Interactive Effects of Nitrogen and Manganese Availability and Temperature on Losses of Carbon and Nitrogen from Leaves

Manganese (Mn) and nitrogen (N) can interact to influence microbial litter decomposition and soil organic carbon storage. Past incubation studies have shown that a combination of limited Mn availability and elevated N decreased enzyme activities in litter, whereas elevated availability of both Mn and N increased enzyme activities in litter. However, little is known about how warming controls the way Mn-N interactions influence losses of C (as CO2) and N (as N2O) during litter decomposition. To investigate these effects, we are conducting a 1-year laboratory decomposition study that used a factorial combination of different levels of aqueous Mn (II) [no addition, low Mn (500 µg/g), high Mn (5000 µg/g)] and NH4NO3 [no addition, low N (5000 µg/g), high N (20000 µg/g)] added to maple leaves sampled from an upland temperate forest in East Tennessee (USA). The study was conducted in 1L mason jars, and each treatment was incubated under three different temperatures (4 oC, 14 oC, and 25 oC). We periodically analyzed CO2 and N2O in the headspace using gas chromatography and calculated their flux rate over time during leaf decomposition. At the end of the 1-year study we will measure total (cumulative) CO2 and N2O losses, as well as elemental C and N , Mn, and the chemical composition of the remaining leaves to examine how leaf chemistry changed with experimental warming and nutrient additions. Preliminary results representing day 3 of the incubation study indicate that Mn additions and temperature interacted to affected CO2 fluxes. Increased temperature (from 4 oC to 25 oC) significantly accelerated CO2 flux rates in the treatment that received high level of Mn addition relative to the treatment that received low level of Mn addition, and CO2 fluxes generally increased with temperature for all treatments. We found no significant interactions between temperature and N or between temperature, N, and Mn for CO2 release. We also observed no production of N2O by day 3. Our preliminary results indicate warming and greater Mn availability can accelerate losses of C as CO2 at the first stage of litter decomposition. We will show how losses of CO2 and N2O were affected by warming and both Mn and N in the first 6 months of the litter decomposition study and discuss potential implications of these results to C and N storage in terrestrial ecosystems.