Atmospheric CO₂ Enrichment and Reactive Nitrogen Inputs Interactively Stimulate Soil Cation Losses and Acidification

Reactive N inputs (Nr) may alleviate N-limitation of plant growth and are assumed to help sustain plant responses to the rising atmospheric CO₂ (eCO₂). However, Nr and eCO₂ may elicit a cascade reaction that alters soil chemistry and nutrient availability, shifting the limiting factors of plant growth, particularly in acidic tropical and subtropical croplands with low organic matter and low nutrient cations. Yet, few have so far examined the interactive effects of Nr and eCO₂ on the dynamics of soil cation nutrients and soil acidity. We investigated the cation dynamics in the plant-soil system with exposure to eCO₂ and different N sources in a subtropical, acidic agricultural soil. eCO₂ and Nr, alone and interactively, increased Ca²⁺ and Mg²⁺ in soil solutions or leachates in aerobic agroecosystems. eCO₂ significantly reduced soil pH, and NH₄⁺-N inputs amplified this effect, suggesting that eCO₂-induced plant preference of NH₄⁺-N and plant growth may facilitate soil acidification. This is, to our knowledge, the first direct demonstration of eCO₂ enhancement of soil acidity, although other studies have previously shown that eCO₂ can increase cation release into soil solutions. Together, these findings provide new insights into the dynamics of cation nutrients and soil acidity under future climatic scenarios, highlighting the urgency for more studies on plant-soil responses to climate change in acidic tropical and subtropical ecosystems.