RADIOCARBON DATING FOR DETERMINATION OF SOIL ORGANIC-MATTER POOL SIZES AND DYNAMICS

The size and turnover rate of the resistant soil organic matter (SOM) fractions were measured by C-14 dating and C-13/C-12 measurements. Thi s involved soils archived in 1948, and recent samples, from a series o f long term sites in the North American Great Plains. A reevaluation o f C dates obtained in the 1960s expanded the study scope. The C-14 age s of surface soils were modern in some native sites and near modern in the low, moist areas of the landscape. They were much older at the ca tena summits. The C-14 ages were not related to latitude although this strongly influenced the total SOM content. Cultivation resulted in lo wer C contents and increased the C-14 age by an average of 900 yr. The 10- to 20-cm depths from both cultivated and native sites were 1200 y r older than the 0- to 10-cm depth. The 90- to 120-cm depth of a culti vated site at 7015 yr before present (BP) was 6000 gr older than the s urface. The nonhydrolyzable C of this depth dated 9035 yr BP. The resi due of 6 M HCl hydrolysis comprised 23 to 70% of the total soil C and was, on the average, 1500 Sr older. The percentage of nonhydrolyzable C and its C-14 age analytically identify the amount and turnover rate of the old resistant soil C.