CALCULATING THE ANNUAL INPUT OF ORGANIC-MATTER TO SOIL FROM MEASUREMENTS OF TOTAL ORGANIC-CARBON AND RADIOCARBON

Measurements of total organic C, deltaC-13 and deltaC-14 are given for topsoils taken from six experimental sites in southern England. At each site, some of the samples were collected before and some after the thermonuclear tests of the early 1960s, so that pre- and post-bomb samples could be compared for radiocarbon content. The current Rothamsted model for the turnover of organic C in soil gave an acceptable fit to the data from five of the sites, apart from one aberrant radiocarbon measurement. The annual input of C to the topsoil was calculated for the five sites from these fits; the values obtained were: 0.15 t C ha-1 a-1 for a site on silty clay loam, kept bare by hand weeding since 1870; 0.2 for unmanured spring barley growing on a sandy loam; 2.95 for a fertilized all-arable rotation on a loamy sand; 1.9 for the same fertilized all-arable rotation on a silty loam and 2.5 from this rotation on a calcareous silty loam. The corresponding values for Net Primary Production at the five sites were 0.15, 0.76, 5.16, 5.71 and 5.46 t C ha-1 a-1. In fitting the model to the radiocarbon data it was necessary to postulate that all these sites contained substantial quantities of biologically-inert organic matter, ranging from 2.2 to 10.0 t C ha-1.