Conservation tillage could enhance soil organic carbon (SOC) sequestration, but is rarely used in cropping systems in Mexico, especially under irrigation. A study was conducted on a clayey, smectitic, isothermic Udic Pellustert to evaluate the use of traditional-deep and no-tillage systems on SOC dynamics for wheat (Triticum aestivum L.)-corn (Zea mays L.) and wheat-bean (Phaseolus vulgaris L.) cropping systems. Experimental design was a randomized block of five tillage/crop-rotation (two crops per year) systems with four replications: (WC-CTb) wheat-corn, burning the residues of both crops, plowing and disking twice (WC-CT) wheat-corn under conventional tillage (plowing and disking twice to incorporate crop residues following the harvest of each crop), (WC-NT) wheat-corn under no-till, (WB-CT) wheat-bean under conventional tillage, and (WB-NT) wheat-bean under no-till. Each crop in the sequence received one of three fertilizer-N rates broadcast as urea: (a) 0, 150, and 300 kg N ha(-1) for corn; (b) 0, 40, and 80 kg N ha(-1) for bean; and (c) 0, 125, and 250 kg N ha(-1) for wheat. The baseline year was 1994, and relative changes were measured from 1994 to 1999 for grain yield and residue production, crop residue C and delta(13)C, SOC, soil C/N ratio, and change in soil delta(13)C. Interaction of cropping system x fertilizer-N rate was highly important to grain yield and crop residue production and amount of crop-residue C produced. High N rates increased SOC sequestration and decreased soil C/N ratios. In WC systems, more negative delta(13)C was associated with higher N rates, indicating increased contribution of wheat (a C(3) plant) residue C relative to corn (a C(4) plant). In WB, N-rate and tillage had no effect on SOC sequestration. Highest rate of SOC sequestration was under WC-NT and when increases in SOC from 1994 to 1999 were annualized was 1.0 and 1.9 Mg SOC yr(-1) in the 0-15- and 15-30-cm depths, respectively. Corresponding SOC in 0-15- and 15-30-cm depths in the WC-CT treatment was 0.2 and 0.6 Mg yr(-1) and amounts in all other treatments were equal or lower than those observed for WC-CT. There was a significant correlation between aboveground crop-residue C produced and amount of SOC sequestered. Results from this study indicate no-till on N-fertilized WC systems can potentially increase SOC sequestration on large areas of irrigated Vertisols in Central Mexico while maintaining high crop yields.
