Functional Relationships of Soil Acidification, Liming, and Greenhouse Gas Flux

Soil acidification can be accelerated by intensive farming or prevented by sustainable management practices. Soil acidification in a managed agricultural production system is caused by the transformation of carbon (C), nitrogen (N), and sulfur (S), which releases protons (H+) to soil solution. Soil acidification decreases soil pH, causing adverse effects on plants and soil microorganisms. Acidification, coupled with aluminum, manganese, and iron toxicities, and phosphorus, calcium, magnesium, and potassium deficiencies, can lead to low soil fertility. Soil acidity influences soil C and N cycles by controlling activities of microorganisms involved in the transformations of these two elements. Traditionally, lime materials are added to neutralize acidic soils and to overcome the problems associated with soil acidification, but they also influence C and N cycles, thereby affecting greenhouse gas (GHG) flux in soils. For example, liming has been shown to decrease nitrification-induced nitrous oxide (N2O) emission from many agricultural lands. However, there are concerns that liming increases the availability of soil nitrate (NO3-), which is a substrate for N2O emission through denitrification. The dissolution of liming materials can act as either a net source or sink for carbon dioxide (CO2). Lime-derived CO2 reacts with microbial respiration-derived carbonic acid in soils to yield carbonate material, serving as a sink of CO2 in soil. In calcareous soils with high pH, agricultural lime (CaCO3) serves as a net sink for CO2 whereas in acid soils it serves as a net source of CO2. In acid soils, increased availability of aluminum (Al3+) ions inhibits activity of methane (CH4) oxidizers. Adding lime to soils has shown to increase CH4 oxidation and reduce GHG emission. The present review brings together basic concepts of soil acidification and recent developments on the implications of liming in relation to C and N transformations and cycling, particularly GHG emissions from soils. Given the major influence of lime addition on soil microorganisms relating to C and N cycles, future research should focus on the role of liming on soil microbial communities to provide insight into combined mitigation of N2O, CO2, and CH4 gases from agricultural soils.