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

Abstract

The role of herbaceous legumes in the nitrogen cycle of burned forest ecosystems 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 using acetylene reduction, nodule biomass, and 15 N natural abundance techniques, and (c) identifying potential environmental controls on the N 2-fixation process. Sites that were regularly burned had a substantially higher frequency, density, and diversity of legumes compared to sites that received only 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 sites. The highest measured acetylene reduction rates, 75.4 and 39 nmol h −1 mg −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 occurred in mid-July and decreased later in the growing season. L. hirta was rarely observed to have nodules. Water stress apparently had a direct effect on N 2-fixation rates during the mid-July assay, and a greater indirect effect on fixation activity later in the season by reducing leaf area in all species. The δ 15 N of the available soil nitrogen (average of −3.9±0.4‰ based on the isotopic composition of non-N 2-fixing reference species) differed significantly from that of atmospheric N 2 (0‰) and estimates of N 2-fixation activity based on the acetylene reduction and nodule biomass assessments were directly correlated with those based on the 15 N natural abundance technique, suggesting that this stable 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 losses due to burning.