Volcanic Deposits Thickness and Distance from Mt Semeru Crater Strongly Affected Phosphate Solubilizing Bacteria Population and Soil Organic Carbon

Volcanic eruptions cause large-scale damage and leave piles of volcanic material that destroy plants, agricultural lands, animals, and soil microorganisms, decreasing soil fertility. Therefore, it is necessary to accelerate soil fertility recovery in post-volcanic eruption areas to resume agricultural activities. This study aims to elucidate the effect of volcanic deposits on soil fertility as well as explore tolerant plants and bacteria after Mt Semeru eruption. Soil, volcanic ash, and plant samples were collected from Pronojiwo Sub-regency, Lumajang Regency, East Java, Indonesia. Soil and volcanic ash chemical properties were analyzed (pH, available and total phosphorus (P), potassium (K), magnesium (Mg), and calcium (Ca) content). Bacteria were isolated and enumerated, then tested for P solubilization (PSB). The result showed that 3 months after Mt Semeru’s eruption, the first succession was fern, moss, and fungi. Some local plants (banana and coconut) emerge new shoots and recover. A high total P (137.32 mg/kg) with neutral pH 6.8 was found in the volcanic ash. Total P and available P were higher at the closest distance from the crater, and soil pH controlled P availability in the soil covered with volcanic deposits. Also, the thickness and distance from the crater strongly affect organic C, which reduces the PSB population from 10³to 10⁴ CFU/g, compared to unaffected areas. The bacteria exhibited P solubilization activities even under harsh environmental conditions. Thus, accelerating soil fertility restoration by adding organic materials and inoculating beneficial bacteria (such as PSB) in the post-eruption area is essential as the bacteria benefit both soil fertility recovery and agriculture sustainability in degraded lands (e.g., post-eruption).