Soil C contents can be raised by land use practices in which rates of C input exceed those of C oxidation. Rates of C inputs to soil can be raised by continuous cropping, especially with perennial legumes, and by soil amendments, especially manure. We have summarized our understanding of the processes by which changes in soil C content are determined by rates of soil C input in the mathematical model ecosys. We compared model output for changes in soil C with those measured in a Gray Luvisol (Typic Cryoboralf) at Breton, Alberta, during 70 yr of a 2-yr wheat (Triticum aestivum L.)-fallow rotation vs. a 5-yr wheat-oat (Avena sativa L.)-barley (Hordeum vulgare L.)-forage-forage rotation with unamended, fertilized, and manured treatments. Model results indicated that rates of C input in the 2-yr rotation were inadequate to maintain soil C in the upper 0.15 m of the soil profile unless manure was added, but that those in the 5-yr rotation were more than adequate. Consequent changes of soil C in the model were corroborated by declines of 14 and 7 g C m-2 yr-1 measured in the control and fertilized treatments of the 2-yr rotation; by gains of 7 g C m-2 yr-1 measured in the manured treatment of the 2-yr rotation; and by gains of 4, 14, and 28 g C m-2 yr-1 measured in the control, fertilized, and manured treatments of the 5-yr rotation. Model results indicated that soil C below 0.15 m declined in all treatments of both rotations, but more so in the 2-yr than in the 5-yr rotation. These declines were corroborated by lower soil C contents measured between 0.15 and 0.40 m after 70 yr in the 2- vs. 5-yr rotation. Land use practices that favor C storage appear to interact positively with each other, so that gains in soil C under one such practice are greater when it is combined with other such practices.
