Abstract
A field experiment was conducted for 8 yr (from 1994 to 2001) in a Black Chernozem (Udic Haploboroll) high in organic C (5.5%) at Melfort, Saskatchewan, Canada, to assess the environmental impact of tillage and crop rotation. Short-term effects on residue cover, soil temperature, moisture, aggregation, bulk density and hydraulic conductivity were measured, and long-term effects on total organic C (TOC) and N (TON), and light fraction organic C (LFOC) and N (LFON) were determined. There were two tillage systems [no tillage (NT) and conventional tillage (CT)] and three 4-yr annual crop rotations (canola-wheat-barley-barley,canola-barley-pea-wheat,and canola-pea-flax-barley). Straw was returned to soil every year in all treatments with the exception of flax residue. Soil samples were taken for analysis of organic C and N after the harvest of canola at the end of second 4-yr rotation cycle. Residue cover on soil was higher under NT than CT. Soil temperature was lower and soil moisture was higher in spring, under NT than CT. The proportion of fine aggregates (< 1.3 mm) in soil was lower and that of medium (2.0–12.7 mm) and large aggregates (> 12.7 mm) was generally higher under NT compared with CT. The mean weight diameter (MWD) of aggregates was also larger under NT compared with CT in most cases. The proportion of fine aggregates was higher after canola compared with pea in some cases, but the proportion of medium and large aggregates was higher after pea compared with canola, and in rotations with 50% than 75% broadleaf crops in 2yr. Soil hydraulic conductivity and bulk density were lower under NT than CT in some instances. Compared with CT, NT had greater mass of TOC, TON, LFOC and LFON in soil. However, the differences were significant only for LFOC and LFON, in dicating that light fraction organic C and N was more responsive to NT than CT compared with total organic C and N. In conclusion, the findings suggest that there is no interaction between tillage and rotation, and that light fraction C and N accumulate when tillage is eliminated despite high levels of soil organic C in the soil. Key words: Aggregation, bulk density, crop rotation, hydraulic conductivity, organic C and N, moisture, residue cover soil, temperature, tillage
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