Seasonal distribution of runoff and soil loss under four tillage treatments in the upper St. John River valley New Brunswick, Canada

The effect of four different tillage treatments on surface runoff and soil loss, their seasonal distribution and temporal variation in soil erodibilit y were examined using runoff-erosion plots (10 m wide x 30 m long), on a Ho lmesville gravelly loam soil, a major soil type used for potato production in New Brunswick. Fall moldboard plowing, fall chisel plowing, spring moldb oard plowing and subsoiling followed by fall moldboard plowing were evaluat ed under barley and fallow conditions on 8 and 11% slopes between 1989 and 1993. With exception of one year, annual precipitation was lower than norma l. However, due to higher rainfall during the cropping season, the calculat ed erosivities were higher than those typically used for conservation plann ing in this region. Runoff data revealed that fall moldboard plowing genera ted the highest runoff. Either performing subsoiling prior to fall moldboar d plowing or delaying moldboard plowing until the next spring, reduced runo ff by approximately 10%. Chisel plowing, which loosens the soil without inv erting it and leaves a large amount of residues on the surface, provided by far the greatest benefit in reducing runoff (20% reduction over fall moldb oard plowing). Soil loss from fall moldboard plowing on the 11% slope under fallow was 2.8 and 2.6 times greater than from spring moldboard plowing an d fall chisel plowing, respectively. The majority of the difference in soil loss occurred during the summer months. The benefit of spring moldboard an d fall chisel plowing was considerably less on the 8% slope in which soil l oss from the fall moldboard plowing was only 24 and 19% higher than spring moldboard and fall chisel plowing, respectively, indicating that the benefi ts are slope dependent and increase with increasing slope from 8 to 11%. Wh en the plots were planted in Chapais barley (Hordeum vulgare L.), soil loss es were negligible. Tillage treatments, and particularly cropping practices , play a major role in seasonal distribution of runoff and soil loss. Under fallow, approximately 79% of runoff and 8.1% of soil loss occurred during the non-cropping season whereas 96% of runoff and 68% of soil loss were fou nd when the plots were planted in barley. The soil erodibility factor was t wo to three times higher during March and April, which coincide with the wi nter-spring thaw period, than during the rest of the year. This seasonal va riation must be considered when using event-based models to predict soil lo sses.