EFFECT OF CROPPING PRACTICES ON THE INITIAL POTENTIAL RATE OF N MINERALIZATION IN A THIN BLACK CHERNOZEM

In pursuit of identifying sustainable agricultural production practices, there is an urgent need to develop or identify parameters that effectively describe and differentiate changes in the quality and quantity of soil organic matter resulting from crop management or cultural practices. The effectiveness of the potentially mineralizable nitrogen (N0) concept for this purpose was examined using data from a 30-yr crop rotation study, carried out on a Rego thin Black Chernozem at Indian Head, Saskatchewan. Although Kjeldahl N and hydrolyzable soil amino-N are effective for demonstrating the beneficial influence of fertilizers, legumes and continuous cropping on total soil organic N content, they are less effective in identifying changes in organic matter quality. We have shown that a parameter which we called the "initial potential rate of N mineralization" [potentially mineralizable N (N0) x rate constant (k) at time = 0] was effective in distinguishing both the absolute and qualitative changes in soil organic N due to various cultural and management practices. The results showed that fertilizers can be as effective as legumes, used either for green manure or for hay, in increasing the quantity and improving the quality of soil organic matter. A 6-yr rotation including 1 yr of fallow, 2 yr of spring wheat (Triticum aestivum L.) and 3 yr of bromegrass (Bromus inermis Leyss) and alfalfa (Medicago sativa L.) hay had a greater quantity and improved quality of soil organic N relative to that of unfertilized continuous wheat (Cont W), but the were equivalent to that of fertilized Cont W.