A Dual Isotope Protocol to Separate the Contributions to Phosphorus Content of Maize Shoots and Soil Phosphorus Fractions from Biosolids, Fertilizer and Soil

Jean Davis¹, Richard J Flavel², Graeme Blair²*

¹Sydney Water, PO Box 399 Parramatta, Australia.

²School of Environmental and Rural Science, Agronomy and Soil Science, University of New England, Armidale, NSW, Australia.

Corresponding Author e-mail: gblair@une.edu.au

DOI : http://dx.doi.org/10.12944/CARJ.6.3.02

Abstract:

Separation of the phosphorus (P) contributions from soil, fertilizer and biosolids to plants has not been possible without the aid of radioisotopes. Dual labelling of soil with ³²P and fertilizer with ³³P isotopes has been used to partition the sources of P in maize (Zea mays) shoots and in soil P pools. Biosolids containing 4.1% P that had been prepared using Fe and Al were applied to a Kurosol soil from Goulburn, NSW, Australia. The biosolids were applied at five rates up to 60 dry t/ha with and without P fertilizer. Phosphorus derived from fertilizer was determined directly with³³ P and that from soil by³² P reverse dilution.  Phosphorus derived from biosolids was estimated as the difference between total P and that derived from the soil plus fertilizer calculated from isotope data. Yield and P content of maize shoots increased linearly with the rate of biosolidsapplication. The proportion of P in the plant derived from biosolids also increased with application rate up to 88% for the soil receiving biosolids at 60 dry t/ha with no fertilizer. The corresponding value with fertilizer applied at 80 kg P/ha was 69%. The proportion of P in the maize shoots derived from soil and fertilizer decreased as biosolids application rate increased. Soil total P, bicarbonate extractable P, Al-P, Fe-P and Ca-P increased with biosolids application rate. The increase in plant P uptake and in bicarbonate extractable P in the soil shows that biosolids P provides a readily available source of P. A decrease in uptake of fertilizer and soil P with increasing biosolids application is attributed to the decrease in the proportion of P from these sources in the total pool of available P, rather than to immobilization of P by Fe and Al in the biosolids.

Keywords:

Aluminium; Biosolids; Fertilizer; Iron; Phosphorus; Radioisotopes

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