Plant and Soil
Extracellular enzymes mediate the decomposition of organic matter and the release of plant-available nutrients. Current theory predicts that enzyme production by soil microbes is regulated by the stoichiometric demands of microbial biomass and the complexity of environmental resources, but most experiments ignore the potential effect of alleviated carbon limitation in the rhizosphere. Our objective was to investigate linkages between enzyme activities, soil nutrient availability and plant roots in a tropical Oxisol. We conducted a greenhouse experiment using soils from the Luquillo Experimental Forest and seedlings of Tabebuia heterophylla. Planted and unplanted pots were fertilized with different combinations of phosphorus and either mineral nitrogen (ammonia chloride) or a nitrogen-rich organic compound (casein). We measured changes in plant and soil nutrients and five extracellular enzyme activities. Phosphatase activity declined by 28% in the P and 40% in the complex nitrogen treatment, while N-acetyl glucosaminidase increased 162% in the complex nitrogen treatment. Beta-glucosidase, beta-xylosidase, cellobiohydrolase and N-acetyl glucosaminidase all increased significantly over time in the simple nitrogen treatment (P < 0.05). Enzymatic responses support microbial resource allocation theory, that is, the concept that soil microbes regulate enzyme production based on scarcity of resources. However, we did not observe any additional effect of roots on extracellular enzyme activities. Enzymatic C:N, C:P and N:P ratios further support the notion that shifts in microbial stoichiometric demand drive responses to nutrients.
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