Nonsymbiotic Nitrogen Fixation in an Oak-Hickory Forest Following Long-Term Prescribed Burning

Seasonal rates of nonsymbiotic nitrogen fixation in the surface soil of oak-hickory forest plots exposed to 30 years of annual and periodic (4-year) prescribed burning were estimated using an in-situ acetylene reduction procedure. Fixation rates were low, averaging 0.1 kg·ha⁻¹·year⁻¹ for all soil cores assayed in April, August, and October 1980 and April 1981 with approximately 11% of samples displaying activity. Burning treatments had no influence on fixation rates or proportion of samples displaying activity. Highest rates were found in autumn with an average rate for active samples of 2.2 kg·ha⁻¹·year⁻¹. Higher autumn fixation rates may be related to higher quantities of high energy foliar leachates which could induce a significant seasonal peak on sites demonstrating higher rates of fixation.