Factors Affecting Nitrogen Fixation by White Clover (Trifolium repens) on Colliery Spoil

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Abstract
The N2 fixation of white clover established on colliery spoil was measured by the acetylene reduction technique at weekly intervals between May and Nov. Spoil chemical characteristics, moisture and temperature, sunshine hours, air temperature and rainfall were monitored over the same period. Residual spoil phosphate levels from 2 rates of phosphate fertilizer applied 5 yr previously affected fixation. High residual phosphate gave significantly higher N fixation than low. These differences resulted in higher spoil N on high phosphate plots. Sunshine hours, spoil temperature 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, temperature and sunshine hours assumed greater importance. The estimated annual N inputs by fixation were 146-147 kg N ha-1 on high phosphate plots and 90-1-3 kg N ha-1 on low phosphate plots. No N fixation was detected on plots not containing white clover. Cutting apparently decreased N fixation rates. Ways of maximizing N2 fixation on colliery spoil are discussed in the light of these findings.