Residual effects of pig slurry and mineral nitrogen fertilizer on irrigated wheat

The residual effects of pig slurry beyond the year of application have yet to be clearly quantified to determine if it is possible to reduce N fertilizer rates for the subsequent crops. The objective of this study was to assess the residual effects of pig slurry (PSRE) and mineral N fertilizer (MRE) applied to maize (Zea mays L.) with respect to the subsequent wheat (Triticum aestivum L.). A wheat crop was established in the Ebro valley (Northeast Spain) following a 6-year maize monoculture (2002–2007). The maize crops had been annually fertilized with a combination of three pig slurry rates (0, 30, and 50 m3 ha−1) and three mineral N rates (0, 100, and 200 kg N ha−1) commonly applied by farmers in the region. The soil mineral N levels before wheat sowing ranged from 94 to 609 kg N ha−1 and increased as pig slurry and mineral N applications to maize increased. The PSRE were high and ranged from 2287 to 3796 kg ha−1 for wheat grain yield and from 36 to 84 kg N ha−1 for grain-N uptake. Results suggested that the PSRE would mainly originate from the carry-over of unused mineral N. Mineralization of the organic N applied with pig slurry, at rates based on European Union Nitrates Directive, contributed with further residual N effects and would allow farmers to reduce N fertilization of wheat about 30 kg N ha−1. A reduction in the typical N rates applied to maize along with additional N fertilization of wheat is necessary to maximize crop profitability and reduce the risk of water pollution with nitrates.

Research highlights
► We quantified the residual N effects of pig slurry and mineral N fertilizer on wheat.
► Residual N effects were high in terms of wheat grain yield and N uptake.
► Residual N effects mainly originated from carry-over of unused mineral N.
► The organic N of the slurry contributed with further residual effects (≈30 kg N ha−1).
► Consider residual N effects of pig slurry to improve N management in crop rotations.