Short-rotation plantations of balsam poplars, aspen and willows on former arable land in the Federal Republic of Germany. II. Nutritional status and bioelement export by harvested shoot axes

As part of a comprehensive interdisciplinary research program the nutritional status of a balsam poplar (P. trichocarpa cv. Muhle Larsen), an aspen (P. tremula x tremwuloides cv. Astria) and a willow clone (Salix viminalis) on former arable soils at three different locations in the FRG was monitored by means of foliar analysis. Bioelement concentrations in shoot axes at the end of the rotations and the nutrient export via harvested biomass are described. The methodological approach is a partly factorial fertilization trial with the nutrients N, P, K, Mg and Ca. The foliar nutrient levels in combination with the results of the fertilization trial, in terms of biomass yield, clearly show the balsam poplars and the aspen to be well supplied with all macronutrients on all sites, independently of the fertilization regimes. Both these species are obviously characterized by an excellent capacity to acquire nutrients which are presumably accumulated in these substrates as a consequence of earlier fertilization and liming. In contrast, the willows proved to be very N-demanding and need repeated fertilization with N at all sites under study to achieve optimum yield. Salix was able to increase growth considerably in the second rotation by about 44-300% as compared with the first rotation cycle. The increase of foliar N (only ca. 6% more with 100 kg N ha(-1) year(-1) than the unfertilized control), however, was rather low as compared to the growth effects caused by the same treatment in the second rotation, leading to low correlative values between foliar N, tree height, and growth. All foliar nutrient levels measured in the experiments are in good accord with values reported in literature for optimal production. In terms of five-year growth periods, total bioelement removals via shoot biomass for all clones are 90-270 kg N, 15-45 kg P, 30-180 kg K, 55-350 kg Ca, and 5-25 kg Mg ha(-1). Nitrogen fertilization of Salix viminalis resulted in higher N concentrations in shoot axes, and N amounts removed via harvest increased as a consequence of N fertilization by ca. 26-47% (50 and 100 kg N applied annually). However, with values between 16% and 39%, only a low percentage of the applied N was incorporated into shoot biomass and, therefore, the efficiency of fertilized N was rather low. (C) 1999 Elsevier Science B.V. All rights reserved.