Soil enzyme activity in response to long-term organic matter manipulation

Enzymes are considered to be a key soil component catalysing important transformations related to decomposition and nutrient turnover, and their activity in soil can be used as a measure of soil health. As part of the Síkfőkút DIRT (Detritus Input and Removal Treatments) Project in a temperate deciduous forest in northern Hungary, we examined the extent to which enzyme activity in soil is influenced by both the quality and quantity of plant detrital inputs. DIRT treatments include doubling of leaf litter and woody debris inputs as well as removal of litter and trenching to prevent root inputs. Our objective was to examine seasonal dynamics of soil phosphatase and β-glucosidase activities and to determine the effects of detrital manipulations on these dynamics. We found that the litter additions did not affect enzyme activities, but removal of roots caused significant decreases in enzyme activities. We conclude that plant-induced changes to soil enzyme activities are driven primarily by readily available, labile carbon provided by root turnover and root exudation rather than by aboveground detrital inputs. However, these results could also have been affected by changes in soil chemistry with detrital input removal: after only 6 years of litter removal, soil cation content decreased and soils became more acidic, both of which could inhibit enzyme activity. The soil phosphatase and β-glucosidase enzymes measured showed similar seasonal dynamics. Both enzymes showed the highest activities in spring coincident with high soil moisture and, presumably, high root activity. The minimal response of soil enzyme activity to dramatic litter additions suggests a level of resilience in ecosystem function in this forest, and suggests that aboveground litter is not a significant source of labile carbon to microbes in the mineral soil.