Microbial immobilisation of phosphorus in soils exposed to drying-rewetting and freeze-thawing cycles

Abstract

Drying-rewetting and freezing-thawing can drastically alter P availability in soil. We studied how these weather events affect microbial immobilisation/mobilisation of P on the four soil types from a climatic gradient with increasing annual mean temperatures and a progressive decrease in precipitation: Podzol, Phaeozem, Chernozem and Kastanozem. Soils were exposed to (1) optimal moisture and temperature, (2) drying-rewetting and (3) freezing-thawing. Soils were treated with a 33P spike immediately after rewetting or thawing to simulate P pulse. Thereafter, P immobilisation by soil microorganisms was estimated by direct fumigation and anion exchange membrane techniques. To ensure correct estimation of microbial P (Pmic), the conversion factors kP were determined individually for each soil by 33P labelling with the correction for 33P sorption and 31P–33P isotopic exchange. The membrane extraction minimised both sorption and isotopic exchange of P released with both sorption and isotopic exchange coefficients close to 0.9 irrespectively of the soil. Specific kP varied from 0.19 to 0.38. Pmic values followed the pattern freezing-thawing < drying-rewetting < optimal conditions, varying from 2.0 to 36.6 mg P kg−1. Intensive microbial immobilisation of 33P after rewetting (up to 41 %) demonstrated a conversion of dissolved P to Pmic potentially available for plant nutrition. Remarkably, no detectable microbial immobilisation of 33P was found in the freezing-thawing treatment. In contrast to drying, freezing decreased total Pmic by up to 7.5 times. Values of membrane-extractable 33P increased in the order drying-rewetting < control < freezing-thawing, and up to 77 % of added 33P was recovered in dissolved P forms after thawing, indicating the potential risk of P leaching after freezing-thawing events.