Enzymatic and Microbial Interactions in Response to Pitch Pine Root Growth

Abstract

AbstractRoot growth stimulates many soil metabolic and enzymatic activities through, in part, loss of photosynthate as root exudates. Because of the abundance and structural simplicity of the carbonaceous compounds comprising root exudates, compared with native soil organic matter, it can be hypothesized that not only the magnitude but also the relationship between various soil microbial and enzymological processes vary with proximity to the growing root. To test this hypothesis, metabolism of amino acids and glucose, microbial growth, microbial biomass C, and potential acid phosphatase, protease, and invertase activities were examined in pitch pine (Pinus rigida Miller) rhizosphere and near rhizosphere soils. The trees were grown in microcosms (total volume ≈145 cm3) in the greenhouse in O or A horizon soils (Lakehurst sand, a mesic, coated Haplaquodic Quartzipsamment) from the New Jersey Pinelands. The microbial and enzymological activities in the organic‐matter‐rich O horizon soil were 2‐ to 25‐fold greater than those detected in the organic‐matter‐poor A horizon soils throughout the study. Phosphatase activity and glucose and amino acid metabolic rates did not correlate with tree root mass, whereas protease activity, microbial growth rate, and microbial biomass C varied with root mass. No effect of proximity to the root on these relationships was detected. In the A horizon soil after 22 wk of growth, amino acids and glucose metabolic rates declined with increasing root mass, whereas the microbial growth rate increased. Phosphatase activity was not detected in the A horizon soil. The interrelationship of these activities was evaluated in O horizon soil. Significant (P <0.05) correlations were detected between microbial biomass C and the various indices of microbial activity, but glucose metabolism and phosphatase and proteolytic activities varied independently of microbial biomass C. In conclusion, significant effects of tree root growth on enzymatic and metabolic activities were detected, but, with a few minor exceptions, relationships between these enzymological and metabolic activities were not altered by proximity to the growing tree root.