and three tillage treatments (moldboard plowing (MP), chisel plowing (CP) and no-tillage (NT)) were compared in a split-plot design. Total organic C was affected by the tillage treatments but not by the rotations. In the topsoil layer (0-7.5 cm), NT and CP treatments had C,contents 20% higher than the MP treatment. In the same soil layer, MBC averaged 300 mg C kg-' in the MP treatrnent and up to 600 mg C kg-l in the NT soil. Hot-water-extractable and acid-hydrolyzable carbohydrates were on average 4O% greater under reduced tillage than under MP. Both carbohydrate fractions were also slightly larger in the rotation than in the soil under continuous barley. The ratios of MBC and carbohydrate C to total organic C suggested that there was a significant enrichment of the OM in labile forms as tillage intensitywas reduced. Alkaline phosphatase activity was 507o higher under NT and 20% higher under CP treatments than under MP treatment and, on average, 15% larger in the rotation than in the continuous barley treatment. Overall, the management-induced differences were slightly greater in the top layer (0-7.5 cm) than in the lower layer of the Ap horizon (7.5-15 cm). All the properties measured were highly correlated with one another. They also showed significant temporal variations that were, in most cases, independent of the treaftnents. Four years of conservation tillage and, to a lesser extent, rotation with red clover resulted in greater OM in the top soil layer compared with the more intensive systems. This organic matter was enriched in labile forms.

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N. 1999. Protected organic matter in water-stable aggregates as affected by mineral fertilizer and manure applications. Can. J. Soil Sci. 79: 419-425. Effects of long-term (18-yr) applications of cattle manure (20 Mg ha -1 yr -1 ) and NPK fertilizer on the labile organic matter (OM) and its protection in water-stable aggregates were investigated in a Le Bras silt loam (Humic Gleysol). Soil from the 0-to 10-cm depth was sampled from the untreated control, NPK, manure and NPK + manure treatments and fractionated into four size classes of slaking-resistant aggregates (>1000 µm, 250-1000 µm, 53-250 µm, <53 µm). Intact and crushed macroaggregates (250-1000 and >1000 µm) and intact microaggregates (<250 µm) were incubated for 21 d at 25°C, and mineralized C and N were determined. The amount of mineralized C in intact aggregates increased with increasing aggregate size irrespective of the agronomic treatments, but there was no consistent trend for total N. Manure application led to an increase in mineralized C in most aggregate fractions. Crushing the macroaggregates enhanced mineralization of C by 14 to 35% and N by 17 to 103%. Additional C and N rendered mineralizable by crushing represents a fraction of the macroaggregate-protected OM. Manure application increased the protected pools of C (up to threefold) and N (up to fourfold) located in the small macroaggregates (250-1000 µm). In contrast, NPK fertilization increased the pool of macroaggregate-protected N by 2.5-fold but had no effect on the protected C. We conclude that manure application contributed to the accumulation of macroaggregate-protected C and N, whereas mineral fertilizers increased the protected-N pool only. Macroaggregates can provide a mechanism for the protection of labile soil OM in an annually tilled cropping system and this mechanism is enhanced with long-term manure application.Key words: Aggregate-protected organic matter, manure application, mineralization, mineral fertilizer, water-stable aggregates Aoyama, M., Angers, D. A., N'Dayegamiye, A. et Bissonnette, N. 1999. Effets de l'application de fumier et de la fertilisation minérale sur la matière organique protégée dans les agrégats stables. Can. J. Soil Sci. 79: 419-425. Cette étude a évalué les effets à long-terme (18 années) de l'application de fumier de bovin (20 Mg ha -1 année -1 ) et de l'engrais minéral sur la fraction labile de la matière organique (MO) et sur la protection physique de celle-ci dans les agrégats d'un loam Le Bras (Gleysol humique). A partir des échantillons de sol provenant de la couche 0-10 cm des traitements témoin, avec fumier, avec engrais minéraux (NPK) et avec fumier+NPK, nous avons obtenu, par tamisage sous eau, quatre fractions d'agrégats stables (>1000 µm, 250-1000 µm, 53-250 µm, <53 µm). Les quantités de C et N minéralisées ont été mesurées dans une incubation (21 jours à 25°C) de macroagrégats brisés et intacts (250-1000 and >1000 µm) et de microagrégats (<250 µm) intacts. Les résultats indiquent que les quantités de C minéralisé augmentent avec la taill...

N. 1999. Soil organic matter quality and aggregate stability under eight potato cropping sequences in a fine sandy loam of Prince Edward Island. Can. J. Soil Sci. 79: 411-417. Soil degradation and soil organic matter (SOM) reduction have long been suspected to occur under intensive potato (Solanum tuberosum L.) production in Prince Edward Island. Thus, this study was conducted to evaluate the effect of eight potato cropping sequences involving barley (Hordeum vulgare L.), ryegrass (Lolium multiflorum L.), red clover (Trifolium pratense L.) and barley underseeded with red clover on aggregate stability and SOM properties of a Charlottetown fine sandy loam (Orthic Humo-Ferric Podzol). Total C and N, light-fraction (density < 1.7 g cm -3 ) C (LF-C) and light-fraction N (LF-N), microbial biomass C (MBC), carbohydrate content, alkaline phosphatase activity (APA) and aggregate stability were determined in the 0-to 15-cm soil layer in the 6th and 10th years of the trial. Average C and N contents were 20 to 27% greater in sequences that included 3 or 4 yr of potatoes and a high frequency of red clover than in a 9-yr potato sequence; and were attributed to the less frequent tillage and higher organic matter inputs associated with the forage component. The response for MBC, LF-C, LF-N and APA was greater than those for total C and N suggesting that the former parameters may better reveal SOM changes caused by variations in potato management than the latter. Light-fraction N was particularly sensitive to the inclusion of perennial legumes in the cropping sequence. Wet-aggregate stability was 33% higher after 6 yr in the sequence with 50% frequency of red clover than in the continuous potato system, but no effect was noted after 10 yr. The results of this study indicate that sequences which are limited to 30 to 40% of potatoes and have a high frequency of perennial forage such as red clover, lead to greater SOM content and quality of a Charlottetown fine sandy loam compared to continuous potatoes. Trends in aggregate stability generally followed those of SOM fractions but the effects were relatively smaller and varied with time. . La production intensive de pomme de terre (Solanum tuberosum L.) est souvent associée à la dégradation et la réduction de la matière organique des sols de l'Île-du-Prince-Édouard. Cette étude, effectuée sur un loam sableux fin de Charlottetown (Podzol humo-ferrique orthique), a évalué les effets de huit séquences culturales incluant la pomme de terre, l'orge (Hordeum vulgare L.), le ray-grass (Lolium multiflorum L.), le trèfle rouge (Trifolium pratense L.) en culture pure ou semé sous orge sur les propriétés de la matière organique et la stabilité des agrégats du sol. Les teneurs en C et N du sol entier et de la fraction légère (densité < 1.7 g cm -3 ) de la matière organique, les teneurs en biomasse microbienne et en glucides extraits à l'acide sulfurique, l'activité de la phosphatase alcaline et la stabilité des agrégats ont été déterminées dans la couche 0-15 cm de profondeur du sol dans la 6 e...

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