Tripled plant productivity and soil microbiome restructuring in a greenhouse volcanic ash fertilization experiment

Volcanic ash is widely held responsible for fertilizing soils, but the processes and conditions leading to volcanic soil fertility remain poorly understood. We report here the results of a greenhouse volcanic ash fertilization (VAF) experiment aimed to explore the impact of basaltic ash addition (0–10 wt%) to soil on plant growth, nutrient uptake, and the soil microbiome. Our four-month experiment with the Coleus species Plectranthus scutellarioides on forest soil revealed a non-linear growth response with distinct growth patterns below and above 3 wt% ash addition, respectively. Low ash VAF (<3 wt% ash) had a negligible growth impact, while high ash addition yielded a threefold increase in biomass production, reproductive effort, and total nutrient uptake in aboveground plant biomass. An increased uptake of nutrients that are not part of the ash itself (e.g., nitrogen) implies that VAF was not a direct nutrient addition process but rather acts indirectly. The soil microbiome composition (16S rRNA; Bacteria/Archaea, and 18S rRNA; Eukaryota, rRNA gene markers) also changed at >3 wt% ash concentrations, with no apparent further change with increasing ash content. Key changes were not only an increased relative abundance of several potentially plant-growth-promoting rhizobacteria and fungi but also a decreased nematode abundance, suggesting that changes in the microbiome are likely a major factor for the substantial VAF effect observed in our experiment. We conclude that VAF with basaltic ash offers substantial potential benefits for agriculture as well as post-eruptive plant productivity on soils possibly associated with significant CO₂ sequestration from the atmosphere. However, VAF is a complex process that does not act by simple nutrient addition from its chemical inventory but rather acts by restructuring the soil growth environment.