Effects of night-time warming on the rates and main chemical components of root exudates produced by Picea asperata seedlings in subalpine coniferous forests
While it is well recognized that root exudates play a crucial role in driving belowground biogeochemical processes,few studies have attempted to examine the effects of elevated temperature on the rates and chemical components of root exudation in forests. In this study,we conducted a night-time warming experiment by using an infrared heating device to explore the ecological consequences of warming on the rates and chemical components of root exudation by Picea asperata seedlings. Root exudates were collected from intact fine roots of plants growing in the warmed plots and the control plots using a modified culture-based cuvette system,developed especially for collecting field-based exudates. The concentrations of total organic carbon (TOC) and total nitrogen (TN) in the root exudates were quantitatively investigated,and expressed as the C and N exudation rates for P. asperata seedlings. Furthermore,the main chemical components of root exudates were quantified using the gas chromatography and mass spectrometry (GC-MS) method. The results showed the following: (1) Experimental warming considerably increased the root C exudation rates (mug C g~(-1) root biomass h~(-1)). In contrast,no significant effects were observed on the root N exudation rates (mug N g~(-1) root biomass h~(-1)),which led to a significant increase in the C ∶N ratio of the root exudates. (2) Experimental warming had significant effects on the relative contents of chemical compounds,and the response magnitude and direction to experimental warming were closely related to the components of the chemical compounds. Specifically,the relative contents of sugars,amino acids,and phenolics were significantly increased by warming,while the relative contents of esters and ethers markedly decreased. (3) Experimental warming had significant effects on the contents of different chemical components in root exudates. For example,the relative contents of two phenolics,2,6-Di-tert-butyl-4-methylphenol and 4-Tert-butylcalix [4] arene,increased by 88.9% and 375.7%,respectively,compared to the control plots,but no significant differences were observed for the other components. Collectively,our results suggested that experimental warming can lead to profound influences on the exudation rates and the relative contents of the specific exudate components,which provides a theoretical foundation that can improve understanding of the soil C-nutrient cycling process mediated by root exudation inputs in subalpine coniferous forests when P. asperata is subjected to environmental changes.