Nitrogen transformation in the rhizospheres of two subalpine coniferous species under experimental warming

Abstract

Tree species can exert a strong influence on rhizosphere nutrient cycling through root and rhizosphere processes and create feedback in the patterns of nutrient cycling in forest ecosystems. In this study, we conducted an experiment to compare the rhizosphere effects of two coniferous species on N transformation as well as their responses to experimental warming using infrared heaters in the Eastern Tibetan Plateau. We examined the potential net N mineralization and nitrification rates, N availability, and microbial biomass C (MBC) and N (MBN) in rhizosphere soils of Picea asperata and Abies faxoniana plots and compared them to bulk soils. The infrared heater increased both the mean air and the soil temperatures by 1.5°C and 2.1°C respectively. Potential net N mineralization and net nitrification rates were generally greater in rhizosphere soils for the two conifers than in bulk soil, especially in the warmed plots. This led to higher NH4+ and NO3− concentrations in the rhizosphere soils. MBC and MBN were markedly higher in the rhizosphere soils relative to bulk soil in the study plots. In the control subplots of P. asperata, MBC, MBN, potential net N mineralization and net nitrification rates in the rhizosphere were 9.6%, 21.7%, 33.3% and 20.1% greater than in the bulk soil, respectively. MBC, MBN, potential net N mineralization and net nitrification rates in the control subplots of A. faxoniana, however, were 2.0%, 7.7%, 22.0% and 11.8% higher, respectively, in the rhizosphere than in the bulk soil; all of the variables were significantly lower than those of P. asperata subplots. Warming significantly promoted N transformation and nutrient availability by enhancing the rhizosphere priming effects for the two conifers, but the magnitudes of the rhizosphere effects on soil N transformation stimulated by warming were generally greater in P. asperata than in A. faxoniana subplots. Differences in the altered morphological and functional characteristics of the roots between the two species under experimental warming could be largely responsible for this variation. Taken together, the results indicated that the two species exhibited similar patterns but with considerably different magnitudes of rhizosphere effects on N transformations in response to experimental warming, implying different capacities of the two conifers to acquire nutrients and thereby altered the competitive and adaptive relationships between the tree species under climate change.