Effect of soil moisture regime on the microbial contribution to Olsen phosphorus values

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Abstract
Amounts of inorganic phosphorus (Pi) extractable with 0.5M NaHCO3 (Olsen P) were measured in a range of soils before and after airdrying. Except for two sandy soils, air-drying caused increases in extractable Pi of 14 – 184%. The increases were related to the microbial P (determined by a CHCl3-fumigation procedure) and the proportion of the microbial biomass killed by the air-drying (determined by a respiration response technique). The microbial populations from soils of regions with a high (> 400 mm) annual water deficit were more resistant to laboratory air-drying and, in general, released less extractable Pi. A procedure to estimate the approximate microbial contribution to Olsen P is suggested. In the soils tested, release of microbial P accounted for up to 95070 of the increases obtained on air-drying of soils, and comprised 4 – 97% of the Olsen P content. It is suggested that soils with > 2% organic C, < 20 µg-1 Olsen P, and a water deficit of <400 mm are likely to have a large microbial contribution to the Olsen P value.