N Stress Alleviation in Crops—A System Approach Analysing Residual N From Winter Crops in a Late-Maize-Wheat Sequence

Nitrogen (N) is crucial for crop production. Crop sequences with different legume participation affect N availability and therefore N fertiliser management. The study aimed to assess the inclusion of winter crops (WC) with different amounts of residues and different C:N ratios on the following: (i) the response to N fertilisation in the following late-maize (Zea mays L.), and to carry that comparison into a subsequent wheat crop (Triticum aestivum L.), and (ii) identify soil N indicators associated with these responses. Two field experiments (E1 and E2) were conducted in the Argentinean Pampas during two growing seasons to evaluate a WC/late-maize-wheat sequence under no-tillage. In each experiment, late-maize was sown after a bare-fallow and three WC: wheat, vetch (Vicia villosa L.) and field pea (Pisum sativum L.), where five rates of N fertilisation were evaluated. An area of late-maize that was not fertilised with N within each previous WC was used to evaluate the response to N fertilisation in the subsequent wheat crop. Indigenous N was estimated by using N uptake in the non-N-fertilised treatments. Soil N indicators and C:N ratio of WC residues were evaluated as indicators of response to N fertilisation in both crops. Significant responses to N fertilisation in grain yield and N uptake were observed in late-maize when bare-fallow and wheat were the previous treatments in both experiments. In contrast, vetch and field pea supplied 32 and 40 kg N ha⁻¹ in E1 and E2, respectively, and showed no response to N fertilisation, satisfying the N required by late-maize. However, this supply was not enough to sustain the N demand of the subsequent wheat, where the response to N addition ranged from 36% to 74% when vetch and wheat were the previous WC, respectively. Only soil inorganic N indicators were associated with indigenous N supply. Moreover, the apparent net WC effect was linked to late-maize (r² = 0.91) and subsequent wheat (r² = 0.67) grain yield response, which was also related to the C:N ratio of the WC residues in late-maize and the subsequent wheat (r² = 0.78), suggesting that mineralisation occurs when C:N ratio is below 18. Consequently, in future studies the C:N ratio of the WC residues can be included in N fertilisation recommendation schemes when late-maize is sown as a double crop in more intensified crop sequences.