While topography can infer erosion potential, the practice of conventional agronomic management can trigger accelerated erosion and pose major threats to soil assets such as biodiversity. The majority of farmlands in Upper-Eastern Ghana are moderately hilly and highly susceptible to erosion. This study pioneered the comparative and interactive effects of topography and conventional versus conservation agriculture practices (reduced tillage, main crop and cover crop, crop residue retention vs. removal) in treatments amended with 0, 40, and 80 kg ha−1 N on soil physicochemical properties and microbiota. Topography imposed profound shifts in soil physiochemical properties and prokaryotic community structure. Foot-slope soils harbored higher prokaryotic richness and diversity compared to the up-slope. Bacillaceae (28.95%) and anaerobic bacteria increased in relative abundance in foot-slope soils, while Micrococcaceae (25.79%) gained prominence in up-slope soils. The effect of tillage was significant in foot-slope while crop rotation was influential in up-slope soils on structuring the prokaryotic community. The interactive effect of slope × tillage was significant in altering soil physiochemical properties, but not prokaryotic community structure. Variation in prokaryotic community composition was explained by soil physiochemical properties (14.5%), elevation as a proxy for topography (11.3%), and spatial distance (10.8%), but rather weakly overall by agronomic practices. Among the soil physicochemical properties, pH, clay content, total C%, volumetric water content, temperature, cation exchange capacity, and NO3−-N were relevant factors influencing the soil microbiota. Geomorphic and soil edaphic properties appeared to interact and were the primary triggers of variation in soil microbiota and their responses to the range of agronomic practices that incorporated conservation management outcomes.

It is well established that land use change (LUC) can impact soil organic carbon (SOC) in tropical regions, but the long-term effects of LUC on soil quality and C cycling remain unclear. Here, we evaluated how LUC affects soil C cycling in the Amazon region using a 100-year observational chronosequence spanning primary forest-to-pasture conversion and subsequent secondary forest succession. We found a surprising increase in topsoil SOC concentrations 60 years following conversion, despite major losses (> 85%) of forest-derived SOC within the first 25 years. Shifts in molecular composition of SOC, identified with diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, occurred in tandem with a significant decline in permanganate-oxidizable C (POXC) and β-glucosidase activity (per unit SOC), interpreted as a deceleration of soil C cycling after pasture grasses became the dominant source of C inputs to soil. Secondary forest succession caused rapid reversal to conditions observed under primary forest for β-glucosidase activity but not for SOC molecular composition (DRIFT spectroscopy), reflecting a long-lasting effect of LUC on soil C cycling. Our results show that rapid changes in the origin of SOC occur following deforestation with legacy effects on some indicators of C cycling (e.g. enzyme activity) but not others (e.g. molecular composition). This approach offers mechanistic understanding of LUC in the Amazon basin and can be used to help explain conflicting reports on how deforestation impacts SOC in the region.