ifoam'96 Book of Abstracts 11th IFOAM Scientific Conference 11-15 August 1996, Copenhagen, Denmark

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Competition for Inorganic N in Clover-Grass. P1; 43

Plant Nutrition Laboratory, Department of Agricultural Sciences, Royal Veterinary and Agricultural University, Thorvaldsensvej 40DK-1871, Copenhagen, Denmark

Under temperate conditions, white clover is the most important fodder legume (Frame and Newbould, 1986) and it is mostly grown in mixture with ryegrass. In organic farming systems, clover-grass pastures with a well-growing clover is of even greater importance. Environmental and management factors will determine a balance between these two species; a balance that is very important in organic agriculture. As the morphology of roots of white clover and ryegrasses differs and as there is a physiological regulation of N uptake on whole plant level (Touraine et al., 1994) the competitive ability might differs between white clover and ryegrass depending on access to inorganic N. However, how these two species compete for inorganic N has only been investigated to a very limited extent.
The objective of the present study was to investigate the uptake and absorption of inorganic N when growing white clover and ryegrass with access to ammoniumnitrate in different concentrations (0, 25, 50, 100, 300, 500 %M) for seven days. Morphological and physiological adaptations of the plants were investigated. The experiment was conducted in a hydroponic system consisting of a 4 l bucket with a lid in which six small containers were placed. The full nutrient solutions were changed every morning.
In all cases white clover absorbed more N from the growth media than ryegrass (P). As total root length of white clover only was higher at N concentrationsd %M NH4NO3 the uptake per unit root length was higher in white clover than in ryegrass (P). The relative growth rate (RGR) was not significantly increased in ryegrass, whereas white clover in all cases had higher RGR than the accompanying ryegrass (P) with some particular stimulus between 25 and 100 %M NH4NO3.
Enzymatic activity of GS were in all cases higher in white clover compared to ryegrass. The enzymatic activity of NR indicated that under the higher nitrate concentrations white clover reduced relative more nitrate in the roots whereas grass tended to reduce nitrate in the shoots. This can disadvanges white clover in the competitive process under more fertile conditions.
It is concluded that under low concentrations of N in the solution (%M), and under otherwise non-limiting growth conditions, white clover has a competitive advantage compared to its associated ryegrass. Measurements under field conditions show that the soil solution under unfertilized white clover-ryegrass sward is characterized by almost equal concentrations of nitrate and ammonium, both present in concentrations less that 50 %M. When white clover-ryegrass mixtures are applied moderate amounts of N fertilizer the concentration of inorganic N in the soil solution can raise up to 500 %M.

Frame, J., & Newbould, P. (1986): Agronomy of white clover. Advances in Agronomy, 40, 1-87.

Touraine, B., Clarkson, D.T. & Muller, B. (1994): Regulation of nitrate uptake at the whole plant level. In: Roy, J. and Garnier, E. (eds.) A whole plant perspective on carbon-nitrogen interactions. SPB Academic Publishing, The Hague, p. 11-30.