Persistence of enzyme function in soils of different terrestrial ecosystems

Background/Question/Methods

Extracelluar enzymes are now seen as central to the decomposition process and to new generations of models describing it. In such models, a critical parameter is the functional lifespan of these enzymes—unfortunately this is unknown. It is unclear how function differs among enzymes and for particular enzymes across ecosystems. In this study, we measured the persistence of enzyme function in soils of different terrestrial ecosystems. The potential functional activities of six hydrolytic enzymes (b-1,4-glucosidase; a-1,4-glucosidase; b-1,4-xylosidase; b-1,4-cellobiosidase; N-acetyl-glucosaminidase and acid phosphatase) and two oxidative enzymes (phenol oxidase and peroxidase) were measured periodically over 12 weeks. We measured activities in the soils of five types of terrestrial ecosystems (chaparral, grassland, temperate forest, tropical, and tussock tundra). To keep soils sterile, samples were kept under constant chloroform fumigation in a pressure cooker. Since no significant increase in either dissolved organic carbon and microbial biomass occurred, we assume that no additional enzymes were produced. Therefore, we can measure the persistence of enzyme function from only the existing enzyme pool. 

Results/Conclusions

Generally, initial potential enzyme activities were highest in tussock tundra soil samples followed by temperate forest soils, grassland, tropical, and chaparral soils. The highest initial potential enzyme activity was acid phosphatase followed by beta-glucosidase and N-acetyl glucosaminidase. Of the hydrolytic enzymes, alpha-glucosidase activity decreased the fastest with decay rates approximately 10X faster. N-acetyl glucosaminidase activity was the most stable followed by phosphatase, beta-glucosidase, cellobiosidase, xylosidase, and alpha-glucosidase activities. Although enzymes in organic soils (tussock tundra) had high initial potential enzyme activities, they appeared to be more sensitive/least stable than the enzymes in mineral soils (chaparral, temperate forest, tropical soils, and grassland). First-order model fits better for most sites and enzymes although the differences in coefficient of determination are small in some cases. Coefficient of determinations ranged between 0.76 to 0.97 for hydrolytic enzymes. Decay rate constants for hydrolytic enzymes ranged from 0.002 – 0.114 day^-1. Decay rate constants for oxidative enzymes ranged from 0.005 – 0.078 day^-1. The production of enzymes is costly to microorganism. The results of this study suggest that while enzyme activities may persist for a long period of time, they are not uniform across ecosystems or type of enzyme.