Annual, seasonal and diel variation in nitrogen fixing activity by Inga jinicuil, a tropical leguminous tree

Patterns in nitrogen-fixing activity by Inga jinicuil Schl., a leguminous shade tree in Mexican coffee plantations, were monitored over a three-and-half year period using acetylene reduction. Year to year variation was unexpectedly large; mean annual fixation equalled 35 kg N ha yr, which constitutes a significant nitrogen input to the coffee ecosystem. Nitrogen-fixing activity occurred throughout the year but was highest during the summer and autumn when precipitation and temperature were at a maximum and when the majority of tree growth and reproduction occurred. I. jinicuil flowered twice annually and nodular activity peaked once during each reproductive cycle, with maximum activity after flowering in the first reproductive cycle and before flowering in the second. Diel fluctuation in nitrogen fixation rates were obtained on most but not all sampling dates, but the observed patterns of activity varied from date to date. Aside from an activity peak that occurred at 1900 hours averaged rates of nodular activity were remarkably constant throughout the day. Nodules from seedlings fixed 35% more nitrogen than 30-yr-old-trees but had a similar diel activity pattern. Overall, the results show that variability in nitrogen-fixing activity was large between years, pronounced but explainable between months, and relatively small between hours of the day. The timing of maximum and minimum activity, both seasonally and daily, differed significantly from what has been reported for most other nitrogen-fixing species.