Nitrogen contribution of pea residue in a hummocky terrain

Topography influences the distribution of soil water and inorganic N which could affect the availability of N from legume residue to the subsequent crop. A study assessed the landscape-scale variability of the N contribution by pea (Pisum sativum L.) to the soil (N2 fixation in residue minus seed N derived from the soil) and the subsequent wheat (Triticum aestivum L.) crop. In situ 15N-labeled pea residue was used to monitor the contribution of pea residue to the succeeding crop. A 100-point sampling grid, with 10-m spacings, was established in a field with hummocky terrain. Each sampling point was classified as a shoulder or a low-catchment or high-catchment footslope. The N contribution by pea to the soil was 45 kg ha-1 in the shoulders and 63 kg ha-1 in the footslopes. Recovery of 15N in the microbial biomass was greater in the footslopes (71%) vs. the shoulders (51%), and related to greater soil water content in the footslopes. The N contribution by pea to wheat, as assessed by the 15N that wheat derived from the residue of the preceding wheat crop, was 11% and similar among landform complexes. Therefore, the N contribution of pea to wheat did not explain the greater N accumulation by wheat in the high-catchment areas than the other landform complexes. The N contribution of pea to a succeeding wheat crop apparently will be a small component of the rotation benefit by pea in a hummocky terrain