Foliar δ13C and δ15N response of lodgepole pine and Douglas-fir seedlings to soil compaction and forest floor removal

The effects of soil compaction, forest floor (FF) removal, and rehabilitation treatments on foliar delta C-13 and delta N-15 of lodgepole pine (Pinus contorta) and Douglas-fir (Pseudotsuga menziesii) were studied on a calcareous soil. Regardless of soil compaction, FF removal (which reduces soil water potential) resulted in less negative foliar delta C-13 values of lodgepole pine (from -25.9 to -23.4 parts per thousand), whereas soil compaction effects on foliar delta C-13 were observed only within the FF intact treatment. This result and the more negative foliar delta C-13 with increasing seedling growth most likely reflected limitation on CO2 diffusion due to water stress caused by those treatments. However, foliar delta C-13 of Douglas-fir (range -25.0 similar to -24.5 parts per thousand) were not affected by the treatments, indicating less susceptibility to water stress. Soil compaction reduced NH4+-N concentrations in the FF (from 48.5 to 28.0) and NO3--N concentrations in the FF (from 13.8 to 6.4) and mineral soil (from 4.3 to 2.1 mg kg(-1)), and FF removal tended to decrease NH4+-N concentrations in the mineral soil. Foliar delta N-15 of both species were not affected by soil compaction but were increased by the FF removal and rehabilitation treatments, indicating that the latter two treatments dramatically altered soil N dynamics.