability of a legume to increase P uptake of an associated species when interplanted. Thus, interplanting or underplanting of legumes to increase nitrogen in plant communities may be used to increase fertilizer phosphorus recovery by target plants as well.

Degree

Ph.D.

Advisors

Pope, Purdue University.

Subject Area

Forestry

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