AGROECOSYSTEM MANAGEMENT EFFECTS ON SOIL CARBON AND NITROGEN

The cumulative effects of long-term ( 1980-1990) tillage and crop rotation management on soil organic C and N concentrations and potential mineralization were determined. Tillage systems studied were conventional moldboard plow tillage and conservation tillage, with various crop rotations including: continuous soybean (Glycine max L.)-wheat (Triticum aestivum L.) cover (SW); continuous com (Zea mays L.)-wheat cover (CW); and com-wheat cover-soybean-wheat cover (CWSW). Surface soil (0-5, 5-10, and 10-20 cm depth increments) organic C and N concentrations and potential mineralization were determined for all tillage/rotation combinations in October 1990. After 10 years, surface soil organic C and N concentrations were 67% and 66% higher, respectively, under conservation tillage than plow tillage to a depth of 10 cm. Potential C and N mineralization followed a pattern similar to organic C and N distribution. However, differences in substrate quality below 10 cm indicated that conservation tillage promoted N immobilization. Crop rotation had less effect than tillage on soil organic C and N amounts and potential mineralization. Rotations with higher frequency of corn (CW and CWSW) had higher organic C and N concentrations and C mineralization than SW; crop rotation had no effect on potential N mineralization. Tillage system apparently influenced soil organic matter concentrations and mineralization via crop residue incorporation rather than any effects owing to concentration differences at the soil surface, while the effect of crop rotation was related to amount of crop residues added between 1980 and 1990.