Estimation of turnover time of microbial biomass potassium in paddy field soil

Soil microbial biomass (microbial biomass) contains substantial amounts of potassium (K) as a reservoir of K in soils. However, information about K dynamics and K flux through the microbial biomass are lacking. In the present study, the turnover time of microbial biomass K was estimated for the first time using paddy field soils. The paddy field soils, either amended with substrates of carbon, nitrogen, and phosphorus, or not, were aerobically incubated for 60 days, during which the amounts of microbial biomass K were periodically determined by the chloroform fumigation-extraction procedure. Microbial biomass K increased to twice (51.4–72.3 mg K kg⁻¹ soil) of that in the unamended control 5 days after addition of the substrates and gradually decreased to the values in the control (25.2–44.0 mg K kg⁻¹ soil) until 60 days of the incubation. The turnover time of microbial biomass K was estimated from the declines in the substrate amended soils. The turnover time of microbial biomass K ranged from 80.4 to 98.5 days, which roughly corresponded to the cultivation period of rice (about 100 days). These results indicated that microbial biomass K plays important roles in the supply of K source as well as the reservoir of K in paddy field soil.