『Abstract
　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.

Keywords: Microbial biomass P; P-Olsen; P-Bray-1; Basaltic soils; Ferralsol; Saline-sodic soil』

1. Introduction
2. Materials and methods
　2.1. Soils and sampling
　2.2. P fractionation
　2.3. Microbial biomass indices
　2.4. Statistical analysis
3. Results
　3.1. Total P and P fractions
　3.2. Microbial biomass indices
　3.3. Interactions between microbial and chemical properties
4. Discussion
　4.1. Inorganic P fractions
　4.2. Organic P fractions
　4.3. Methods for determining microbial biomass P
5. Conclusions
Acknowledgements
Appendix A. Supplementary data
References