Interactions between nitrogen, phosphorus and potassium determine growth and N2-fixation in white clover and ryegrass leys

Variations in yield, N-2 fixation and above-ground accumulation of nitrogen (N), phosphorus (P) and potassium (K) in white clover and ryegrass grown separately or in mixture were investigated under field conditions over two consecutive years at different supplies of N, P and/or K. Ryegrass-clover mixtures consistently out-yielded the pure stand clover and pure stand ryegrass in terms of dry matter. Improved N supply favoured ryegrass, and ryegrass in mixture with clover accumulated substantially higher amounts of N, P and K than ryegrass in pure stand. Conversely, the growth of white clover in mixture with grass was significantly depressed by N application, particularly when P and K were also applied. Plots of dry matter, N, P and K in white clover versus ryegrass in mixtures followed log-normal relations demonstrating the superior responsiveness of ryegrass to improved availability of N, P and/or K. Although competition for P and/or K reduced the N-2 fixation rate in the mixture, the effect on the total above-ground N accumulation was insignificant. The proportions of P and K in the shoot dry matter of ryegrass in pure stand were only half of those of ryegrass in mixture with clover, while white clover co-cultivated with ryegrass had lower P concentration in dry matter, showing a P deprivation of clover growing in mixture with grass. Na was able to replace K under the competitive conditions in the mixture. In conclusion, the results show that growth and nutrient acquisition of clover and ryegrass interact in a complex manner involving competition, facilitation and complementarity.