Influence of sorghum crop residue management on the recovery of 15N labelled fertilizer by wheat in Mexico

El Bajio (ca. 5 x 10(5) ha) is one of the main cereal producing regions of Mexico. During the past 30 years, a cereal-cereal rotation has been practiced with constantly increasing annual rates of N-fertilizer application: from 120 kg N ha(-1) in 1960 to 320 kg N ha(-1) in 1998. Furthermore, in order to avoid overlap between the fall-winter and spring-summer cycles, crop residues are burned. A field experiment was conducted in Abasolo, Guanajuato, Mexico, to compare the effects of incorporating or burning sorghum crop residues on the availability of fertilizer N for a succeeding crop of winter wheat. The wheat received 320 kg N ha(-1), either all applied at sowing or in two split applications of 160 kg each (at sowing and at 50 days after sowing), as ammonium sulphate enriched in N-15. I,,parallel experiment, a fertilizer N balance was carried out using a single application at sowing of 320 kg N ha(-1) as ammonium sulphate enriched in N-15. Dry matter and grain yields obtained ranged from 4 to 6 t ha(-1) for dry matter and from 4.1 to 6.1 for grain, which are normal for this region. Wheat total-N uptake was 46 kg N ha(-1) higher with the burned residue than with the incorporated residue. The values of %N in the plant derived from fertilizer were not significantly affected by residue treatment (%NdFf 25 to 28). The fraction of plant N derived from soil (Ndfs) was 72 to 75% for both residue treatments. Higher recoveries (25 to 49 kg N ha(-1)) of fertilizer N were observed when it was applied in two applications. Higher values were registered with burned residues. More fertilizer N was found in the surface 30-cm layer (102 kg N ha(-1)) with incorporated residues vs. burned residues (30 kg N ha(-1)). From 30 to 90 cm, 67 to 80 kg N (25% of the applied fertilizer N) was observed regardless of residue treatment. Other factors, which are under study, such as poor water management in this region seem to play a role in the low N fertilizer efficiency observed.