Biomass and nitrogen dynamics of hybrid poplar on two different soils: implications for fertilization strategy

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Abstract
One concern of short-rotation intensively cultured hardwood plantations is the accelerated loss of nutrients due to removal of large amounts of biomass and short harvest cycles. In this study, we examine simultaneously the nitrogen trends in all the various pools (trees, weeds, soil, and soil water). We compare the N trends and fertilization rates with leaf N status and tree growth on two soil types over 5 years. The data show a gradual transition from an early herbaceous plant dominated system to a tree-dominated system later in the rotation. On the more fertile site, fertilization was not beneficial to tree growth the 1st year, or even the 2nd, if all weeds were kept out. However, when weeds were present, fertilization was very beneficial to tree growth in the 2nd and 3rd years. After crown closure, fertilization declined in importance as herbaceous vegetation was eliminated, tree foliage biomass stabilized, and N cycling increased. Foliar N was strongly related to fertilizer rate during years 2 and 3 on the febrile site and during the entire 5 years on the infertile site. From these relations and others, we infer possible critical times for N fertilization during the first 5 years of a rapidly growing hybrid poplar plantation from the time of initial planting through complete canopy closure and shading out of the herbaceous competition.