Dynamics of nutrient cycling on postharvested white spruce sites in interior Alaska

Abstract

Various field and laboratory methods were used to characterize nutrient cycling on two mature white spruce sites, one recently harvested site and three 14-year-old harvested white spruce sites colonized by different plant communities and presenting different intensity of soil disturbance. Study sites were chosen on upland south facing sites and presented conditions of reduced environmental variability. Soil analysis showed no changes in pools of soil nutrient unless the forest floor was removed. On the other hand, some differences in the dynamics of nutrients were seen: (1) sites where the forest floor was removed showed low N mineralization rates; (2) N mineralization rates appeared faster in the surface soil of the recently harvested site than in mature white spruce sites; (3) the surface soil of sites regenerating to aspen showed the highest N mineralization rates of all 14-year-old sites. Field soil temperature, and field soil moisture content as well as N and lignin concentrations of the forest floor could not explain the differences in N mineralization rates between sites. This suggests that species colonization may influence N dynamics and that N cycling rate on regenerating sites is controlled by a small pool of rapidly cycling N. The determination of nutrient uptake and return by vegetation growing in the field indicated that nutrient cycling was much faster in 14-year-old aspen stands than on any other regenerating or mature site. The measurement of element availability with ion exchange resin bags indicated an increased leaching of nitrate, phosphate and sulfate at springtime, the second summer following harvesting. Poor correlations were obtained between conventional soil testing and ion exchange resin bag determinations. Comparisons between field and laboratory nutrient availability indices indicated that sites colonized by sprouting aspen exhibited the highest N cycling rates seen in this study. This observation makes aspen an interesting species to consider for mixed species management strategies.