Soil Organic Matter Response to Crop Rotation and Nitrogen Fertilization.

Crop management to improve soil C and N storage is necessary for increased soil productivity. Continuous corn (Zea mays L.) (CC) and corn-soybean [Glycine max (L.) Merr.] (CS) rotations with N fertilizer rates of 0 to 250 kg N ha^-1 in corn were sampled after 8 yr at three locations in Illinois. Soil organic C (SOC) and total N (TN) were greater following CC in the 0- to 15-cm and 15- to 30-cm depths at DeKalb, but were not affected by crop rotation at Dixon Springs or Urbana. The fine-textured soil and the cool climate at DeKalb likely limited decay, allowing differences in SOC and TN to develop between rotations. Particulate organic matter (POM) C (POM-C) and POM N (POM-N) were also greater following CC in both depths at DeKalb and in the surface 15 cm in the coarser-textured soil at Dixon Springs. Higher SOC, TN, POM-C, and POM-N following CC were attributed to greater crop-derived C returned to the soil over the previous 8 yr, and to slower decomposition of corn residue than soybean residue. Overall, N fertilization resulted in a linear increase in POM-N and a linear decrease in the POM C/N ratio, but had no effect on SOC, TN, or POM-C. These results indicate that crop rotation has a greater influence on soil C and N than N fertilization, and that transition from the CS rotation to CC can improve C and N storage in soils where decomposition of organic matter is restricted by soil texture and climate.