Abstract
Land management practices can vastly influence belowground plant traits due to chemical, physical, and biological alteration of soil properties. Beneficial Pseudomonas spp. are agriculturally relevant bacteria with a plethora of plant growth promoting (PGP) qualities, including the potential to alter plant physiology by modulating plant produced ethylene via the action of the bacterial enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase (acdS). This study evaluated the impact of land management legacy on the selection and function of wheat root associated culturable pseudomonad isolates. Three distinct previous land uses prior to wheat culture (grassland, arable, and bare fallow) were tested and culturable pseudomonad abundance, phylogeny (gyrB and acdS genes), function (ACC deaminase activity), and the co-selection of acdS with other PGP genes examined. The pseudomonad community could to some extent be discriminated based on previous land use. The isolates from rhizosphere and root compartments of wheat had a higher acdS gene frequency than the bulk soil, particularly in plants grown in soil from the bare fallow treatment which is known to have degraded soil properties such as low nutrient availability. Additionally, other genes of interest to agriculture encoding anti-fungal metabolites, siderophores, and genes involved in nitrogen metabolism were highly positively associated with the presence of the acdS gene in the long-term arable treatment in the genomes of these isolates. In contrast, genes involved in antibiotic resistance and type VI secretion systems along with nitrogen cycling genes were highly positively correlated with the acdS gene in bare fallow isolated pseudomonad. This highlights that the three land managements prior to wheat culture present different selection pressures that can shape culturable pseudomonad community structure and function either directly or indirectly via the influence of wheat roots.
Highlights
Intensive croplands have been expanding since the 1960s, helping to achieve food security by increasing productivity (Agren et al, 2013)
This study focused on the impact of land management on populations of the soil and rhizosphere dwelling model bacterium Pseudomonas spp. in terms of abundance, phylogeny, function (ACC deaminase activity) and the co-selection of ACC deaminase structural gene (acdS) with other plant growth promoting (PGP) genes
Across all three land managements (Figure 1), abundance of Pseudomonas spp. was similar in bulk soil and rhizosphere communities whilst pseudomonads associated with wheat roots were significantly more abundant (107–108 mean CFU g−1 root)
Summary
Intensive croplands have been expanding since the 1960s, helping to achieve food security by increasing productivity (Agren et al, 2013). Intensive land use has been linked to soil erosion and compaction, altered nutrient cycling, depleted organic matter levels, salinization, acidification, and pollution (Pretty and Shah, 1997; Doran and Zeiss, 2000; European Commission, 2002; Kibblewhite et al, 2008; Muhammed et al, 2018) These impacts can disrupt soil ecosystem services, thereby threatening the sustainability of the food system for future generations (Edwards, 2002). A diversity of plant species in such systems offer long-term topsoil cover, along with a reliable source of photosynthetically fixed carbon and organic matter inputs compared to high intensity managements (Piccolo et al, 2008) This can increase soil organic carbon (SOC) levels, aggregate stability, soil pore structure, and biodiversity of micro and macrofauna (Ding et al, 2013; Muhammed et al, 2018)
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