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. 1998. Soil biological activity in recent clearcuts in west-central Alberta. Can. J. Soil Sci. 78: 69-76. Soil biota response to changes in the soil physical environment following forest harvesting is relatively unknown in boreal forests. Soil biological activity was measured at four sites with Luvisolic soil following clear-cut forest harvesting. Aerobic respiration rate and cellulose decomposition in flooded soils were measured on soil samples collected from treatment plots subjected to tree removal only and tree removal associated with three levels of skidding activity immediately after clear-cut harvesting and after 1 and 2 yr. More than half of variation in respiration and cellulose decomposition rates was related to soil properties. Soil respiration rate increased significantly after 1 yr but was not affected by skidder traffic. Cellulose decomposition was highest in soil with air-filled porosity <0.10 m 3 m -3 , and increased significantly with skidder traffic. Air-filled porosity measured in the field at the time of harvest indicated a poorly aerated environment that becomes wetter in subsequent years. The results imply that soil had biota well adapted to poor soil aeration. The development of a fully anaerobic soil environment following forest harvesting only occurred on compacted soil after heavy precipitation, but partial anaerobiosis of these boreal forest soils was common. Although partial anaerobiosis increased decomposition rate, it is considered sufficient to adversely affect the growth of plant roots and change the availability and mobility of nutrients.Key words: Forest harvesting, soil compaction, biological activity, respiration rate, decomposition rate, Alberta, boreal forests Startsev, N. A., McNabb, D. H. et Startsev, A. D. 1998. Activité biologique du sol dans des coupes à blanc récentes dans le centre-ouest de l'Alberta. Can. J. Soil Sci. 78: 69-76. On connaît relativement peu de choses sur la réponse de la faune et de la flore des sols forestiers aux modifications physiques du milieu tellurique résultant de la coupe du bois. Nous avons mesuré l'activité biologique du sol à 4 emplacements sur sols luvisoliques à la suite de coupes à blanc de la forêt. Le taux de respiration en aéro-biose ainsi que de décomposition de la cellulose en sols ennoyés étaient mesurés sur des échantillons de sol prélevés dans des parcelles exposées à l'enlèvement des arbres seul ou combiné à trois intensités de débardage mécanique, immédiatement après la coupe et 1 et 2 ans plus tard. Plus de la moitié de la variation des taux de respiration et des taux de décomposition de la cellulose était reliée aux propriétés du sol. Le taux d'activité respiratoire du sol augmentait significativement après un an mais ne semblait pas influencé par l'intensité des activités de débardage. L'activité cellulolytique la plus forte s'observait dans le sol ayant une porosité en air inférieure à 0,10 m 3 m -3 et elle augmentait significativement selon l'intensité des activités de débardage. La porosité en air mesuré à l'époque de l...

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When fertilizer is broadcast in boreal forest stands, the applied nutrients must pass through a thick layer of either feather moss or leaf litter which covers the forest floor. In a growth chamber experiment we tested the transfer of N through living feather moss or aspen litter when fertilized with urea ((NH 2 ) 2 CO) or NH 4 NO 3 at a rate of 100 kg ha −1 and under different watering regimes. When these organic substrates were frequently watered to excess they allowed the highest transfer of nutrients through, although 72% of the applied fertilizer was captured in the substrates. In a field experiment we also fertilized moss and aspen litter with urea ((NH 2 ) 2 CO) or NH 4 NO 3 at a more operationally relevant rate of 330 kg ha −1 . We captured the NO 3 − or NH 4 + by ion exchange resin at the substrate-mineral soil interface. In contrast to the growth chamber experiment, this fertilizer rate killed the moss and there was no detectable increase in nutrient levels in the aspen litter or feather moss layers. Instead, the urea was more likely transferred into the mineral soil; mineral soil of the urea treatment had 1.6 times as much extractable N compared to the NH 4 NO 3 treatment. This difference between the growth chamber and field studies was attributed to observed fertilizerdamage to the living moss and possibly damage to the litter microflora due to the higher rate of fertilization in the field. In addition, the early and substantial rainfall after fertilization in the field experiment produced conditions for rapid leaching of N through the organic layers into the mineral soil. In the field, only 8% of the urea-N that was applied was captured by the ion exchange resin, while 34% was captured in for the NH 4 NO 3 fertilization. Thus, the conditions for rapid leaching in the field moved much of the N in the form of urea through the organic layers and into the mineral soil before it was hydrolyzed.

Two experiments were conducted to examine the fate of N in feathermosses when mosses were dehydrated and then rewetted. In the first experiment, dry feathermoss shoots were sealed in containers where the N2 was replaced by He. After rewetting and incubating for 20 h, gas in the containers had significant amounts of N2 and N2O and had elevated levels of CO2 compared with mosses maintained in a moist condition or dried mosses that had not been rewetted. In the second experiment, samples of feathermoss were subjected to five dry–wet cycles. By the fifth cycle, there was a 9% reduction in mass of the moss and a similar decline in quantity of N. The results of this study suggest that in forests undergoing dry–wet cycles, there is a potential for N volatilization directly from the moss layer. After drying and rewetting, the moss releases sugars and soluble forms of N into the substrate, creating a suitable environment for aerobic denitrification. Thus, more N is lost from an already N‐deficient ecosystem.