EFFECT OF CARBON ADDITIONS ON SOIL LABILE INORGANIC, ORGANIC AND MICROBIALLY HELD PHOSPHATE

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Abstract
Investigations of the rate of P movement between soil inorganic, organic and biomass P compartments were carried out to clarify aspects of P cycling in soil systems. Organic carbon, as dried grass (33% C, 0.11% P) and cellulose (43% C), was added at a rate equivalent to 4000 kg organic material (OM)∙ha−1 every 30 days for 9 mo to the Ap horizon of a Chernozemic Black soil kept at field capacity moisture content and 24 ± 2 °C. In a third treatment, cellulose was added at the same rate with P (20 kg∙ha−1) at KH2PO4. Approximately 39% and 22% of the P added in grass and with cellulose, respectively, was found in organic P forms after 9 mo incubation. The remainder was found in NH4Cl-, NH4F- and NaOH-NaCl-extractable P forms which constituted part of the labile inorganic P pool and could be extracted by an anion exchange resin. Increases of biomass P during the first 4 or 5 days of each incubation period after residue addition were found to average 12 μg P∙g−1 in the first 3 mo incubation period. After this period, there was a smaller response in microbial P attributable to additions of grass or cellulose.