N-2-fixation by native herbaceous legumes in burned pine ecosystems of thesoutheastern United States

The role of herbaceous legumes in the nitrogen cycle of burned forest ecosy stems in the Georgia Piedmont was evaluated by (a) measuring the frequency, density, and diversity of populations in sites with different histories of burning, (b) assessing the N-2-fixation activity of the dominant species u sing acetylene reduction, nodule biomass, and N-15 natural abundance techni ques, and (c) identifying potential environmental controls on the N2-fixati on process. Sites that were regularly burned had a substantially higher fre quency, density, and diversity of legumes compared to sites that received o nly a single burning treatment. The N-2-fixation activity of three dominant : species, Desmodium viridiflorum L., Lespedeza hirta L., and L. procumbens Michx., was intensively examined in one of the regularly burned study site s. The highest measured acetylene reduction rates, 75.4 and 39 nmol h(-1) m g(-1) nodule dry mass, and nodule biomass values, 56.3 and 5.2 mg dry mass/ plant, for D. viridiflorum and L. procumbens, respectively, generally occur red in mid-July and decreased later in the growing season. L. hirta was rar ely observed to have nodules. Water stress apparently had a direct effect o n N-2-fixation rates during the mid-July assay, and a greater indirect effe ct on fixation activity later in the season by reducing leaf area in all sp ecies. The delta(15)N of the available soil nitrogen (average of -3.9+/-0.4 parts per thousand based on the isotopic composition of non-N-2-fixing ref erence species) differed significantly from that of atmospheric N-2 (0 part s per thousand) and estimates of N-2-fixation activity based on the acetyle ne reduction and nodule biomass assessments were directly correlated with t hose based on the N-15 natural abundance technique, suggesting that this st able isotope approach has potential applicability in these regularly burned forest ecosystems. The results of this study indicate that nitrogen inputs via N-2-fixation by enhanced legume populations may balance nitrogen losse s due to burning. (C) 1999 Elsevier Science B.V. All rights reserved.