Microbial Biomass Potassium in Calcareous Soils

Background and objectives: Soil microbial biomass is a functional part of soil organic matter. Precise determination of nutrients in the soil microbial biomass has attracted many attentions due to its major role in soil ecosystem function and services. All the soil biomass measurement techniques represent a number of advantages and disadvantages and need to be implicated under controlled conditions. Measurement techniques are generally supposed to be accurate and precise and can be used widely for a large number of various soils. Chloroform-fumigation extraction (CFE) technique has been widely used in order to determine carbon, nitrogen, sulfur and phosphorus. There is a lack of information regarding the size of microbial biomass K (Kmic) in calcareous soils. Therefore, a study was conducted to investigate the relative size of Kmic in calcareous soils compared to other forms of soil K.
Materials and methods: For this purpose, 10 calcareous soil samples were collected from 0-15cm depth of calcareous soils of Isfahan and Chaharmahal-va-Bakhtiari provinces. Some Physical and chemical properties of soil such as pH and Electrical Conductivity (EC), soil organic C content (SOC), calcium carbonate equivalent and texture of the soils were determined by standard methods. Soil exchangeable and non-exchange K were measured by 1 mol L-1 CH3COONH4 and boiling HNO3, respectively. Microbial biomass K was determined by chloroform fumigation extraction method, using the suggested microbial biomass K conversion factor.
Results: Our findings indicated that soil exchangeable K was ranged from 205.3 to 458.1 mg kg-1 and Soil nonexchangeable varied from 338.8 to 1028 mg kg-1. The values of Kmic was ranged from 30.3 to 209.4 mg kg-1 with a mean of 120.4 mg kg-1 which forms 12.9% of total exchangeable and nonexchangeable K on average. Calculating the relative amounts of different soil K pools revealed that, exchangeable K was 2.6 times greater than that of Kmic while, nonexchangeable was 5.1 times greater than that of Kmic. No significant correlation was observed between Kmic and other K pools in soil.
Conclusion: Overall, from a quantitative point of view, Soil microbial biomass K is a considerable source of K that is not possible to be predicted from other K sources in soil and is hypothesized to be a part of K labile pools in calcareous soils. In conclusion, Kmic should be considered as a major K pool in soil which has already been neglected and there is a need to be precisely determined in order to accurate evaluation of plant available K pool in soils.