Implications of topography and agronomic practices for soil biogeochemical and microbiological properties in Upper Eastern Ghana hilly farmland

Abstract

While topography can infer erosion potential, implementation of convention practices can trigger accelerated erosion, and both pose major threats to soil abiotic properties and microbiota. The majority of farmlands in the Upper-Eastern of Ghana (UE) are undulating, moderately hilly, comprised of soils with low organic matter and light texture which make them highly susceptible to erosion and accelerated erosion. Considering the importance of erosion and its repercussions on soil C cycle, fertility, and health, this study was conducted to trace the erosion and unravel its consequences for soil biogeochemical. The current study also aims at attaining deeper insights into the effects of topography and various agronomic practices, comparatively and interactively, on the soil microbial community and their plant growth-promoting potentials. Accrual of clay particles, the higher mass of C (MWC%) and N (MWN%), NH₄⁺-N, NO₃^--N, and other eroded cations in footslope soils served as evidence of ongoing erosion/deposition processes in the field. Erosion homogenized ¹³C values along the slope while accelerating N cycling at the footslope positions. The regular occurrence of anoxic conditions in frequently waterlogged footslope soils led to a higher relative abundance of anaerobic bacteria compared to upslope. A higher relative abundance of denitrifiers suggested the presence of an open N cycle within the footslope sediments. Evidently, geomorphic features imposed profound shifts in soil ¹⁵N natural abundance, as well as in soil edaphic, and pedoclimatic properties. This in turn fueled spatial heterogeneity in the soil prokaryotic community diversity and composition, as well as in the plant growth-promoting potential of the sorghum rhizobiome. Besides, implications of various agronomic practices (conventional vs. reduced tillage, crop rotation with cover crop cultivation and residue return vs. no cover crop and no residue return, and soil amended with 0, 40, and 80 kg ha^-1 nitrogen) for soil edaphic properties and microbiota appeared to be modulated by the prepotent impact of topography. Albeit the interactive impact of slope × tillage changed soil physiochemical characteristics and soil ¹³C values, the given interactive effect was less effective in altering prokaryotic community diversity and assemblages. The potent effect of topography, heterogeneous cropping regimes prior to initiating our experiment, the homogenizing nature of the agricultural practices, and inherited spatial heterogeneity at the study site can explain the diluted implications of agronomic practices for the soil microbiota. Among the soil edaphic properties, pH, clay content, MWC%, volumetric water content, temperature, cation exchange capacity (CEC), and NO₃^--N were identified as the influential factors for structuring soil microbiota, while Fe, Al, CEC, pH, Nmin, and temperature appeared to be effective on the metabolic potential of the sorghum rhizobiome. Among the plant growth-promoting potentials of sorghum rhizobacteria, indole production was strongly contingent upon nutrient availability within the rhizosphere and thus improved at the footslope position. However, the phosphate solubilizing potential of rhizobiome gained prominence at the eroding up-slope positions. Noticeably, the prokaryotic biogeographical pattern, community composition, and the metabolic potentials of the sorghum rhizobiome were all primarily shaped by environmental filtering as the underlying factor.