Comparison of soil acidification rates under different land uses in Indonesia

Aims Increased nitrification risks accelerating soil acidification under land-use change from tropical forest to cropland or plantation, but acidity might be neutralized by net mineralization of soil organic matter (SOM) that has accumulated under the former fallow vegetation. We aim to analyze key drivers of soil acidification and acid neutralization under different crops and fallow vegetation. Methods We compared 30 year changes in soil C stocks, pH, and exchangeable Al under different land-uses (primary dipterocarp forest, Macaranga forest, Imperata grassland, Imperata grassland converted to Acacia plantation, Imperata grassland converted to oil palm plantation) in Indonesia and identified the major drivers of acidification using proton budgets in soil. Results Nitrification is major driver of acidification in soil profiles under N-fixing Acacia or fertilized oil palm plantation. Protons generated by product removal and nitrification in oil palm plantation are neutralized partly by fertilizer and mineralization of the native SOM that has formerly accumulated under grassland. The remaining acidity results in accumulation of soil exchangeable Al. The SOM storage under short-term (< 10 yr) grass or long-term (> 10 yr) Macaranga forest increases without intensive acidification, while long-term Acacia plantation maximizes both SOM storage and acidification. Conclusion Magnitude of soil acidification is regulated largely by the balance between proton generation by nitrification of fertilized N and biologically-fixed N and proton consumption by net mineralization of the native SOM that has formerly accumulated under grass fallow. SOM accumulation under Imperata grassland or Macaranga forest could mitigate soil acidification in Acacia or oil palm plantation.

Automated summary

The study shows that nitrification is a major driver of soil acidification in Acacia plantation or fertilized oil palm plantation. Soil organic matter accumulated under Imperata grassland or Macaranga forest can partially neutralize the acidity generated by nitrification, mitigating soil acidification in these areas.