Modelling nitrogen turnover and production in an irrigated short-rotation forest

Abstract

Biomass production and the amount of nitrogen in an established short-rotation forest were simulated on a daily basis, using a model based on leaf photosynthesis, allocation of assimilates within the plant, plant uptake of nitrogen and supply of nitrogen through decomposition of organic matter. The model was developed for willow stands with 1-year-old shoots and has, except for the areal leaf weight (the ratio between leaf biomass and leaf area), only climatic factors and fertilization rate as input variables. The model was constructed by combining two submodels, a biomass production submodel and a nitrogen turnover submodel. The model is valid for horizontally uniform stands in which growth is not limited by shortage of water or nutrients other than nitrogen. The simulated stem, leaf and leaf fall dry matter, as well as the stem nitrogen content of 1-year-old shoots of a Salix viminalis stand in southern Sweden, agreed well with measured data. The canopy nitrogen content was overestimated. This was probably caused by inaccurate estimates of the mineralization rate of the litter where the role of large soil animals, such as earthworms, was difficult to evaluate. A sensitivity test showed that stem production is most affected by modifications of the “growth parameters”. Only the sensitivities to the parameters of the leaf area development are of equal significance. Stem production is most sensitive to changes in the light extinction coefficient, while the plant nitrogen content is most influenced by the initial amount of litter. If the nitrogen supply to the plant is decreased, production becomes more sensitive to variations in the “decomposition parameters”. Different approaches to nitrogen partitioning within the plant showed that leaves have a lower priority for nitrogen than the roots. Nitrogen uptake by the canopy after fertilization can be 30% higher than that needed to supply the newly formed leaves with “maximum” nitrogen content.