Soil Acidification:

Natural Processes and Human Impact

Abstract

In humid regions, soil acidification is a natural process accelerated by acidic deposition and intensive agriculture. Proton budget analysis has revealed that excess cation uptake by plants is the dominant proton source in most forest ecosystems. However, translocation of temporary acids (e.g., organic acids) and distribution of root biomass contribute to the temporal and spatial heterogeneity of proton generation and proton consumption in soils, resulting in different pedogenetic acidification. In Spodosols, protons associated with plant uptake and organic acids released from organic horizons promote acidification of surface horizons. Incipient podzolization is similarly caused by plant uptake and organic acids in highly acidic Inceptisols and Ultisols (soil pH<4.5). In less acidic Andisols and Ultisols, weathering reactions and leaching are caused by carbonic acid dissociation. In glacial till and loess soils exhibiting low acid neutralizing capacity, acidification is accelerated by acidic deposition. In tropical forest soils from highly weathered parent material, timber harvesting leads to depletion of basic cations and soil acidification. In Asia, the conversion of traditional agricultural systems to continuous cropping systems has resulted in soil acidification through nitrification of excess fertilizer nitrogen (esp., ammonium sulfate) within the last few decades. The sensitivity of soils to human impact is greater in soils exhibiting a lower acid neutralizing capacity and soil organic matter (SOM) content. The impact of agriculture on soil acidification may be ameliorated by improving nitrogen fertilization strategies and maintaining SOM levels through proton budget analysis.