Effect of land-use history on the potential for carbon sequestration in an Alfisol

The potential of agricultural soils to sequester soil organic carbon (SOC) is affected by changes in the capacity of the mineral matrix to hold and protect SOC that are brought about by agriculture. It is well established that microaggregates provide much of the capacity of soils to protect SOC. In this study, a loss of the capacity of soils to sequester SOC following agriculture was established by comparing samples of an Alfisol, in (1) its virgin condition, (2) following relatively light treatments for agriculture, (3) following conventional cultivation for similar to 120 years, and (4) after 18 years under zero tillage following similar to 100 years under conventional cultivation. It appears that material has been lost from clay and silt fractions relative to sand-size material. Microaggregates in all of the cultivated soils studied were less stable to the 'chemical hammer' of osmotic pressure than those in the virgin soil. The introduction of no-till methods did not restore the stability of microaggregates over a period of 18 years following similar to 100 years of conventional cultivation. Pore size distributions from mercury intrusion showed that cultivation always led to a decrease of pore volume in the range of pores between 0.03 and 100 mu m. Transmission electron microscopy showed that a large proportion of microaggregates, previously composed of clay and silt-sized particles, had disintegrated as a result of conventional soil management over the long-term. Surface analysis by scanning electron microscopy and photoacoustic infrared spectroscopy both showed that intimate associations between fine and coarse material have been broken down by intensive soil management practices. The effects on particle size distributions by chemical disruption with either or both hydrogen peroxide and acid ammonium oxalate, and also with alkaline sodium hexametaphosphate, suggested that microaggregates are stabilised by a combination of organic matter, poorly ordered minerals and clay self-associations, regardless of the agricultural history of the soils. Microaggregates appear to be broken down by agriculture through the loss of any of these aggregating agents. The capacity of soils to form microaggregates will also be affected by loss of soil. It appears that both soil, as a whole, and also clay- and silt-size material, selectively, has been lost through wind erosion following prolonged cultivation. Agriculture has led to a decrease in the capacity of the Alfisol to resequester SOC that is effectively irreversible by farming practices. Even so, the resilience of the soil appeared to be improved by changes in farming practices. (C) 2010 Elsevier B.V. All rights reserved.