Soil organic 14C dynamics:: Effects of pasture installation on arable land

In a study addressing composition and recovery of soil carbon following pasture installation on arable land, radiocarbon isotope ratios were measured in size- and density-separated soil organic matter (SOM) fractions in a pasture and maize plot. The average soil carbon age increased with depth from 444 yr in the 0-30-cm layer to 2456 yr in the 60-80-cm layer in the pasture soils, and from 42 to 1625 yr in the maize-cultivated soil. Weight fractionation of the macro-organic matter (size >150 mu m) in a light (density <1.17 g cm(-3)) intermediate (1.17 g cm(-3) < density < 1.37 g cm(-3)), and heavy fraction (density >1.37 g cm(-3)) resulted in markedly different ages for different fractions with ages increasing from 2 yr in the light fraction to >3000 yr in the heavy fractions. (13)C and (14)C (accelerator mass spectrometry (AMS)) isotope ratios in the <20 mu m fraction in the 60-80-cm layer indicated that vertical displacement of colloidal organic material occurred during maize cropping. The physical fractionation method, in combination with natural level (13)C and (14)C analysis, resulted in a better insight in carbon dynamics that occur after the conversion of arable land to pasture.