Seasonal Patterns Of Nitrogen Mineralization And Nitrification Following Harvesting In The White Spruce Forests Of Interior Alaska

Abstract

The effects of commercial timber harvesting upon nitrogen transformations were evaluated for the forest floor and mineral soil of mature white spruce (Picea glauca (Moench.) Voss) forest sites in interior Alaska. Analyses of forest floor and mineral soil incubated in situ in mature forest and two recently harvested areas of different ages indicated an ammonium-dominated soil system for the unharvested area. Statistically, logging had no effect upon mineralization, nitrification or on the average NH(,4)-N or NO(,3)-N produced on incubation in the forest floor. In the mineral soil the same patterns were apparent with the exception that the NO(,3)-N produced on incubation was significantly greater in the harvested areas. However, the lack of a large number of incubation periods (n = 8), over the entire course of study (22 months of continuous incubation) likely contributed to this result. While mineralization (ammonification) rates did not differ between areas, there were times in mid-summer when nitrification rates in the clearcut areas far exceeded those in the control area. Thus, although the net amount of NH(,4)-N produced was not different between areas, the amount of nitrogen moved from the organic pool into available pools (net nitrogen mineralization) was greater in the harvested areas, primarily because of mid-summer nitrification in the latter. Increased soil moisture and temperature regimes due to harvesting were thought to be the prime factors responsible. Nitrogen cycling in the mature white spruce forest was typical of a steady-state system; cycling was very tight. Field estimates of mineralization for the forest floor (2.48 g/m('2)/yr) and the mineral soil (0.48 g/m('2)/yr) fit well into a schematic cycle developed around other estimates of N-pools and fluxes derived from the literature. This was also true of nitrification rates which were very low in both horizons. In the cleared areas, mineralization rates in the forest floor varied from 0.11 to 3.59 g/m('2)/yr and in the mineral horizon from 0.42 to 0.54 g/m('2)/yr. Nitrification rates varied from 1.49 to 3.01 g/m('2)/yr in the forest floor and from 0.24 to 0.27 g/m('2)/yr in the mineral soil. The effects of changes in abiotic and biotic controls on nitrogen mineralization and nitrification due to harvesting, and the implications of whole-tree vs. conventional logging are also discussed.