The impact of the 1991 Plinian eruption of Mt. Pinatubo, Philippines on soil development, phytolith accumulation, phosphorus status, and crop productivity

The catastrophic 1991 eruption of Mt. Pinatubo in the Philippines inflicted severe damages to agriculture. It blanketed the landscape with a thick layer of unconsolidated silica-rich tephra. The remobilization of this material by rain produced debris flows (or lahars) that flooded and buried the lowland soils. Nearly three decades after this catastrophic event, agriculture has not yet fully recovered and adapted, making the livelihoods of farmers more vulnerable to a future shock. The objectives of this Ph.D. research were to assess the impacts of the eruption on soil properties and development. We investigated (i) the weathering of the pyroclastic (tephra and lahar) deposits from which a new soil is being formed; (ii) the accumulation of biogenic silicon (phytolith) and (iii) the status of phosphorus, a typically limiting nutrient, in the newly formed soils under different vegetation covers. These analyses were compared to the properties of the ~500 years old soils buried by the 1991 volcanic products. Finally, we interviewed the small-scale farmers in order to identify the main problems that they face while reestablishing agriculture in the tephra- and lahar-impacted area. We show that very limited weathering has taken place in the recent tephra and lahar deposits. However, in most cases, the ~500 years buried soil exhibits roughly similar properties, suggesting slow weathering reactions despite a tropical climate. The early weathering of the fresh pyroclasts seems to promote biogenic silicon accumulation in the newly formed soil. Phosphorus is not limiting plant growth as the tephra and, to a lesser extent, the lahar materials contain ample bioavailable phosphorus. The farmers complain about the highly heterogeneous nature of the substrate on which they now have to cultivate. This exhibits peculiar hydrological properties that are probably responsible for poor or excessive drainage conditions (depending on the location), high acidity and reduced nutrient retention. Our results provide new insights into the protracted and largely overlooked effects of tephra and lahar deposits on agricultural soils. It paves the way for future studies aimed at favouring recovery and adaptation in tropical regions devastated by volcanic eruptions.