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PHOSPHORUS MANAGEMENT

Relationships between Immobilized Phosphorus Uptake in Two Grain Legumes and Soil Bioactive Phosphorus Pools in Fertilized and Manure-Amended Soil

^a USDA, ARS, Grazinglands Res. Lab., El Reno, OK 73036
^b USDA, ARS, Environmental Management & Byproducts Utilization Lab., BARC-East, Bldg. 306 Powder Mill Rd., Beltsville, MD 20705

^* Corresponding author (thanh.dao{at}ars.usda.gov).

Mixing P-immobilizing additives with manure has raised concerns of irreversible reduction in P availability to growing crops. A potted plant growth experiment was conducted to characterize cattle manure P mineralization as modified by iron amendments and uptake by pigeon pea (Cajanus cajan L. Millsp.) and soybean [Glycine max (L.) Merr.]. Triple superphosphate, untreated, or manure amended with Fe at 1:1 or 1:3 molar ratio of manure P:Fe, was applied to Dale silt loam (fine-silty, mixed, superactive, thermic Pachic Haplustolls) at the rate of 20 mg kg^–1. Whole plants were harvested at three stages of development (vegetative, flowering, and physiological maturity) to determine and correlate P uptake to changes in soil bioactive P pools over the growing season. Dry matter production was unaffected by Fe at both application rates. Phosphorus solubility and plant uptake were reduced at the 1:3 P:Fe molar ratio rate, in spite of the legumes' reported ability to secrete siderophores. Within-season changes in bioactive P fractions indicated that P was taken up from the inorganic (water-extractable P [WEP] + inorganic ligand-exchangeable P [EEP_i]) pools. Comparing soil P pools in cropped and noncropped soils, the decrease in the organic phosphohydrolase-labile P (PHP) pool corroborated the fact that the PHP pool replenished the WEP + EEP_i fractions. Although Mehlich 3 P and EEP_i were related, the Mehlich 3 P pool showed no significant relationship with whole plant or any plant part P. The latter did not perform as well as the EEP_i pool in assessing plant availability and enzymatically mediated turnover of immobilized P.

Abbreviations: CDTA, 1,2-cyclohexanediamine-tetraacetate • DAP, days after planting • EDTA, ethylenediamine-N,N,N',N'-tetraacetate • EEP, ligand-exchangeable phosphorus • P, phosphorus • PHP, phosphohydrolase-labile P • WEP, water-extractable phosphorus

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Received for publication December 18, 2007.