Canada bluejoint foliar δ¹⁵N and δ¹³C indicate changed soil N availability by litter removal and N fertilization in a 13-year-old boreal plantation

Abstract

Canada bluejoint grass [Calamagrostis canadensis (Michx.) P. Beauv., hereafter referred to as bluejoint] outcompetes overstory tree species such as white spruce [Picea glauca (Moench) Voss] by creating a thick litter layer and competing for the available nitrogen (N). This study was conducted to investigate the effects of bluejoint litter layer (with or without litter removal) and N fertilization on soil water and N availabilities using principal component analysis (PCA) and foliar δ¹⁵N and δ¹³C of bluejoint in a plantation in north-central Alberta, Canada. PCA using soil properties and understory growth data demonstrated that N fertilization was more effective in changing the soil environment and resource availabilities for bluejoint growth than litter layer removal. The increase in soil N availability by N fertilization was linked with increased bluejoint foliar δ¹⁵N (by around 3‰) in fertilized plots, as a result of greater N isotopic fractionation in the fertilized plots. The more negative δ¹³C (by around 1‰) of bluejoint in litter layer-removed plots suggested that litter layer removal increased soil water availability, indicating that the litter layer reduced soil water availability on the site. Therefore, results from this and previous studies showed that the litter layer decreased both soil water and N availabilities. Although the exact mechanisms of the benefit of the litter layer for bluejoint remains unknown, bluejoint likely adversely impacted tree growth by competing for N due to its strong N acquisition ability under soil resource-limiting conditions.

Key words:

• Canada bluejoint grass (Calamagrostis canadensis (Michx.) P. Beauv.)
• δ¹³C
• δ¹⁵N
• understory competition
• white spruce (Picea glauca (Moench) Voss)

ACKNOWLEDGMENTS

Initial funding for this study was provided by the Natural Science and Engineering Research Council (NSERC) of Canada, the Canadian Foundation for Innovation, the Weyerhaeuser Company Ltd. and Weldwood of Canada Ltd. We thank Dave Swindlehurst, Weyerhaeuser Company Ltd., and Paul Godin, Millar Western Forest Products Ltd., for assistance in site selection and in providing information on site history.