Carbohydrate concentrations and enzyme activities as influenced by exchangeable cations, mineralogy and clay content

Enzymes can exist in bound or free form within the soil, but the impact of interactions between extracellular enzymes with the amount and type of clay minerals and exchangeable cations on the activities of enzymes and decomposition or retention of carbohydrates in soil is still poorly understood. Appropriate amounts of homoionic Na-, Ca- and Al-clay minerals from Georgia kaolinite, Illinois illite and Wyoming montmorillonite were mixed with pure sand to prepare artificial soils with different clay contents, exchange cations and clay types to examine the effects of exchangeable cations, mineralogy and clay content on the concentrations of hot water and dilute acid extractable carbohydrates and activities of acid and alkaline phosphatases and CM-cellulase. There was a significant effect of clay content on the concentrations of carbohydrates and activities of enzymes. The concentrations of carbohydrates increased when the clay contents of soils increased from 0 to 5 and 10%, respectively, showing that the clay contents influence the capacity of soils for stabilization of carbohydrates. But opposite trends were found in enzyme activities. The enzyme activities decreased significantly as the clay contents of the artificial soils increased. The concentrations of carbohydrates and activities of enzymes were significantly affected by exchangeable cations. In soils with 5 and 10% clay, the concentrations of carbohydrates were maximum in Al-soils and minimum in Ca-soils, in contrast, the activities of enzymes were maximum in Ca-soils and minimum in Al-soils. There was a significant effect of clay mineralogy on the concentrations of carbohydrates and activities of enzymes. The concentrations of carbohydrates were highest in soils with Wyoming montmorillonite clay mineral and lowest in soils with Georgia kaolinite clay mineral. But the activities of enzymes were lower in soils with Wyoming montmorillonite clay mineral than soils with Georgia kaolinite and Illinois illite clay minerals, indicating the influence of specific surface area (SSA) and cation exchange capacity (CEC) of clay minerals on carbohydrate retention and enzyme activities. The results of this study indicate that carbohydrates are stabilized in soils through the interaction with clay minerals and a small amount of clay (5%) significantly increases carbohydrate retention and reduces enzyme activities in soils. Exchangeable cations exert their influence on enzyme activities and hence carbohydrate dynamics by controlling the activities of enzymes through modifying the physicochemical characteristics of soils.