Linking the carbon cycle to climate change: Effects of warming and altered precipitation on organic matter decomposition.

摘要

Untangling the effects of multiple factors of climate change on terrestrial carbon stocks is complex due to the differential responses of heterotrophic (Rh) and autotrophic (rhizosphere; Rr) respiration. Our lack of understanding of the relative sensitivities of these responses limits our ability to predict soil carbon loss in future climate scenarios. I measured soil and heterotrophic respiration monthly, and used these values to estimate Rr. The heterotrophic respiration in this mesic ecosystem strongly responded to precipitation. During the summer, when Rh was highest, there was threshold, hysteretic responses to soil moisture: R h decreased sharply when volumetric soil moisture dropped below ∼15% or exceeded ∼26%, but Rh increased more gradually when soil moisture rose from the lower threshold. The effect of climate treatments on the temperature sensitivity (Q10) of Rh depended on the season where high warming decreased Q10 in spring and fall and drought decreased Q10 in fall alone. To my knowledge this was the first study that identified the seasonal variation in the temperature sensitivity of microbial respiration in the field. Currently, most biogeochemical models represent the relationship between soil organic matter decomposition and warming using a temperature function with a fixed Q10 and my research supports the argument that that models with seasonally varying parameters may be more accurate than those with constant parameters. The Q10 values of Rh from this study and from future work in other biomes could be used to develop a temporally variable Q10 function that responds to abiotic conditions.;In this mesic ecosystem, both Rs and Rr responded strongly to precipitation. Drought reduced Rs and Rr, both annually and during the growing season. Annual cumulative Rs responded non-linearly to precipitation treatments; both drought and supplemental precipitation suppressed Rs compared to the ambient treatment. Cumulative winter Rr increased by about 200% in the high warming (∼3.5oC) treatment. This carbon loss, presumably to maintenance respiration, should reduce net primary production (NPP) in the subsequent season, thereby affecting the longer-term carbon balance. The effect of climate treatments on the temperature sensitivity of Rs depended on the season. Drought decreased apparent Q10 in fall compared to the other precipitation treatments. These results highlight the non-linear responses of soil respiration to soil moisture, and to my knowledge quantify for the first time the loss of carbon through winter rhizosphere respiration due to warming.;Since plants form the substrate for decomposition process, the quality of plant tissue is an important determinant of the stability of carbon under future climate. The effect of climate change on the preferential decomposition of labile (easily degradable) and recalcitrant compounds in organic matter is still a matter of debate. I studied how warming and altered precipitation affected the decomposition of recalcitrant matrix in litter and the associated changes in microbial extracellular enzyme activity using three litter types (the shrub-like Polygonum cuspidatum, at two stages---newly senesced litter (referred hereafter as NEW litter) and standing litter that has been decomposing for a year (OLD litter) and the grass Poa trivialis) the BACE. The OLD litter was enriched in recalcitrant compounds compared to NEW as indicated by the initial 13C-NMR spectral analysis and C:N ratios. After three years of decomposition in the BACE plots, using DRIFT (Diffuse Reflectance Infrared Fourier Transform) spectroscopy, I found that OLD litter with a high proportion of recalcitrant compounds responded faster to precipitation compared to NEW litter with relatively higher proportion of labile compounds. Supplemental precipitation along with high warming conditions accelerated the degradation of lignins and phenolic bands. This study also revealed a non-linear response of microbial enzymes and change in fungal biomass (ergosterol content) due to warming and altered precipitation. I conclude that warming along with changes in precipitation would alter the decomposition of recalcitrant compounds in plant litter, thus changing the amount and quality of carbon available for sequestration. (Abstract shortened by UMI.).