Management effects on the dynamics and storage rates of organic matter in long-term crop rotations

Factors controlling soil organic matter (SOM) dynamics in soil C sequestration and N fertility were determined from multi-site analysis of long-term, crop rotation experiments in Western Canada. Analyses included bulk density, organic and inorganic C and N, particulate organic C (POM-C) and N (POM-N), and CO2-C evolved during laboratory incubation. The POM-C and POM-N contents varied with soil type. Differences in POM-C contents between treatments at a site (deltaPOM-C) were related (r(2) = 0.68) to treatment differences in soil C (deltaSOC). The CO2-C, evolved during laboratory incubation, was the most sensitive indicator of management effects. The Gray Luvisol (Breton, AB) cultivated plots had a fivefold difference in CO2-C release relative to a twofold difference in soil organic carbon (SOC). Soils from cropped, Black Chernozems (Melfort and Indian Head, SK) and Dark Brown Chernozems (Lethbridge, AB) released 50 to 60% as much CO2-C as grassland soils. Differences in CO2 evolution from the treatment with the lowest SOM on a site and that of other treatments (deltaCO(2)-C) in the early stages of the incubation were correlated to deltaPOM-C and this pool reflects short-term SOC storage. Management for soil fertility, such as N release, may differ from management for C sequestration.