Changes in extracellular enzyme activities and kinetics with depth across contrasting parent materials in the Luquillo Critical Zone

Background/Question/Methods

Extracellular enzymes in soils mediate the decomposition of organic matter and catalyze key transformations in carbon, nitrogen and phosphorus cycling. However, most studies of extracellular enzyme activity have focused exclusively on the upper 15 cm of the soil, despite the fact that microbial communities influence biogeochemical processes throughout the soil profile. The goal of this study was to determine how extracellular enzyme activity changes with depth in soils from two contrasting parent materials and forest types at the Luquillo Critical Zone Observatory (LCZO) in northeast Puerto Rico. We collected soils at 10 cm increments from the top of the A-horizon to a depth of 140 cm and measured total potential activities of six enzymes: α-glucosidase, β-glucosidase, β-xylosidase, cellobiohydrolse, N-acetyl glucosaminidase and acid phosphatase. The Michaelis-Menten kinetic parameters V_max and K_m were also measured for acid phosphatase. 

Results/Conclusions

For all enzymes, total potential activity declined exponentially with depth. When normalized on a per unit carbon basis, however, only β-glucosidase and N-acetyl glucosaminidase declined significantly with depth (P<0.05). V_max and K_m both declined significantly with depth for acid phosphatase (P<0.001). We propose that changes in potential enzyme activity are controlled by changes in total biomass, while specific enzyme activity is governed by carbon quality and substrate availability.  The observed decline with depth in both kinetic parameters for acid phosphatase suggests a tradeoff between maximal rate of substrate turnover (V_max) and substrate binding affinity (K_m). In tropical subsoils, microbial communities may be producing distinct isoforms of the phosphatase enzyme that are adapted to operating efficiently in a low organic-P environment.