SEASONAL PATTERNS OF NITROGEN FIXATION AND DRY MATTER PRODUCTION BY CLOVERS GROWN IN THE PEACE RIVER REGION

Abstract

Alsike clover (Trifolium hybridum L.) and red clover (T. pratense L.) were established in field plots on an Orthic Gray Luvisol (Hazelmere CL) and a Black Solod (Landry CL) in 3 consecutive yr, 1972, 1973 and 1974. Data were collected in the 2 yr following the year of establishment. The factors measured included seed and forage yield, dry matter production rate, potential nitrogenase activity (acetylene reduction), soil temperature, air temperature, soil moisture and NH⁺₄-N and NO⁻₃-N content of the soil. Based on calculations using the theoretical C₂H₂:N₂ ratio of 3:1, alsike clover annually fixed 20.8–143.0 kg N/ha, and red clover fixed 15.3–77.3 kg N/ha on the Orthic Gray Luvisol. Also, alsike clover fixed more N₂ earlier in the growing season than red clover. The estimate of annual N₂(C₂H₂) fixation by both clovers on the Black Solod was less than one half that on the Orthic Gray Luvisol. There was little difference between the N₂(C₂H₂) fixation by clover crops harvested for forage and those used for seed production. However, the amount of N₂(C₂H₂) fixed always equalled or exceeded the amount of nitrogen removed in the seed, but only occasionally exceeded the amount removed in the forage. Yield and N₂(C₂H₂) fixation varied widely among years, suggesting considerable climatic influence. However, the measured climatic and soil factors were not consistently correlated with potential nitrogenase activity. The seasonal pattern of potential nitrogenase activity appeared to be influenced by the phenological development of the plant. Activity commenced early in May, increased to a maximum about mid-June (flower initiation), and then decreased and generally remained low for the remainder of the growing season. Significant deviations from the usual seasonal profile of potential nitrogenase activity occurred in years with periods of moisture stress or with above average precipitation and soil heat units.