Changes in soil organic carbon storage under grassland as evidenced by changes in sulphur input–output budgets

Information about temporal changes in soil organic carbon (C) pools may be obtained indirectly from changes in input–output budgets of organically combined nutrients such as sulphur (S). Sulphur budgets were therefore evaluated for Northern Ireland (NI) for the period 1940–1990, inclusive. These budgets indicated that the land or soil had acted first as a sink but then as a source for S, and that reserves of soil S built up between 1940 and 1965 were totally depleted by the mid-1980s. Pooled data from six long-term soil-monitoring sites on undisturbed grassland suggested that negative S budgets from the late-1970s onwards had been due to the net mineralization of soil organic matter and thus were indicative of net losses of organic C from surface soil horizons. There was some evidence that the decline in rainfall and fertiliser S inputs from the mid-1960s may have precipitated the breakdown of soil organic matter.