The Fateof Agricultural “Lime” Amendments: Implications forTerrestrial Carbon Sequestration

Hamilton, S.K., G.P. Robertson, and A.L. Kurzman

Presented at the All Scientist Meeting (2002-10-04 )

Soil organic matter dynamics and greenhouse gas emissions generated by row-crop systems under various management schemes have been investigated intensively for over a decade at the Kellogg Biological Station’s Long-Term Ecological Research (KBS LTER) site.  Recent synthesis of this information permitted estimation of the global warming potential of various row-crop management systems (Robertson et al. 2000), and revealed that soil amendments of “lime” [usually limestone CaCO₃ or dolomite CaMg(CO₃)₂] appear to play a major role in the carbon budgets of Midwestern agricultural soils.  Massive amounts of carbonate minerals are mined and spread over agricultural soils to counteract acidification, and this represents a major yet overlooked anthropogenic flux of carbon associated with intensive agriculture.  Robertson et al.’s calculations assumed that the inorganic carbon in these minerals eventually becomes carbon dioxide as the lime is consumed, yet in fact the fate of this material is not clear.  New measurements of soil solution chemistry in experimental treatments at the KBS LTER suggest that added lime can act as either a source or a sink for atmospheric carbon dioxide depending on the biogeochemical reactions in the upper meter of soil (especially those affecting the acid-base chemistry). New research is being planned that will investigate the fate of lime amendments in conventional row cropping systems to determine whether they result in a net source or sink for carbon dioxide, and to discover the mechanisms that are involved.  Our global hypothesis follows:Differences across treatments in the rate and nature of reaction of lime with the soil solution are explained primarily by acidifying reactions mediated by microbes in association with various plant-soil systems, and particularly by differences in rates of nitrification associated with fertilization or nitrogen fixation.  A reduction in the rate of acidifying processes and/or an increase in the availability of lime in the soil will shift the net carbon balance associated with liming from a CO₂ source to a CO₂ sink via the promotion of carbonic acid weathering. One way to examine this hypothesis is through controlled experimental additions of lime to cropping systems at the KBS LTER site.  We are currently evaluating the options for such experiments and planning for the collection of pre-liming data.  The experiments would entail monitoring soil solution chemistry over several years following the lime addition.  Ideally, the loss of carbon and other elements from the soil profile via gas emissions, infiltrating waters, and harvest of crops will be monitored, and the inorganic carbon remaining in the soil profile will be measured.  This would allow construction of a mass-balance budget for the carbon added as lime.  In addition, a suite of biogeochemical variables should be measured to construct an acid-base budget for the soil profile, which will reveal the major chemical and biological processes that control the dissolution of lime and the fate of the resultant solutes.  This would reveal the fate of lime amendments under tillage and no-till management and also examine how interactions between lime and N fertilization determine the ultimate fate of the carbon in the lime. If the hypothesis proves to be supported, then management options to ameliorate the global warming potential associated with the widespread use of limestone amendments could be considered, and these might be easier to implement than options to enhance soil carbon storage.  Such options may entail adding more lime rather than less because the dominant reaction of lime dissolution at circumneutral pH — carbonic acid weathering — actually sequesters soil carbon dioxide as bicarbonate, which is then subject to transport downward by infiltrating waters.