Quantifying the contribution from the soil microbail biomass to the extractable P levels of fresh and air-dried soils

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Abstract
Levels of P extractable in 0.5 M NaHCO3 and the microbial biomass C were measured on a range of 15 New Zealand pasture soils, with and without an air-drying treatment. The microbial biomass C, estimated using a modified substrate-induced-respiration method, was decreased 11-68% by air-drying. Two soils showed no increase in extractable inorganic phosphorus (Pi) levels after air-drying, but increases for the other 13 soils ranged over 14-184%. In general, the biomass C was not related to the overall Pi levels of moist or air-dried soils. However, the increase in P after air-drying was related to the microbial P content of the moist soil, estimated by CHCl3 fumigation, and the proportion of the biomass C killed by air-drying. A comparison of the actual measured Pi levels of dried soils and those estimating allowing for the microbial contribution, showed excellent agreement, strongly suggesting that the whole of the Pi increase on air-drying of soils was derived from killed microbial cells. The microbial contribution to Pi levels in NaHCO3 extracts of dried soils ranged over 4-76% and was therefore a significant large proportion in some soils. A large microbial contribution to Pi in extracts is most likely on air-dried soils from under pasture with > 2% organic matter and NaHCO2-extractable P values of < 20 .mu.g g-1.