SYMBIOTIC NITROGEN-FIXATION OF WHITE CLOVER IN A MIXED SWARD IS NOT LIMITED BY HEIGHT OF REPEATED CUTTING

The aim of this study was to investigate whether symbiotic nitrogen fi xation in white clover nodules limits nitrogen supply and hence clover growth by repeated defoliation at two cutting heights. Other possible factors governing symbiotic nitrogen fixation in the field were also elucidated. Using N-15, a 2-year field experiment including white clov er (Trifolium repens L. cv. Ladino) and perennial ryegrass (Lolium per enne L. cv. Bastion) in monocultures and in mixtures was conducted in Eschikon, Switzerland. The effect of two cutting heights (4 cm and 10 cm above ground level) on the performance of symbiotic nitrogen fixati on of white clover in the different sward-types was investigated. Afte r each harvest, the plots were fertilized with 3 g N m-2 (equivalent t o 30 kg N ha-1 cut-1 or 210 kg N ha-1 year-1). In both years, white cl over grown in a mixture with grass received a significantly higher per centage of nitrogen from symbiotic fixation compared with clover grown in monoculture. This phenomenon is attributed to the strong competiti veness of ryegrass in soil nitrogen uptake. Consequently, white clover in the clover-ryegrass mixture was more dependent on symbiotic nitrog en fixation than that grown in monoculture. The cutting height did not preferentially influence symbiotic nitrogen fixation, as opposed to t he uptake of mineral nitrogen from the soil. From this finding it is s uggested that symbiotic nitrogen fixation did not limit the supply of nitrogen to clover and hence its growth. It is proposed that symbiotic nitrogen fixation in white clover is regulated by the demand for nitr ogen rather than by the availability of carbohydrate reserves in the s tolons. Symbiotic nitrogen fixation should thus be looked upon as an i ntegrated plant growth factor and not as an isolated phenomenon.