Estimating C inputs retained as soil organic matter from corn (Zea Mays L.)

In agroecosystems, the annual C inputs to soil are a major factor controlli ng soil organic matter (SOM) dynamics. However, the ability to predict soil C balance for agroecosystems is limited because of difficulties in estimat ing C inputs and in particular from the below-ground part. The objective of this paper was to estimate the proportion of corn residue retained as SOM. For that purpose, the results of a C-13 long-term (15 yr) field study cond ucted on continuous silage corn and two silage corn rotations along with da ta from the existing literature were analyzed. The total amount of corn-der ived C (0-30 cm) was about 2.5 to 3.0 times higher for the continuous corn treatment (445 g m(-2)), compared to the two rotational treatments (175 and 133 g m(-2) for the corn-barley-barley- wheat and corn-underseeded barley hay-hay rotations, respectively). Assuming that the C inputs to the soil fr om silage-corn was mainly roots and would have been similar across treatmen ts on an annual basis, the total amount of corn-derived C for the two rotat ional treatments was approximately proportional to the number of years the silage-corn was present in the rotation (4 yr). The results from the curren t study indicate that about 17% of root-derived C is retained as SOM. This value is higher than those reported in the literature for long-term studies on shoot-derived C (range of 7.7 to 20%, average of 12.2%), which is in ag reement with previous studies showing that more C is retained as SOM from r oots than from shoots.