Non-symbiotic nitrogen-fixing bacterial diazotrophs closely associated with the roots of grasses probably contribute most of the new nitrogen acquired to sustain productive natural grasslands, yet their ecology is poorly understood, especially in southern Africa. We looked for genetic evidence, using qPCR and gel electrophoresis, for the presence of the bacterial nifH gene associated with the roots of four grass species (20 plants each) in a mesic grassland in South Arica, which would indicate the potential for N fixation by diazotrophs. Grasses most tolerant of low N (Aristida junciformis) were predicted to harbour the most diazotrophs, especially compared to those most responsive to fertiliser N (Eragrostis curvula). However, the nifH gene was found in all 80 root samples and did not differ in copy number between species. Sequencing of a representative sample confirmed the identity of the nifH gene. The recently burned half of the grassland had 60% more relative nifH gene copy numbers than the area burned 15 months previously, suggesting that grass growth stimulated by fire could recruit diazotrophs. Given their ubiquity and importance in the N economy of grasslands, research is required to characterise root-associated diazotroph communities, quantify their N fixation rates, and understand their environmental controls.
Elephant grass genotypes display a variety of morphological differences, influencing the nutritive value of the forage. This study evaluated the histological arrangements of the leaves and stems of different elephant-grass genotypes, two tall-sized (Elephant B and IRI-381) and two dwarfs (Mott and Taiwan A-146 2.37), during a two-year trial. The grasses were harvested at 60-day intervals for two years. Biometric analyses of the stems and leaves were performed based on histological measurements. An in vitro dry matter digestibility (IVDMD) assay of the forage was performed. Among the elephant grass genotypes, the lignified cells and vascular bundles of the stems had higher variation than the leaves. Tall-sized genotypes displayed more lignified tissues in stems than the dwarfs. The transversal area occupied by vascular bundles and lignified cells were higher in Elephant B (44 911 μm2 and 35 895 μm2) (p < 0.05), compared to the dwarfs. Forage IVDMD was higher in leaves (699 g kg−1 of dry matter [DM]) than in the stems (678 g kg−1 of DM), considering all genotypes (p < 0.05). We did not observe any direct influence of genotype on forage digestibility, despite some differences in the histological arrangements.
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