Retention and turnover of corn residue carbon in some eastern Canadian soils

Agricultural practices determine the level of soil organic C by influencing the amount of residue returned to, and retained by, soil. We conducted field experiments on four soils in Ontario and Quebec to test the hypothesis that short-term changes in soil organic C and the proportion of corn (Zea mays L.)-residue C retained in soils are affected by soil texture, fertility, and tillage management. We measured the amount of stover to estimate C input into the soils, and total soil C and (13)C natural abundance to determine the soil C derived from corn residues. Only the sandy clay loam exhibited changes in total soil C, related to levels of fertilizer N and quantity of residues returned to the soil. The proportion of soil C derived from corn residues varied from 4 to 19%, depending on the time under continuous corn. Our data indicate a rapid increase in the proportion of corn-derived C in the early stage of cropping to corn, but a markedly reduced rate of increase beyond 12 yr. Measured delta(13)C of archived samples of tao soils with different clay contents (71 and 28% clay) but similar residue inputs (11 Mg stover ha(-1) yr(-1)) indicated that the rate of increase in the proportion of corn-derived C was higher in the soil,vith higher clay content (2.4 vs. 1.9% yr(-1)). The results of this study are consistent with the hypothesis that the quantity of crop residue inputs and soil texture greatly influence the retention and turnover of crop residue C.