Relationships between P fractions and the microbial biomass in soils under different land use management

The first aim of the present study was to investigate the relationship between microbial biomass P and P fractions, especially the organic P (P-o) fractions, in a group of 21 German soils that were relatively low in P availability to plants due to their geogenic origin, clay quality, pH, or level of fertilisation. These German soils were compared with an alkaline saline-sodic soil from Pakistan and a relict Ferralsol. The second aim was to reassess Olsen and Bray-1 extractants for determining microbial biomass P by the fumigation extraction method. The strong positive relationships with resin and 0.5 M NaNCO3 extractable inorganic P (P-1) suggest that 1 M HCI-P-1 is a major source of these two labile P fractions. The fraction NaNCO3-P-o was the smallest Po fraction, but that with the most specific features. It significantly increased with increasing soil organic C content in the order arable