Nitrogen use and efficiency in a rotation with and without incorporation of crop residues

A field experiment was conducted to study nitrogen (N) use and efficiency in a crop rotation with and without incorporation of residues under irrigated conditions. Nitrogen-15-labelled microplots were installed on a long-term experiment in which six rotations at two rates of fertilization are being examined. The rotation without incorporation of residues was maize-wheat-red clover-red clover; and the one with incorporation of residues was maize-wheat-common bean-barley. Treatments during the first growing season, when labelling maize (Experiment 1), were as follows: i) Treatment 1 (T1), determine the fate of 15N-labelled fertilizer in the rotation with incorporation of residues. During subsequent seasons, only unlabelled residues were applied to these plots; ii) T2, determine the fate of 15N-labelled residue with residues. Labelled residues were applied in the second growing season and, thereafter, unlabelled residues were used; iii) T3, determine the fate of labelled 15N fertilizer in the rotation without residue incorporation - the control treatment for maize. During the second growing season (Experiment 2), the 15N-labelled experiment was repeated and wheat (second crop in the rotation) was labelled. To label maize and wheat, 300 and 160 kg N ha-1 as 15N-enriched urea and ammonium sulphate were applied, respectively. The labelled residues of maize and wheat were applied before sowing at 8,000 and 4,000 kg ha-1, respectively. The same amounts of unlabelled residues were applied afterwards where appropriate. Results from both experiments showed similar trends and did not support the hypothesis that increased inputs of carbon lead to increased retention of N in the system. Possible explanations are discussed. (author)