EFFECTS OF HISTORICAL LAND CLEARING ON ORGANIC-MATTER AND NITROGEN LEVELS IN SOILS OF THE FRASER LOWLAND OF BRITISH-COLUMBIA, CANADA AND WASHINGTON, USA

Two soils derived from glacial deposits and one from alluvium were examined for changes in organic matter (OM) and nitrogen levels between 1943 and 1983 across the international boundary in the Fraser Lowland of western North America. Duplicate samples were collected from the upper 0.2 m from 60 randomly selected sites on the five land clearing periods (1943-1955, 1955-1966, 1966-1976, 1976-1983, uncleared) on each of three parent materials: glacial outwash, alluvium, and glacialmarine deposits in the United States and Canada. The land clearing periods were determined from medium scale aerial photographs taken in 1943, 1955, 1966, 1976, and 1983. Cultivation results in a 20% loss of OM after 35 yr on all soils. The largest loss is in the first 15 yr on all soils. Changes in N levels on the three parent materials is irregular. C:N narrows on all soils from about 15:1 to about 12:1.