Warming and nitrogen fertilization effects on winter wheat yields in northern China varied between four years

• Wheat grain yield was decreased under warming treatment in 3 out of 4 years.
• Warming limited yield gain from nitrogen fertilization under normal winter weather.
• Wheat yield was increased by warming following an unusual cold and snowy winter.
• Warming improved nitrogen utilization in a year with unusual cold and wet winter.
• Early growth phases were shortened whereas later ones were extended under warming.

Global warming is expected to affect wheat productivity significantly, but with large regional differences depending on current climatic conditions. We conducted a study that aimed to investigate how wheat growth and development as well as yield and yield components respond to warming combined with nitrogen fertilization. Infrared heaters were applied above the crop and soil to provide a warming of around 2 °C at 5 cm soil depth during the whole winter wheat growing season from 2008 to 2012 at a site near Shijiazhuang in the North China Plain. Two temperature levels (warming and ambient) for winter wheat were compared in a factorial combination with (N2, 240 kg N ha−1 y−1) and without nitrogen fertilizer (N1) in a field experiment. Measurements showed that the infrared heater increased soil temperature by 1.6 to 2.2 °C in N2 and by 1.3 to 2.0 °C in N1 depending on soil depth (0.05 to 0.40 m). The volumetric water content decreased significantly before heading by 9.3, 3.9, 2.4 and 1.2 vol% in the soil depth of 0.10, 0.20, 0.40, 0.60 m in N2 and by 5.9, 1.4, 1.3 and 1.2 vol% in N1 from heating compared with no heating. The duration of the entire growth period was shortened by on average 7 days in the warming compared with control treatment. The early growth stages before re-greening in spring were shortened by 12 to 18 days, whereas the later stages were prolonged by up to 6 days. Warming reduced grain yield by 36%, 39% (P < 0.05) and 12% for N2 and 33%, 7% and 10% for N1 in 2009, 2011 and 2012, respectively, which can be considered years with normal winter weather. However, warming increased grain yield by 1% and 31% (P < 0.05) in N1 and N2, respectively, in a year with unusually cold and snowy winter conditions (2010). Warming increased plant height and 1000-grain weight, but reduced spike number per m2. This suggests that the wheat yield loss may be related to reduction of spike number, which was affected by decreased soil water content under warming. Warming tended to give larger yield reductions at higher nitrogen fertilizer rates, and this may be related to larger water consumption with both higher nitrogen and temperature leading to water shortages. These effects indicate that wheat yield loss from warming was primarily associated with more severe water shortage from greater evapotranspiration under warming. The large crop canopy in the fertilized plot may further have enhanced evapotranspiration and thus severity of the drought leading to larger yield reduction in fertilized plots. Yield increased under warming when water was not a limited factor in a year with unusual cold and wet winter.