Diurnal and seasonal patterns of nitrogenase activity of red alder in comparison with white clover in silvopastoral agroforestry systems

Simultaneous measurements were made to assess the diurnal and seasonal patt erns of nitrogenase activity of red alder (Alnus rubra Bong.) and white clo ver (Trifolium repens L.) growing together in a silvopastoral agroforestry system using the acetylene reduction assay. Diurnal measurements were made in the summer and autumn at 3-h intervals whereas seasonal nitrogenase acti vity was assessed based on observations made at midday in July, September a nd January to represent the summer, autumn and winter seasons, respectively . No obvious diurnal patterns of nitrogenase activity were found in either red alder or white clover in summer and no significant variations in nitrog enase activity were observed between day and night. However, in autumn, pro nounced diurnal patterns were observed in both species. Significantly highe r rates of nitrogenase activity per unit dry weigh (dwt) of nodules were de tected at 1500 hours in red alder, whereas, in white clover: significantly higher rates were obtained at 2100 hours. There was no significant correlat ion between diurnal nitrogenase activity and air temperature, photosyntheti cally active radiation and soil temperature at 10 cm depth in either red al der or white clover. Seasonal rates of nitrogenase activity showed signific antly higher activity in summer, which subsequently decreased in autumn, to reach very low levels in the winter. The rates of nitrogenase activity of white clover were consistently higher than those of red alder both diurnall y and seasonally. In the three seasons sampled, the average nitrogenase act ivity for white clover was 66.42 mu mol C2H4 g dwt(-1) h(-1), which was 3.5 times higher than the 18.67 mu mol C2H4 g dwt(-1) h(-1) obtained for red a lder.