Condensed tannins in senesced foliage and soil organic layer in response to fertilization in boreal forest ecosystem

M-ECOL

anglais. Anthropogenic nitrogen deposition rate in boreal forests increased over the last century and significantly impacted many parts of these ecosystems. Boreal forests contain a large amount of carbon (C), and nitrogen (N) is a limiting factor for plant growth. Consequently, anthropogenic N deposition is likely to boost productivity and C sequestration in the boreal ecosystem. Some boreal regions are experiencing a significant amount of N deposition. On the other hand, forest fertilization is used in areas with limiting N supply to enhance timber volume and C sequestration as a means of climate change mitigation. As a result, other parts of the boreal ecosystem may be considerably affected due to nutrient enrichment. Condensed tannin (CT) is a carbon-based secondary metabolite known to provide herbivore defence to the plants, available in boreal plant species with a high concentration. Change in CT concentrations with N deposition in green leaves has been reported in earlier studies. This experiment was carried out to comprehend better how the change of resource condition affects tannins concentration in senesced foliage and soil organic layer as well as nitrogen pool and carbon storage in the boreal forests' soil. I conducted this research in an old Picea abies forest in South-Eastern Norway, which has been fertilized annually since 2003 with 150 kg N ha-1. The forest has 20 established experimental plots, ten controls, and the other ten are fertilization application plots. I measured CT concentration, C and N concentration in senesced foliage of Picea abies and Vaccinium myrtillus as well as in soil organic layer and pH value for soil. Fertilization increased foliar N concentration, decreased C: N ratio, while foliar CT did not change in senesced foliage. Increase of N concentration and decrease of C: N ratio were in accordance with the resource-based ecological theories on plant defence, i.e., a higher resource availability condition causes improved growth rather than improving plant defence. However, no response in CT concentrations to fertilization suggests an inherent phenological accumulation pattern of tannins. As expected, soil organic layer CT declined in fertilized plots, likely due to a shift in ground vegetation from Ericaceae dominance to graminoid dominance. I also found an increase in C and N concentration and decreased corresponding C: N ratio in fertilized plots. N content significantly increased, whereas C content was nonresponsive. Organic layer pH value responded negatively to fertilization, which can be explained by increasing low pH containing Picea abies litterfall. Overall, tannins are well known to immobilize nutrients, and N availability in this study reduced the CT concentration of the soil organic layer and increased the N concentration, thereby increasing the decomposition of organic matter. Therefore, my study findings may imply that N deposition and forest fertilization facilitate organic matter decomposition when climate change also speeds up this process.