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 (Po) 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 NaHCO3 extractable inorganic P (Pi) suggest that 1 M HCl–Pi is a major source of these two labile P fractions. The fraction NaHCO3–Po 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 < grassland < forest soils and was also closely related to the soil microbial biomass. The high percentage of NaOH–Po is a characteristic feature of the three basaltic soils, ranging from 19 to 41% of total P. Microbial biomass P-Olsen and P-Bray-1 showed a highly significant linear relationship (r = 0.98, P < 0.001, n = 23) and did not differ between the two extractants. P-Bray-1 has a slightly lower CV and thus advantages in soils low in microbial biomass P such as saline-sodic soils, but also in P fixing soils such as Ferralsols and basaltic soils.

► NaHCO3–organic P (Po) was most closely related to microbial biomass P.
► 1 M HCl inorganic P (Pi) dominated the saline-sodic soil.
► NaOH–Pi and 6 M HCl-Pi characterised the Ferralsol.
► NaOH–Po characterised the basaltic forest soils.
► Microbial biomass P can be accurately determined by Olsen and Bray-1 extractions.