Atmospheric CO 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 CO (eCO). However, Nr and eCO 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 eCO on the dynamics of soil cation nutrients and soil acidity. We investigated the cation dynamics in the plant-soil system with exposure to eCO and different N sources in a subtropical, acidic agricultural soil. eCO and Nr, alone and interactively, increased Ca and Mg in soil solutions or leachates in aerobic agroecosystems. eCO significantly reduced soil pH, and NH-N inputs amplified this effect, suggesting that eCO-induced plant preference of NH-N and plant growth may facilitate soil acidification. This is, to our knowledge, the first direct demonstration of eCO enhancement of soil acidity, although other studies have previously shown that eCO 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.