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Nitrogen (N) fertilizer plays an important role in wheat yield, but N application rates vary greatly, and there is a lack of data to quantify the residual effects of N fertilization on soil N availability. A 17-yr experiment was conducted in a semiarid area of the Loess Plateau of China to assess the effects of N fertilization on spring wheat (Triticum aestivum L.) grain yield, N uptake, N utilization efficiency, and residual soil nitrate. Treatments included a non-N-fertilized control and annual application of 52.5, 105.0, 157.5, and 210.0 kg N ha−1 in the first two years (2003 and 2004). In the third year (2005), the four main plots with N fertilizer application were split. In one subplot, N fertilization was continued as mentioned previously, while in the other subplot, N fertilization was stopped. The concentration of NO3-N in the 0–110 cm depth soil layers was significantly affected by N application, with higher N rates associated with greater soil NO3-N concentration. With the annual application of N over 17 years, residual soil NO3-N concentration in the 100–200 cm soil layer in the last study year was significantly greater than that in the non-N-fertilized control and was increased with rate of N application. There was a significant positive relationship of soil NO3-N in the 0–50 cm and 50–110 cm soil layers at wheat sowing with wheat grain N content and yield. Wheat grain yield in the third year (2005) was significantly, i.e., 22.57–59.53%, greater than the unfertilized treatment after the N application was stopped. Nitrogen use efficiency decreased in response to each increment of added N fertilizer, and was directly related to N harvest index and grain yield. Therefore, greater utilization of residual soil N through appropriate N fertilizer rates could enhance nitrogen use efficiency while reducing the cost of crop production and risk of N losses to the environment. For these concerns, optimum N fertilizer application rate for spring wheat in semiarid Loess Plateau is about 105 kg N ha−1, which is below the threshold value of 170 kg N ha−1 per year as defined by most EU countries.

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Agriculture in rainfed dry areas is often challenged by inadequate water and nutrient supplies. Responses to these challenges include adequate fertilization, but it is unknown whether different nitrogen (N) rates from that commonly used in the Loess Plateau can alleviate this issue. Field experiments were conducted over three cropping seasons to investigate the effect of different N fertilization levels on soil water dynamics, photosynthetic activity, and grain yield of maize (Zea mays L.) grown in the Western Loess Plateau of China. Fertilizer was applied at planting at rates between 0 and 300 kg N ha−1 at regular increments of 100 kg N ha−1 (referred to as N0 and N300, respectively), and treatments were arranged in a complete randomized block design. Results showed that water use efficiency in the N200 and N300 treatments was ∼60% higher than N0 and N100 (P < 0.05), which translated into increased crop biomass and therefore grain yield (≈70–80%). These observations were consistent with all measurements of photosynthetic traits and suggested that, under the conditions of this study, the ecophysiological response of the crop may be optimized at N application rates in the range of 200 to 300 kg ha−1, depending on soil water availability. Agronomic efficiency calculations at this level of N fertilization reported consistently higher values and consequently suggested that environmental losses of applied fertilizer N were small. The current experimental results for the N application rates in the range of 200 to 300 kg ha−1 could be useful for improving N fertilizer and soil water management practices of maize production while maintaining a relatively stable yield level in rainfed dry areas. Future work should focus on optimizing timing and improving the placement of fertilizer N applied to maize.

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