The 12-year effects of different regimes and frequencies of repeated fertilization (applied periodically and yearly) on forest floor and mineral soil properties were evaluated at two study sites in central British Columbia. When applied at 6-year intervals to 10- to 12-year-old lodgepole pine and spruce plantations, two applications of urea (totalling 400 kg N/ha), with and without other added nutrients, had few measurable effects on forest floor or mineral soil properties 12 years after initial fertilization. Conversely, 12 years of annual nutrient additions (775-1600 kg N/ha in total) had significant effects on several forest floor and mineral soil properties, but the effects were different at the two study sites. At Crow Creek, yearly fertilization of spruce resulted in larger forest floor mass, lower carbon/nitrogen ratio, lower ph, higher mineralizable
nitrogen (N), and larger pools of total N, carbon (C), and sulphur (S) in the
forest floor and mineral soil. Pools of forest floor total phosphorus (P) and
potassium (K) were larger in annually fertilized plots than in control plots
after 12 years. Extractable P, N, and S, and exchangeable K and magnesium (Mg)
levels were also higher in intensively fertilized forest floors and mineral
soils at Crow Creek. These results indicate that large and frequent nutrient
additions may increase the rate of N cycling and site quality and may also
promote above- and below-ground C sequestration. The low levels of soil NO3-N at Crow Creek indicate minimal nitrification and/or rapid N uptake or immobilization of added N in vegetation and soils. Also, the continuous input of organic C from above- and below-ground sources may increase the immobilization capacity in these repeatedly fertilized soils. The apparent high retention capacity of the added N at Crow Creek and low levels of
NO3-N indicates that this system is not N saturated. In contrast, yearly fertilization of lodgepole pine at Kenneth Creek had no measurable effects on forest floor mass, C/N ratio, mineralizable N, and total N, C, and S. Relatively high forest floor and mineral soil
NO3-N levels in heavily fertilized treatment plots may indicate N saturation and high
NO3 - leaching potential. The coarse-textured soils, relatively high precipitation, and poor tree growth may have contributed to high leaching losses in repeatedly fertilized treatments at Kenneth Creek.
Results indicate that large, and frequent, nutrient additions may be an effective way to build up soil organic matter reserves and sequester atmospheric C on sites where increased tree growth stimulates litter production, root growth, and understorey development. Intensive fertilization may, therefore, be a biologically viable management option for rehabilitating forested sites that have been degraded from poor management practices such as severe broadcast burning or scarification and for reducing the contribution of greenhouse gas emissions to climate change. However, our results indicate that not all forested sites offer equal opportunities in this regard. Also, the amount of greenhouse gas emissions through all stages of forest fertilization must be considered when evaluating the net effect of intensive fertilization on C sequestration.
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Updated April 01, 2009