This study investigates the effects of perennial grass species richness levels and consortium of three bacteria on properties of a slightly alkaline soil and on biomass production of grasses. Four species of perennial aromatic grasses namely: vetiver (Vetiveria zizanioides), lemongrass (Cymbopogon citratus), palmarosa (Cymbopogon martinii), and citronella (Cymbopogon winterianus) were planted in all possible combinations at one, two, three, and four species richness levels in a randomized block design with three replications and two treatments, these were PGPR+ (with plant growth promoting rhizobacteria) and without (PGPR−). Species and richness level specific changes in soil physicochemical properties: electrical conductivity (EC); bulk density (BD); total soil organic carbon (TOC); available nitrogen (Nav), microbial colony forming unit (CFU); microbial biomass carbon (MBC), and enzymatic alkaline phosphatase (ALKP) and dehydrogenase activity (DHA) were observed. The highest reduction in soil pH and EC and increase in soil TOC, CFU, Nav, MBC, and enzymes activities were found at two species richness levels of vetiver and lemongrass in both PGPR+ and PGPR− plots followed by monoculture of vetiver. The highest annual total dry biomass 22.8 and 27.5 Mg ha−1 was produced by monoculture of vetiver in PGPR− and PGPR+ plots, respectively, with similar values at two species richness levels of vetiver and lemongrass in PGPR− and PGPR+ plots. The highest plant carbon was found in vetiver monoculture and two species richness levels of vetiver and lemongrass. The soil carbon sequestration potential was highest at two species mixtures of vetiver–lemongrass and lemongrass–palmarosa as well.

The cultivation of perennial aromatic grasses on marginal lands is a socioeconomically and ecologically sustainable approach. However, the studies on assessment of the potential efficacies of consortium of plant growth promoting rhizobacteria (PGPR) in enhancing the productivity of perennial aromatic grasses on marginal lands are rare. In this study, therefore, we studied the effect of PGPR consortium (Pseudomonas protegens and two strains of Bacillus paramycoides) on yield and economics of four perennial aromatic grasses (Vetiveria zizanioides, Cymbopogon citratus, C. martinii, and C. winterianus) in slightly alkaline (pH >8.50) soil. A field experiment was established in a complete randomized block design with two treatments (i.e. with PGPR consortium and without PGPR consortium) and three replications. The changes in growth (height, number of slips, and leaf area index), productivity (fresh shoot-root biomass and oil yield), quality (important secondary metabolites), and economics (cost of cultivation, gross, and net return) in response to PGPR were observed at three harvests. The species-specific effects of PGPR were found on the growth and productivity of perennial aromatic grasses. On an average, the highest increase in growth and productivity with the effect of PGPR was found in V. zizanioides followed by other grasses. The highest oil yield 16.38, 71.14, 53.13, and 49.49 kg ha À1 were recorded at third harvest in PGPR consortium treated plots for V. zizanioides, C. citratus, C. martinii, and C. winterianus, respectively. The agronomical economics show the highest net return of 2094.95 $US in the cultivation of V. zizanioides with the application of PGPR.

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