EFFECT OF UNDERSOWING BARLEY WITH ANNUAL RYEGRASSES OR RED-CLOVER ON SOIL STRUCTURE IN A BARLEY-SOYBEAN ROTATION

Barley (Hordeum vulgare L.), in short-term barley-soybean (Glycine max L.) rotations in Atlantic Canada, was undersown with annual ryegrasses (Lolium multiflorum Lam.) or red clover (Trifolium pratense L.) to assess the effects of undersowing on the distribution of carbon within soil aggregates and on aggregate stability. The soil was a structurally unstable fine sandy loam (Podzoluvisol). Soil structural stability was assessed at the micro- and macro-aggregate scale using turbidimetry and wet sieving, respectively. Both ryegrass and red clover provided a six- to 11-fold increase in root biomass compared with barley alone. After two cycles of the rotation, soil organic carbon concentration was increased in both 1-2 mm and 4.7-9.5 mm macro-aggregates in the undersown treatments, but not in the whole soil. Micro-aggregate stability tended to increase where undersowing was used and was related to soil organic carbon (r=0.56, P=0.05) and root biomass (r=0.75, P=0.01). Macro-aggregation showed significant differences between undersown species, but none of the forages increased the level of structural stability compared with barley alone.