Changes in leaf litter chemistry and extracellular enzyme activity during decomposition of natural leaf mixtures at two northwest Ohio forests

Background/Question/Methods The general, conceptual model of leaf litter decomposition predicts increasing litter recalcitrance with mass loss, with nutrient limitations often controlling decay rates in early stages and lignin concentrations dominating the late stages. The activities of extracellular enzymes (EEA) responsible for resource acquisition are predicted to track these changes in litter chemistry, with hydrolytic carbon and nutrient acquiring enzyme activities peaking early in decay and oxidative enzymes responsible for degrading recalcitrant compounds peaking late. However, this conceptual model is based on single species litter experiments whereas natural ecosystems often have mixed species litter. Studies of mixed species litter decay to date have concentrated on mass loss dynamics with little attention to litter chemistry or EEA. Mass loss patterns in mixed species litter have not consistently followed the general conceptual model. The purpose of this study was to examine patterns of changing litter chemistry and EEA during decomposition of natural leaf mixtures in two oak dominated forests in northwest Ohio.

Results/Conclusions Over a two-year period, litter decaying in the urban, Stranahan Arboretum showed rapid loss of soluble compounds but little evidence of a faster relative loss of holocellulose than lignin. EEA showed a seasonal pattern more strongly related to litter moisture than litter chemistry. Spring flooding of the study site and a dense population of exotic earthworms (Amynthas agrestis) may have affected decay at the Arboretum, i.e., urease activities were very high in litter at this location. In contrast, litter decaying in the rural, Oak Openings Metropark showed expected patterns of change in litter chemistry, with holocellulose decaying more rapidly than lignin. However, EEA was strongly related to litter nitrate concentration, rather than carbon chemistry. At both sites the ratio of carbon to nutrient acquiring enzyme activities behaved roughly as expected with a general increase in relative carbon-acquiring enzyme activities with progressive decay, i.e., increasing litter recalcitrance. Enzyme activity ratios suggested that the Arboretum site was more likely limited by carbon, and perhaps phosphorus, whereas the Oak Openings site was more limited by nitrogen availability. These results add to a growing body of literature suggesting that the current conceptual model for leaf litter decomposition may not be adequate to address patterns of decay in more natural mixes of leaf litter in many ecosystems.