Effects of elevated nitrogen on the interaction between microbial activity and plant litter chemistry during decomposition of Acer saccharum litter

Background/Question/Methods

Despite over a century of study, fundamental questions remain about plant litter decomposition, a key control on carbon (C) sequestration. The interaction between plant litter chemistry and microbial activity during decomposition is strongly influenced by N availability. Thus, anthropogenic nitrogen (N) deposition has the potential to alter litter decomposition rates. With soil organic matter (SOM) containing more than twice as much C as the atmosphere or terrestrial vegetation, the relationship between changes in decomposition rates and carbon dioxide (CO₂) efflux in response to N deposition is a topic of growing concern. Currently, there has not yet been a comprehensive quantitative framework established for how N enrichment affects the magnitude and direction of microbial activity, and more research is needed into how variations in plant litter chemistry influences microbial responses to elevated N.

To help provide mechanistic insight into this variation, a long term laboratory incubation with Acer saccharum litter and sandy, low C soil from the Oak Openings Region of Toledo, Ohio was conducted with and without added N. N deposition was simulated at a rate equivalent to 9 kg N/ha/yr, the annual ambient deposition rate for this region. Respiration, microbial biomass and extracellular enzyme activities, and extractable inorganic nutrients were monitored throughout the incubation.

Results/Conclusions

Over the 19 mo. incubation, N addition increased the activities of microbial extracellular enzymes that degrade cellulose and chitin, while the effects of N amendment on lignin degrading enzyme activities were variable. Microbial biomass C and N peaked early for all groups but was ultimately reduced by elevated N. Microbial respiration peaked within the first 3 days of the incubation, and the magnitude of this peak was increased by N amendment. These results suggest N-addition to fresh litter in concentrations similar to that of potential anthropogenic deposition has significant effects on microbial biomass, respiration, and extracellular enzyme activities with respect to microbial activity and SOM decomposition. Understanding these relationships will help predict how disturbances to the soil C pool, such as N deposition, will alter the conditions under which soils will behave as a source or sink for atmospheric C.