RAPID N2FIXATION IN PINES? RESULTS OF A MAINE FIELD STUDY

Abstract

A recent study of the nitrogen budgets of pitch pines (Pinus rigida Mill.) and red pines (P. resinosa Ait.) growing in model ecosystems in New Hampshire (USA) reported very high nitrogen gains. Thus N2 fixation associated with the rhizosphere of pines may contribute to their success in nitrogen-limited ecosystems. We used the acetylene-reduction assay with carbon monoxide controls to determine whether high rates of N2 fixation are associated with pine forests in Maine. We measured nitrogenase activity in cores of the forest floor and the mineral soil beneath for five pitch pine, seven red pine, and six eastern white pine (P. strobus L.) stands. The stands represented 8 of the 15 biophysical regions of Maine (USA) and included a variety of age classes. We found only very low rates of nitrogenase activity, which varied seasonally with the highest values during the warmer months. The highest mean nitrogenase activity observed was 449 nmol C2H4·m−2·h−1. We estimated that <6 mg N/m2 was fixed annually by non-symbiotic diazotrophs in the soil horizons sampled. These rates are much smaller than atmospheric N inputs in Maine (300–450 mg N·m−2·yr−1) and consequently have little impact on the N budgets of the pines. Based on our results and the literature, it is likely that only low rates of N2 fixation are associated with pines.