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Effects of intensive fertilization on soil nutrient cycling in lodgepole pine and interior spruce forests in the central interior of British Columbia

Harrison, D.. 2011. University of Victoria

Abstract

The growth and productivity of British Columbia's interior forests is largely limited by soil nutrient availability. Fertilization has been shown to be an effective silvicultural tool for increasing the development of immature stands throughout the region. This has lead to increased interest in long-term, repeated fertilization as a means of addressing timber-supply shortfalls as a result of the current mountain pine beetle (Dendroctonus ponderosae) outbreak. However, there is little information related to the impacts of repeated fertilization on the cycling of nutrients in many of these stands. This study makes use of a long-term (13-15 year) fertilization experiment in two lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm) and two interior spruce (Picea glauca [Moench] Voss and Picea engelmannii Parry) forests in the central interior of British Columbia subject to two levels (periodic and annual) of nitrogen(N)-based fertilization. The primary goal of the project was to examine the effects of different fertilizer regimes on aspects of soil chemistry. Specifically, this project was concerned with the impacts of repeated fertilization on: 1) soil carbon (C) and N cycling, and 2) soil base cation (e.g., Ca, Mg & K) availability. Soil and foliar nutrient regimes were quantified throughout the 2008 and 2009 growing seasons using ion-exchange membrane (IEM) plant root simulator (PRS) probes and traditional soil and foliar analyses. Fertilization increased N cycling at all sites, with generally elevated soil and foliar N and significant soil-foliar N relationships in several cases. Nitrate (NO3-) increased in the fertilized plots in several cases; however, there was minimal evidence of NO3- leaching. Greater than 90% of fertilizer-N inputs were retained onsite, suggesting these forests are not N-saturated. Soil, tree and total ecosystem C generally increased in response to fertilization, with the spruce sites exhibiting greater C accrual per unit of fertilizer N than the pine sites. Further, significant linear relationships between soil C and N were evident at all sites. At sites with poorly buffered soils (pH < 4), fertilizer treatments generally led to increased soil acidification and decreases in soil and foliar Ca. Decreases in soil Ca may have been due to significant increases in sulfate leaching; whereas foliar Ca decreases appear to be related to compromised uptake systems, potentially from increased soil aluminum. Buffering capacities, rather than forest type, appear to be the best predictor of soil and foliar Ca responses to fertilization. Despite significant changes in soil chemistry at all four sites, it does not appear that current fertilization rates are detrimentally affecting tree growth.