Direct vs. Indirect Nitrogen-15 Approaches to Estimate Nitrogen Contributions from Crop Residues
- F. C. Stevenson ,
- F. L. Walley and
- C. van Kessel
The benefit of including a legume in a cropping system often has been attributed, in part, to N2 fixation and the additional N that legume residues can supply to the succeeding crop. In this study, we estimated the N contribution from pea (Pisum sativum L.) to a succeeding wheat (Triticum aestivum L.) crop directly, using 15N-labeled crop residues, and indirectly, using the A value. The A value provides an index of soil N availability and was used to indirectly estimate the N contribution of crop residues to the subsequent crop. Pea-wheat and wheat-wheat rotations were established at four sites using a small-plot research approach and at one site using a landscapescale research approach. Nitrogen-15-labeled residue and 15N-labeled fertilizer were applied to unlabeled areas within each rotation following the first growing season. When estimated directly using 15N-labeled residues, pea residue contributed 10 kg N ha-1 to the subsequent wheat crop, whereas wheat residue contributed only 3 kg N ha-1. According to indirect estimates of N contribution, the A value in the second year of the pea-wheat rotation was, on average, 119 kg N ha-1 higher than for the wheat-wheat rotation. The difference in A values was equivalent to an additional contribution from pea, relative to wheat residues, of 4 kg N ha-1 in the small-plot study and 38 kg N ha-1 in the landscape-scale study. The direct approach considers only the N contribution from the aboveground portion of the previous crop residues. In contrast, indirect estimates of the N contribution from crop residues based on the A value includes the contribution of all belowground N sources and the influence of non-N rotation effects, such as root diseases, on the ability of a succeeding crop to access soil N reserves.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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