Microbial and hydrolase activity after release of low molecular weight organic compounds by a model root surface in a clayey and a sandy soil

The stimulatory effects of low molecular weight organic compounds (LMWOC) such as glucose, citrate, oxalate, glutamate, on microbial and hydrolase activity, in the rhizosphere and bulk soil layers of a clayey and a sandy soil were studied. Incubation units reproducing the rhizosphere environment were used, allowing additions and precise sampling of soil. All LMWOC stimulated respiration in both soils although the degree of LMWOC mineralization was different in the two soils. The double-stranded DNA (dsDNA) content increased significantly in the rhizosphere layer of the clayey soil treated with glucose or glutamate, but not with citrate or oxalate. In the sandy soil, a significant increase in the dsDNA content was only observed in the rhizosphere layer upon release of oxalate, whereas differences of the other treatments over the control were not significant. Hydrolase activity in the rhizosphere soil layer was generally stimulated. The alkaline phosphatase activity was significantly stimulated by glucose, glutamate and citrate in both the clayey and sandy soils, whereas the acid phosphatase activity was significantly stimulated by citrate in the clayey soil and by glutamate, citrate and oxalate in the sandy soil. Phosphodiesterase activity was significantly stimulated by glutamate and citrate in the clayey soil and by citrate only in the sandy soil. Urease activity was significantly stimulated by glucose and citrate in the clayey soil, whereas, the stimulation of urease activity in the sandy soil upon release of glucose, citrate and oxalate was not significant. No response to LMWOC release was observed for proatease activity in the rizosphere or bulk soil layer of both soils. It was concluded that in the rhizosphere, different LMWOC commonly released in the root exudates stimulate soil microbial activity and hydrolase activity differently depending on the compound released and soil type. (c) 2007 Elsevier B.V. All rights reserved.