Effects of experimental warming on soil N transformations of two coniferous species, Eastern Tibetan Plateau, China

Abstract

Previous research on the effects of tree species on soil processes has focused primarily on the role of leaf litter inputs and relatively few studies have considered the importance of plant roots and their associated ecological processes, especially under climate change. We therefore conducted an experiment to compare the impacts of two coniferous species via roots on soil N transformations and their responses to experimental warming using infrared heaters in the Eastern Tibetan Plateau. The infrared heater on average enhanced both air temperature and soil temperature by 2.0 °C and 3.7 °C, respectively. Warming did not affect soil organic C (SOC), total N (TN), microbial biomass C (MBC) and N (MBN), or their ratios (MBC/MBN) in both coniferous species plots. Effects of experimental warming on soil N availability varied with tree species and sampling dates. There were higher NO3− and lower NH4+ concentrations in the Picea asperata than in the Abies faxoniana plots irrespective of warming treatment or sampling date, possibly caused by higher gross nitrification and denitrification rates in the P. asperata. Experimental warming significantly increased the net mineralization, net nitrification and denitrification rates on most sampling times in both species plots. Responses of gross nitrification to experimental warming significantly differed between the two species, and depended strongly on seasons. Gross nitrification and denitrification rates were markedly greater in the P. asperata than in the A. faxoniana plots, with P. asperata being more sensitive than A. faxoniana in response to experimental warming. Differences in the root morphology (i.e., root length, root type) and activity (i.e., root exudation, fine root vigor) between the two species could be largely responsible for the variation in the soil N cycling and its response to experimental warming. Taken together, our results indicate that tree species can differ in their effects on soil transformations and nutrient availability via roots and associated microbial processes. Further research is required regarding the exact mechanisms of tree species effects via roots on soil processes and function under climate change.