Early plant–soil functional responses to volcanic ash mixing: a Cucurbita moschata bioassay with and without rhizosphere–root inoculation

Volcanic ash is often mixed into topsoil after ashfall to restore field operability, but early changes in soil function can be difficult to diagnose with routine fertility tests during crop establishment. Here, we developed a rapid 28-day greenhouse bioassay using Cucurbita moschata Duchesne to detect early functional responses to volcanic ash incorporation (0%, 5%, 10%, and 30%, v/v) into a soil-peat substrate, with and without a rhizosphere–root inoculum. We measured plant growth and architecture, key substrate properties (water holding capacity, pH, electrical conductivity, organic carbon, total nitrogen, available phosphorus, exchangeable cations, and cation exchange capacity), respiration-based microbial functional indices, and leaf elemental composition, and we derived biomass allocation indices and aboveground nutrient content per seedling. Across ash rates, inoculated plants showed higher shoot and total dry biomass and greater shoot length than non-inoculated controls, with estimated marginal means indicating increases of approximately 31% in shoot biomass and approximately 26% in total biomass when averaged across ash rates. Across planted treatments, increasing ash generally reduced water holding capacity (WHC) and cation exchange capacity (CEC) while increasing substrate pH and available phosphorus within 28 days; respiration indices responded to both ash dose and inoculation, with ash-rate dependence in respiration responses. Leaf elemental patterns also shifted, with consistently higher leaf phosphorus under inoculation and marked treatment-dependent responses in micronutrients, especially iron, manganese, and zinc. Overall, this short-term bioassay captures short-term physicochemical and biological re-equilibration after ash–soil mixing and reveals biologically mediated shifts in establishment, providing a practical basis to compare ash incorporation rates and management options in subsequent, longer-duration trials.