Decomposition dynamics of plant litter subjected to warming and altered precipitation in an old-field ecosystem

Background/Question/Methods

Decomposition of plant litter not only recycles nutrients locked in the senesced plant biomass back to the ecosystem, thus sustaining ecosystem productivity, but also releases carbon dioxide to the atmosphere. Predicted climate warming may increase litter decomposition rates and CO₂ efflux, resulting in a positive feedback to climate change. We examined how warming and altered precipitation affected the decomposition of two types of Polygonum cuspidatum (Japanese knotweed) litter (stem litter that was either newly senesced or one year old), at the Boston-Area Climate Experiment (BACE), in Massachusetts, USA. We placed litter bags in an old-field ecosystem exposed to four levels of warming (up to 4^oC) and three levels of precipitation (ambient, drought (-50%) and wet (+50%) treatments. The two litter types varied in the amount and quality of carbon as indicated by initial C/N ratio and alkyl/O-alkyl ratio. We measured decomposition at 4 times over a period of 3 years. 

Results/Conclusions

After 13 months, one-year-old P. cuspidatum litter in the drought treatment had more mass remaining than that in the ambient and wet precipitation treatments.  This trend remained the same after 38 months. Mass loss of newly senesced P. cuspidatum litter did not vary with warming and precipitation until 27 months of field incubation.  These responses were further analyzed in relation to the quality of litter based on DRIFT (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) and 13C NMR (Nuclear Magnetic Resonance Spectroscopy). Precipitation and warming treatments significantly altered the chemistry of carbon compounds in decomposed tissue of one-year old P. cuspidatum after 3 years of decomposition, as indicated by principal component analysis of 13 different DRIFT peaks. Our results suggest that the differential response of the two litter types to climate treatments could be due to the difference in initial litter quality. These results emphasize the significance of precipitation in determining litter decomposition dynamics under future climate scenarios.