The impact of shrub encroachment and litter mixing on microbial exoenzyme activity

Background/Question/Methods

Arctic warming has increased the density and cover of deciduous shrubs, which has global, regional and local implications for feedbacks to further climate warming. Enhanced deciduous shrub growth increases litter quantity and is believed to reduce litter quality. Increasing amounts of deciduous shrub litter could result in mixing effects, changing microbial decomposition of litter from other tundra species, altering the balance between C release and C sequestration in Arctic tundra.  Complimentary investigations (cf. Jennie McLaren, ESA abstract #42917) have revealed that birch (Betula nana) slows the mass loss of leaf litter of three co-dominant species over the winter. In this study, we hypothesized that Betula would slow the production of microbial exoenzymes, the primary mechanism for organic matter decomposition. We investigated the impact of B. nana on the potential hydrolytic exoenzyme activity rate of two evergreen shrubs (Ledum palustre and Vaccinium vitis-idaea) and a graminoid (Eriophorum vaginatum), after one winter and after one year.  Species leaf litter and roots were incubated in litter bags singly and in mixture with Betula, allowing us to determine the seasonal species-specific mixing effects on exoenzyme activity rates. A phosphoester-, a chitin-, a starch- and three cellulose-hydrolyising enzymes were assayed.

Results/Conclusions

Our results indicate strong seasonal and species effects on leaf enzyme activity, strong species effects on root enzyme activity, and contrasting effects of Betula mixing on leaves vs. roots. Betula leaves had the highest exoenzyme activity after one year, for all assayed enzymes.  Betula enhanced the exoenzyme activity of all species when in mixture.  These effects were strongly significant in the fall, whereas overall leaf exoenzyme activity was slow at thaw with no dominant species effect.  Root exoenzyme rates were highest in Eriophorum, and equally active at thaw and in the fall.  After one year, there was a negative mixing effect of Betula roots for all enzymes assayed on Eriophorum roots, and for cellulose-hydrolyising enzymes on Vaccinium roots. The positive fall mixing effect of Betula on leaf decomposition activity contrasts with the mass loss results (negative winter mixing effect). Whereas mass loss measures cumulative decomposition, enzyme activity rates combine the cumulative effect of decomposition with sample-time activity, indicating in this study that species effects may be most pronounced in the early winter. Our results demonstrate that deciduous shrub encroachment is likely to alter both microbial activity and litter decomposition rates and may have profound effects on the tundra C balance.