Direct vs. Indirect Nitrogen‐15 Approaches to Estimate Nitrogen Contributions from Crop Residues

Abstract

AbstractThe 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.