Distribution of microbial- and root-derived phosphatase activities in the rhizosphere depending on P availability and C allocation – Coupling soil zymography with 14C imaging

Abstract

Despite its importance for terrestrial nutrient and carbon cycling, the spatial organization of microbial activity in soil and in the rhizosphere is poorly understood. We related carbon allocation by roots to distribution of acid and alkaline phosphatase activity in the rhizosphere of Lupinus albus L. To do so, we further developed soil zymography – an in situ method for the analysis of the two-dimensional distribution of enzyme activity in soil – integrating fluorescent substrates. Soil zymography was combined with 14C imaging, a technique that gives insights into the distribution of photosynthates after labeling plants with 14C. Both acid and alkaline phosphatase activity were up to 5.4-times larger in the rhizosphere than in the bulk soil. While acid phosphatase activity (produced by roots and microorganisms) was closely associated with roots, alkaline phosphatase activity (produced only by microorganisms) was more widely distributed, leading to a 2.5-times larger area of activity of alkaline than of acid phosphatase. These results indicate a spatial differentiation of different ecophysiological groups of organic P mineralizing organisms. The spatial differentiation could be either between microorganisms and L. albus or between microorganisms that produce exclusively alkaline phosphatases on the one hand, and L. albus and root associated microorganisms that produce acid phosphatases on the other hand. The spatial separation of different organic P mineralizing organisms might alleviate a potential competition between them. While alkaline phosphatase activity strongly decreased with P fertilization, acid phosphatase activity was not affected by fertilization, suggesting that alkaline phosphatase-producing microorganisms react more strongly to it than other organic P mineralizing organisms. Alkaline phosphatase activity was high in parts of the rhizosphere where relatively little recent photosynthates were allocated, indicating that rhizodeposition and the activity of alkaline phosphatase-producing microorganisms are not directly related. Our study indicates, first, a spatial differentiation of organic P mineralization by various ecophysiological groups that react differently to inorganic P fertilization and second, that rhizodeposition and alkaline phosphatase-producing microorganisms were not directly related. Finally, we conclude that soil zymography with fluorescent substrates is a very promising approach for studying the distribution of a broad range of extracellular enzymes at microscales.