Between Management Extremes: Moderation Sustains Plant Productivity, Rhizosphere Microbiome, and Nutrient Cycling in Drying Savannahs

Grassland management practices, including frequent plant biomass removal, have intensified globally to enhance productivity. However, intensification leads to shifts in plant composition and plant–microbe interactions, with poorly understood implications for ecosystem functions, such as nutrient cycling, and their stability under climatic stress. We hypothesised that biomass removal frequency has an intermediate optimum at which plant–microbe–soil interactions stabilise ecosystem functions under drought, whereas both low and high removal frequencies reduce resilience to climatic stress. In a native managed African tropical grassland, we applied four above-ground biomass removal frequencies (1×, 2×, 3×, and 6× cuts annually). Intact soil-core mesocosms were grown under controlled conditions and subjected to drought stress in a split-plot, completely randomised design, combined with a ¹³CO₂ pulse-labelling approach. We determined plant productivity, photosynthetic ¹³C assimilation, belowground C allocation, arbuscular mycorrhizal fungi (AMF) colonisation, microbial biomass carbon (MBC), extracellular enzyme activities (EEAs), and rhizosphere microbial community structure to assess the impacts on C allocation and nutrient cycling. Under well-watered conditions, high biomass removal frequencies (3× and 6×) increased shoot productivity and ¹³C assimilation relative to low frequencies (1× and 2×). The EEAs (C, N, and P cycling) and proportion of ¹³C in rhizodeposits increased progressively with an increase in cutting frequency. Low cutting frequencies promoted fungal-dominated rhizosphere communities, particularly saprotrophic fungi, whereas high frequencies favoured bacterial dominance. Drought stress significantly reduced plant productivity, ¹³C assimilation and root biomass at high cutting frequencies. In addition, drought reduced ¹³C incorporation into total phospholipid fatty acids (PLFA) by 53% at high cutting frequencies and by 22% at low frequencies. Notably, despite significant reductions in root biomass and ¹³C assimilation under drought, root AMF colonisation and ¹³C allocation to soil AMF were consistently higher under drought and progressively increased with decreasing cutting frequency. This reflects a greater plant reliance on microbially mediated nutrient and water acquisition during drought. Overall, our results demonstrate that biomass removal frequency modulates drought impacts on rhizosphere nutrient cycling via shifts in plant functional traits, from resource-conservative (“slow”) to resource-acquisitive (“fast”) species, alongside a reshaping of soil microbial communities from oligotrophic to copiotrophic dominance. These findings highlight the inherent trade-offs between ecosystem productivity and an enhanced resilience to increasingly frequent and intense climate-change-induced stresses, underscoring the need for locally adapted management practices.