Evidence for carbon sequestration by agricultural liming

Agricultural lime can be a source or a sink for CO 2 , depending on whether reaction occurs with strong acids or carbonic acid. Here we examine the impact of liming on global warming potential by comparing the sum of Ca 2+ and Mg 2+ to carbonate alkalinity in soil solutions beneath unmanaged vegetation versus limed row crops, and of streams and rivers in agricultural versus forested watersheds, mainly in southern Michigan. Soil solutions sampled by tension indicated that lime can act as either a source or a sink for CO 2 . However, infiltrating waters tended to indicate net CO 2 uptake, as did tile drainage waters and streams draining agricultural watersheds. As nitrate concentrations increased in infiltrating waters, lime switched from a net CO 2 sink to a source, implying nitrification as a major acidifying process. Dissolution of lime may sequester CO 2 equal to roughly 25–50% of its C content, in contrast to the prevailing assumption that all of the carbon in lime becomes CO 2 . The ∼30 Tg/yr of agricultural lime applied in the United States could thus sequester up to 1.9 Tg C/yr, about 15% of the annual change in the U.S. CO 2 emissions (12 Tg C/yr for 2002–2003). The implications of liming for atmospheric CO 2 stabilization should be considered in strategies to mitigate global climate change.