When I used 0.5g of air-dried soil and added 1ml of chloroform to fumigation it overnight in a fume hood, then extract with 0.5mol/L NaHCO3 within pH=8.5, there was no significant change in absorbance, even smaller.
@ Xiao, you are following the correct method. If you dont get any change in absorbance then microbial P in your soil is very low. You may try again following the suggestions like removing resin-extractable P from the soil before lysing microbial cells with CHCl3, or measuring total P rather than inorganic-P in NaHCO3 extracts, and prolonging NaHCO3 extraction from 30 min to 16 h.
Microbial biomass phosphorus (MBP) is one of the most active forms of phosphorus (P) in soil. MBP plays an important role in the biogeochemical P cycle. Generally Phosphorous can be used to determine by Bray tests colorimetric, which means the amount of light passing through a liquid is measured to determine P levels. Mehlich 3 is also a commonly used test for phosphorus.
https://www.sciencedirect.com/science/article/pii/0038071782900013#:~:text=Biomass%20P%20is%20calculated%20from,amount%20extracted%20from%20unfumigated%20soil.
Biomass P is calculated from the difference between the amount of inorganic P (Pi) extracted by 0.5 (Spm) NaHCO3 (pH 8.5) from fresh soil fumigated with CHCl3 and the amount extracted from unfumigated soil.
https://www.sciencedirect.com/science/article/pii/0038071782900013
Biomass P can be calculated from: biomass P =EpKp×100R, where Ep = (inorganic P released by CHCl3 and extracted by distilled water) minus (inorganic P released from unfumigated soil),
Where, Kp = Biomass P released by CHCl3 and extracted as inorganic P by distilled water (Kp = 0.40 at 25°C),
R = percentage recovery (82.0–91.2%) of added P. Compared to the original CHCl3-fumigation extraction (FE)-method, the AEM-method is simpler, more accurate, and more flexible, allowing assay of biomass P in soils of high P retention capacity, e.g. andosols. The use of AEM allows quick and efficient adsorption of P in the soil suspensions, and is analytically very convenient.
https://www.sciencedirect.com/science/article/pii/003807179500057L
Good morning Xiao, you did not write that you proceeded with the molybdate reaction. Is that the case? Paul.
It may be possible that inorganic P once released after the chloroform cell lysis is adsorbed by soil particles. The method by Brookes et al to determine microbial biomass P includes a control with added inorganic P to take into consideration the relative adsorption
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