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Effects of historical land use change on soils in the Fraser lowland of British Columbia and Washington

University of British Columbia

Four objectives were established for this study: 1) to examine historical changes in land clearing during the period 1943 to 1983, 2) to determine the effects of the conversion from woodland to agriculture on soil properties during this period and its influence on soil genesis, 3) to determine the effects of land use on soil variability, and 4) to determine the effects of a political boundary on all of the above. The study is unique in its examination of temporal changes in soil properties based on the interpretation of historical land use changes and the effects of an international boundary and consequent management differences on several parent materials. Soils were collected from the surface 0.2 m by stratified random sampling using a 900 m² grid. Plots were chosen in duplicate and represent the two countries (Canada and the United States), the three parent materials (alluvium, outwash, and glacialmarine drift), and the five land clearing age groups (cleared between 1943 and 1955, between 1955 and 1966, between 1966 and 1976, between 1976 and 1983, and not cleared, i.e., woodland). The soils were analyzed for pH in H₂O and CaCl₂, calcium, magnesium, potassium, phosphorus, organic matter (OM), and nitrogen. Clearing and the development of land on outwash soils after the mid-19^0s was much more rapid and management more intensive in Canada than in the USA as a result of the rapid growth in local population, markets, and technology. Most of the alluvial soils were cleared prior to 1920 on both sides of the International Boundary and have been used predominantly for dairy farming since that time. Few differences have occurred in clearing practices by country after 1920. During the 1940s glacialmarine soils were largely in woodland and subsequent clearing was slow. They are used principally for woodland and pasture. The most important variables for distinguishing the variation as a whole are pH, Ca, Mg, K, OM, and N. Only pH and OM are important in expressing a linear relationship with time-since-clearing. Time-since-clearing can be predicted from soil properties, but the degree of predictability, which is commonly 60%, depends on parent material and country. Variability is in the order alluvium < outwash < glacialmarine. In comparison to CVs reported in the literature, those in this study are lower for OM and N, and comparable for bulk density, pH, Ca, Mg, K, and P. No pattern with time occurs among the four cultivated age groups, nor is there a time trend by land use for any of the parent materials. Variability differences due to country are also mixed in spite of management differences. Commonly 50 to 80% of the variation within all cultivated soils combined is within any individual 0.09 ha cultivated plot. Many variables on each parent material have CVs larger than 80 and so require more than 1000 samples to estimate their respective means within 5% of their original values and more than 60 samples to estimate the means within 20%. The principal anthropogenic effects on soil genesis are the following: 1) pH increases dramatically on all soils from initial levels in woodland. 2) The levels of cations are determined by the initial fertility of the soil. Both outwash and glacialmarine soils have low initial fertility and increase 2 to 15 times after cultivation in proportion to the degree of management. 3) Cationlevels decrease from initial levels on alluvial woodland soils, which have high fertility. A quasi-steady state for most variables is reached in about 15 to 25 years. 4) Cultivation results in losses of OM of 20% after 35 years on all soils, the largest loss coming in the first 15 years. 5) N levels are inconsistent. 6) Steady state for OM and N is not apparent within 35 years. 7) C:N narrows on all soils from about 15:1 to about 12:1. 8) Cultivation increases bulk density by 26% to 58%. 9) Differences by country are greatest on outwash soils, where the higher intensity of management in Canada leads to higher levels of all soil chemical properties. Trends in soil properties with time are similar by country, differing mainly by degree.

Text

2010-08-06

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http://hdl.handle.net/2429/27101

Soil Science

University of British Columbia

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