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

Amounts of inorganic phosphorus (P i ) extractable with 0.5M NaHCO 3 (Olsen P) were measured in a range of soils before and after airdrying. Except for two sandy soils, air-drying caused increases in extractable P i of 14 - 184%. The increases were related to the microbial P (determined by a CHCl 3 -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