Volume Matters: Dilution of Soil Inoculum Reduces Positive Plant-Soil Feedback in Pinus radiata Seedlings

Soil conditioning can generate persistent plant-soil feedbacks (PSF) that influence plant performance under subsequent growth conditions, yet the role of soil inoculum volume in mediating these effects remains poorly understood. Here, we tested how inoculum volume influences the relative strength of a known positive PSF effect. We performed a plant-soil feedback experiment with Pinus radiata D. Don in two phases: one, a "conditioning phase", and two, a "feedback phase", where inoculum from the first phase was used in different dilutions to test the growth differences resulting from conditioning. To understand how inoculum volume affects subsequent growth in the feedback phase, seedlings (n = 12 per treatment) were grown in soil from phase one using different volumetric dilutions; 100% conditioned soil, 50% conditioned soil + 50% inert media, or 25% conditioned soil + 75% inert media. Positive plant-soil feedbacks were observed in undiluted soils: seedlings produced 40-65% greater biomass and experienced 50-70% lower mortality compared to the lowest inoculum treatment. However, this response varied with dilution; the strength of plant-soil feedbacks decreased with increasing dilution of inoculum. These findings highlight soil inoculum volume as an important, but often overlooked, factor in plant-soil feedback experiments and applied soil management. Our study provides experimental evidence that effective soil conditioning depends on both conditioning and a required minimum inoculum volume to confer measurable benefits to future plantings.