Soil silicon dynamics following long-term litter and nutrient manipulations in a lowland tropical forest

Silicon (Si) has an important role in forest ecology, but controls on Si availability in tropical forests remain poorly understood. For example, it remains unclear to what extent recycling from litterfall maintains Si availability in the soil, or whether Si is influenced by the dynamics of other essential nutrients. To address this, we quantified soil Si pools in two long-term experiments in lowland tropical forest in Panama: (i) a litter manipulation experiment involving 15 years of litter addition and removal, and (ii) a fertilizer experiment involving 20 years of nitrogen, phosphorus, and potassium addition. We quantified available Si extracted in 0.01 M CaCl2, reactive pedogenic Si extracted in acidic ammonium oxalate, and biogenic Si extracted in hot 1% Na2CO3. Fifteen years of litter manipulation reduced available Si by 17% where litter was removed and increased it by 53% where litter was added. Litter addition also increased reactive pedogenic Si by 43%, but there was no change with litter removal. Twenty years of nitrogen addition reduced available Si by 22% and pedogenic Si by 25%, but there were no significant responses to the addition of phosphorus or potassium. The decline in Si with nitrogen addition appears to be related to the mobilization and leaching of available Si during soil acidification. Biogenic Si did not respond to any treatment, including long-term litter addition involving an estimated annual return of 7.64 g Si m-2 in leaf litterfall, suggesting that extraction with Na2CO3 is unsuitable for quantifying phytoliths in clay-rich tropical soils. Overall, these findings demonstrate the importance of litterfall in maintaining Si availability in tropical forests and highlight a rarely considered consequence of atmospheric nitrogen addition and associated soil acidification for the tropical Si cycle.