Atmospheric CO2 enrichment and reactive N inputs interactively increase cation release and soil acidification

Background/Question/Methods

Atmospheric CO₂ enrichment often stimulates plant growth and C allocation belowground. CO₂-enhancement of root and microbial activities increases H⁺ production, which may increase cation dissolution and replacement from soil colloids. However, it remains unknown whether and how elevated CO₂ will, along with reactive N, impact the dynamics of soil nutrient cations and soil acidity. We conducted two independent but complimentary experiments to examine the interactions of elevated CO₂ and reactive N inputs on soil cation release, and the resulting effects on soil acidity in an upland system. We measured the effect of combined elevated CO₂ and O₃ on Ca²⁺ and Mg²⁺ in soil solutions in a field wheat-soybean system, using both lysimeters and PRS probes. We also examined the effect of N inputs on wheat growth and soil cation release through manipulating N species (NH₄⁺- vs. NO₃^--N) and quantities (80 vs. 160kg N per ha.) under ambient and elevated (ambient plus 200ppm) CO₂ concentrations.

Results/Conclusions

Our results showed that atmospheric CO₂ enrichment and N inputs, alone and in concert, increased Ca²⁺ and Mg²⁺ in soil solutions or leachates. Elevated CO₂ increased soluble Ca²⁺ and Mg²⁺ in soil leachates by 132% and 96%, respectively, and this effect was higher under NH₄⁺-N than NO₃^--N treatment. Elevated CO₂ also caused a small, but significant decrease in soil pH by 0.2 units, and chemical N inputs amplified this effect. This is, to our knowledge, the first demonstration of elevated CO₂ enhancement of soil acidity. Together, these findings provide new insights into the dynamics of cation nutrients and soil acidity under future climatic scenarios.