Encephalartos ghellinckii-microbe symbiosis, soil nutrient inputs and enzyme activities in KwaZulu-Natal and Eastern Cape grasslands.

Abstract

Cycads are an ancient group of gymnosperms, with over 300 species classified into three families and 10 genera. Originating around 300 million years ago, they are now primarily found in tropical and subtropical regions, where they play crucial ecological roles in carbon sequestration, nutrient cycling, and soil stabilization. A key adaptation of cycads is their symbiotic relationship with nitrogen (N)-fixing bacteria in their coralloid roots, which convert atmospheric N into forms such as ammonia that the plants can assimilate for growth. This trait, which predates its occurrence in legumes and enables cycads to thrive in nutrient-deficient soils, such as those in grassland ecosystems. However, most studies on the role of plant symbiotic relationship with N-fixing bacteria have focused on angiosperms, with little knowledge on their relationship with cycads. Also, over 70% of the cycad species are threatened with extinction, and their loss would have significant ecological consequences due to their vital ecosystem services. This study aims to investigate the symbiotic interactions between Encephalartos ghellinckii and N-fixing bacteria, the role of soil microbial communities and enzyme activities, in regulating soil nutrient availability and influencing the plant’s N source reliance. By examining these factors, this research seeks to understand the contribution of the microbes to the survival of E. ghellinckii in nutrient-deficient, acidic grasslands. To achieve this, microbial diversity in the coralloid roots, rhizosphere, and non-rhizosphere soils of E. ghellinckii was assessed across four sites (Cathedral Peak, Thabankulu, Oribi Gorge, and Umgano). The study also examined extracellular enzyme activities associated with these microbes and their contribution to soil nutrient availability. Additionally, soil nutritional status, including pH, nutrient concentrations, total cations, and exchange acidity, was analyzed to determine its influence on cycad-microbe symbiosis. The N source preference of E. ghellinckii was calculated from atmospheric derived N and plant N concentration. Bacterial strains identified in E. ghellinckii coralloid roots, rhizosphere, and non-rhizosphere soils included Bacillus, Rhizobium, Paraburkholderia, Enterobacter, Caballeronia, Klebsiella, Pseudomonas, Inquilinus, and Arthrobacter. These microbes contributed to N fixation, N cycling, and phosphate solubilization. No significant differences were observed in phosphorus-cycling (acid and alkaline phosphatase), nitrogen-cycling (nitrate reductase), and carbon-cycling (β glucosidase) enzyme activities between rhizosphere and non-rhizosphere soils. Increased enzyme activity was associated with increased nutrient availability and responses to nutrient limitations, highlighting the complex interactions governing soil nutrient dynamics. Isotope δ15N analysis revealed that E. ghellinckii derived more than 80% of its N from atmospheric fixation, while less than 20% was soil-derived. A positive correlation was observed between soil phosphorus (P) concentrations and the proportion of N fixed from the atmosphere across all sites. These findings highlight the role of microbial symbiosis and associated enzyme activities in enhancing nutrient availability in acidic, nutrient-poor grassland soils. Furthermore, this association facilitates E. ghellinckii’s ability to access and utilize atmospheric N, supporting its persistence in nutrient deficient ecosystems.