INFLUENCE OF FERTILIZER AND STRAW BALING ON SOIL ORGANIC-MATTER IN A THIN BLACK CHERNOZEM IN WESTERN CANADA

Heavy crop residues usually present disposal problems for producers. A 30 yr study carried out on a thin Black Chernozem at Indian Head, Saskatchewan, Canada was used to study how fertilization and straw removal influenced soil organic matter of a follow-wheat-wheat (Triticum aestivum L.) rotation. When straw was incorporated, fertilizer increased soil organic N (P < 0.01) and tended to increase soil organic C (P < 0.14) in the top 15 cm. When straw was removed there was a tendency for soil organic N to be depleted (5.7% decrease, significant at P < 0.20); but surprisingly, soil organic C was not affected. C and N balance sheets, developed using plausible assumptions, were used to estimate what should have happened to soil organic matter. The estimates compared favourably with measurements, except for C lost when straw was removed. We estimated that there should have been a 9% loss in organic C in the latter case, compared to when straw was not removed. We hypothesized that, in situ, C from roots contribute more to maintenance of soil organic matter than does straw.