Factors affecting nitrogen fixation by white clover (Trifolium repens) on colliery spoil.
N fixation by white clover established on colliery spoil in S. Yorkshire was measured weekly between May and Nov. Residual spoil P levels from 2 rates of P fertilizer applied 5 yr previously affected fixation. High residual P gave significantly higher N fixation than low, resulting in higher spoil N on high P plots. Sunshine hours, spoil temp. and moisture levels most affected N fixation over the sampling period. Between June and Sept., fixation was most highly correlated with rainfall between 12 and 27 days prior to sampling. When moisture supplies were adequate, temp. and sunshine hours were more important. The estimated annual N input by fixation was 146-167 kg N/ha on high-P plots and 90-103 kg N/ha on low-P plots. No N fixation was detected on plots not containing white clover. Cutting apparently decreased N fixation rates. Ways of maximizing N fixation on colliery spoil are discussed in the light of these findings.