Protease and deaminase activities in wheat rhizosphere and their relation to bacterial and protozoan populations

Protease and deaminase activities and population dynamics of bacteria and protozoa were measured in the rhizosphere of wheat to study their interactions with the mineralization of nitrogen. The experimental design allowed the separation of roots and soil material by means of a gauze. The most pronounced 'rhizosphere effect' was detected for all the measured variables in the soil closest to the gauze. The number of bacteria was significantly higher in the presence than in the absence of plants up to 4 mm away from the soil-root interface and the closer to this interface the higher the number. Protozoan and bacterial population dynamics were positively correlated; generally, populations of flagellates and amoebae were comparable and their sum accounted for the population of total protozoa. For both enzyme activities the rhizosphere effect extended up to 2 mm away from the soil-root interface. The histidinase activity was of bacterial origin, while it is likely that bacteria, protozoa and root hair all contributed to the overall caseinase activity. Decomposition of root exudates and native organic matter in the rhizosphere, reflected by a growing microbial population, is associated with nitrogen mineralization through increases in casein-hydrolysing and L-histidine-deaminating activities. The adopted soil-plant microcosm is suitable for the study of the rhizosphere effect over time of incubation and distance gradient from the soil-root interface.