Abstract: Based on a long-term straw mulching experiment with micro-plot fertilization in the dryland winter wheat monoculture system, we investigated the effects of nitrogen topdressing on photosynthesis and yield formation. There were four treatments, including no mulching as control (CK), straw mulching at rates of 9000 (HSM) and 4500 kg·hm^-2 (LSM) during growing season, and at a rate of 9000 kg·hm^-2 during summer fallow (FSM). Topdressing was applied at the booting stage. Without topdressing, soil NO₃^--N and NH₄⁺-N concentrations were lower under HSM and LSM than under CK. Leaf photosynthesis rate, stomatal conductance, transpiration rate, and water use efficiency were lower under HSM, LSM and FSM than that under CK. Wheat grain yield was lower by 28.43% and 21.74% under HSM and LSM than under CK (P<0.05), respectively. With topdressing, soil NO₃^--N and NH₄⁺-N concentrations and photosynthetic characteristics of winter wheat flag leaves in HSM, LSM and FSM treatments were significantly higher than those without topdressing. There was no significant difference in wheat thousand-grain weight, aboveground biomass, grain yield, and harvest index among all the treatments. Ourresults indicate that nitrogen topdressing at late growing stage of winter wheat would be a reliable measure to solve soil nitrogen deficit caused by straw mulching, promote leaf photosynthesis after anthesis, and reduce the potential risk of yield reduction resulted from straw mulching.

Key words: nitrogen topdressing, straw mulching, winter wheat, photosynthesis, grain yield.