Long-term changes in soil carbon under different fertilizer, manure, and rotation: Testing the mathematical model ecosys with data from the Breton plots

Soil C contents can be raised by land use practices in which rates of C inp ut exceed those of C oxidation. Rates of C inputs to soil can be raised by continuous cropping, especially with perennial legumes, and by soil amendme nts, especially manure. We have summarized our understanding of the process es by which changes in soil C content are determined by rates of soil C inp ut in the mathematical model ecosys. We compared model output for changes i n 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 rotat ion vs. a 5-yr wheat-oat (Avena sativa L.)-barley (Hordeum vulgare L.)-fora ge-forage rotation with unamended, fertilized, and manured treatments; Mode l results indicated that: rates of C input in the 2-yr rotation were inadeq uate to maintain soil C in the upper 0.15 m of the soil profile unless manu re 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 manur ed 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 measure d 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 oth er, so that gains in soil C under one such practice are greater when it is combined with other such practices.