Rhizosphere priming effects differ between Norway spruce (Picea abies) and Scots pine seedlings cultivated under two levels of light intensity

摘要

The rhizosphere priming effect (RPE), which occurs in the presence of growing plant roots, may either stimulate or reduce the decomposition of soil organic matter (SOM). The carbon and nitrogen cycling in forest soils could be more rapid or slower due to the RPE, depending on the tree species and their growing conditions. Norway spruce (Picea abies) and Scots pine (Pines sylvestris) are two commercially important tree species in the boreal region, and their photosynthetic rate depends upon light intensity. Since photosynthesis is an important determinant of below-ground plant C allocation and the root exudation rate, we hypothesized that the RPE would also differ between Norway spruce and Scots pine. The hypothesis was tested in a greenhouse experiment using a combined (CO2)-C-13 pulse-chase and N-15 pool-dilution approach designed to quantify the mot exudation rate, SOM decomposition, gross N mineralization, and the RPE. We found that spruce induced a positive RPE while pine induced a negative RPE. Our results also showed that the light intensity could influence the RPE in a species-dependent way. Spruce induced higher priming of SOM decomposition and gross nitrogen mineralization when exposed to low light intensity, while there was no connection between the RPE and light intensity on soil carbon and nitrogen cycling processes under Pine. Furthermore, the species-specific variation in carbon and nitrogen cycling was related to soil nitrogen availability rather than mot exudation rates, apparently because severe nitrogen limitation led to plant-microbial competition for nitrogen and reduced SOM decomposition and gross nitrogen mineralization rates. We conclude that the environmental factors that influence nitrogen availability need to be integrated into our understanding of the RPE in forest soils.