Pool size of microbial biomass potassium in various farmland soils

Potassium (K) is the major intracellular cation in all kinds of organisms including soil microorganisms. This study for the first time presented the potassium pool size in microbial cells in various farmland soils and the effect of land use and organic matter application on the contents. Microbial biomass potassium was determined for several paddy field soils under various managements in different areas, together with some upland, orchard, and pasture field soils, to estimate the potassium pool size in microbial cells in the farmland soils. The contents of microbial biomass potassium ranged from 9 to 65 mg K kg(-1) soil in the paddy field soils and showed no difference among soil groups of the paddy fields. The biomass potassium content was higher in the pasture field (183-187 mg K kg(-1) soil) than that in the paddy, upland (8-51 mg K kg(-1) soil) and orchard (25-50 mg K kg(-1) soil) field soils. The average ratio of microbial biomass potassium to exchangeable potassium was 0.25 for all the soils and the average values for paddy field (0.31) and pasture field (0.53) soils were higher than those for upland field (0.06) and orchard field (0.08) soils. Positive correlations were found between the contents of microbial biomass potassium and microbial biomass carbon and nitrogen. Application of organic matter significantly increased microbial biomass potassium in the farmland soils. These findings indicate that microbial biomass potassium plays an important role in potassium supply to crops as a reservoir of potassium and also suggest that the contribution of microbial biomass potassium to the potassium source for crops could be relatively higher in paddy field and pasture field soils than upland field and orchard field soils.