Tracing the activity of extracellular enzymes in dead roots during summer drought
In the semi-arid, annual-dominated grasslands of California, the rapid release of CO2 from soil following rewetting (Birch effect) is correlated with rooting distributions. This observation led us to hypothesize that the extracellular enzymes associated with soil decomposer communities degrade dead roots during the dry season, leading to accumulation of bio-available carbon. To test this hypothesis, we made measurements of extracellular enzyme activities in root litter throughout the dry season in a grassland near Santa Barbara, CA, coupled with an extended incubation of litter under continuous chloroform, as a way of assessing enzyme activity in the absence of enzyme production or microbial uptake. Throughout the dry season, roots exhibited significantly larger potential activities of carbon degrading enzymes than stems. However, these potential activities did not translate to in situ enzyme activities estimated by accumulation of water-extractable enzyme products under continuous chloroform. In fact, roots incubated under continuous chloroform in the presence of water showed a net decrease in water extractable C, while potential enzyme activity remained constant.
These results suggest that dead roots do not degrade significantly via enzymatic reactions during the dry season, and indicate that enzymes and/or their products may be chemically stabilized in root litter. Furthermore, these results suggest that potential enzyme activities may not translate to observable accumulation of enzyme products in fresh plant litter, even when water is present.