The amount and type of C-containing amendments applied to soil can have an influence on soil organic carbon (SOC) levels. To test the hypothesis that amendment type is more important than amount, we applied 250 g C m-2 as manure, legume foliage, wheat (Triticum aestivum L.) residue, municipal biosolid, wood sawdust, brassica (Brassica napus L.) residue, composted wheat residue, sucrose, and cotton linters to both fallow soil and an annual winter wheat crop for five consecutive years. After an additional 3.5 yr with no inputs and all plots being fallow, the SOC of biosolid, manure, and wood amended plots were significantly (P < 0.0001) greater than the unamended check. The application of biosolid increased SOC 492 g m-2, and manure increased SOC 316 g m-2, over the fallow check plots in the top 300 kg m -2 of soil (approximately 0-25 cm). The increase in SOC relative to the check ranged from 0 to 39% of the amendment C applied. The SOC content was 482 g m-2 greater under continuous winter wheat than under fallow. The amendment and wheat crop effects on soil C and N changed little during the 3.5 yr after treatments ended, indicating that decomposition occurred soon after application. Wood sawdust was unique in that it increased SOC even though it was low in N content, and it changed the soil C/N ratio from 12.3 to 13.4. This field research demonstrated that amendments applied at the same C rate can have variable effects on SOC accretion.
