Soil Organic Matter Turnover in Long‐term Field Experiments as Revealed by Carbon‐13 Natural Abundance

Abstract Cultivation of C‐3 (Calvin cycle) plants after turning under a native prairie of grasses possessing C‐4 metabolism (Hatch‐Slack cycle) allows partitioning of soil organic matter as to origin. Contrasting natural 13 C abundance for the two different contributing kinds of plant residues provides an in situ labeling of the soil organic matter. A study to exploit this approach used soil from a long‐term experimental field. Samples were collected under different continuous crops and at different dates and were analyzed for total C and for natural isotope composition of C. The results demonstrated an initial rapid loss of prairie C, suggesting the presence of an easily mineralizable component that was exhausted in 30 to 40 yr. The extent of this was dependent upon the type of cultivation. A pool of stable organic matter of prairie origin persisted under all systems during the approximate 100 yr of cultivation, which was no less than 50% of the current level. The half‐life, under cultivation, of the easily mineralizable compartment was shown to be 10 to 15 yr. Mechanical fractionation of soil mineral particles and associated organic matter showed a heterogeneity with regard to stability. The clay fraction included both the most stable C, and some labile C, whereas clay associated with fine silt demonstrated a turnover that suggested slow replacement of the whole of the fraction. The overall results demonstrate the potential value of long‐term field experiments for studying soil organic matter turnover by the natural 13 C abundance technique.