## 1 Introduction  
With the continuous increase in global population and the improvement of people's living standards, there is an urgent need for a significant increase in wheat production. The wheat production of China increased at a faster rate than most countries, especially in terms of grain yield per unit area. The main reason for this were not only the cultivation of new varieties, but also the improvement of agricultural management technology [1]. Among the main factors leading to grain yield increase, the contribution rate of fertilizer practice was 57.4%, and irrigation management accounted for 7.9% [2]. In pursuit of high grain yield, excessive use of N fertilizers and irrigation usually resulted in low N use efficiency, serious water resource waste, and adverse effects on food security and sustainable agricultural development [3, 4]. Reducing the N application rate is the simplest and most direct method to improve N use efficiency [5]. However, N is a crucial element driving wheat growth, and its reduced application rate directly affects leaf area, photosynthetic capacity, and radiation interception, thereby affecting grain yield [6, 7]. Among them, photosynthesis is a key process for biomass accumulation in wheat, closely related to sustainable development and food security, as it affects nutrients, water, and land use efficiency [8, 9]. After anthe, the accumulation of organic matter in grain mainly depended on photosynthesis, and a higher photosynthetic rate was an important guarantee for higher yield. It was reported that about one-third of the dry matter in grain came from the remobilization of substancesstored in nutrient organs at pre-anthesis, while two-thirds came from the accumulation of photosynthetic products in flag leaves after anthesis [10]. The application of N enhanced the light energy harvesting capacity of wheat leaves, improved the conversion efficiency of light energy and the proportion of open parts of photosystem II (PSII), reduced non radiative heat dissipation, and enabled plant to more effectively use the captured light energy for photosynthesis, thereby increasing the net photosynthetic rate [11]. Pre-anthesis photosynthesis of leaves and the remobilization of pre-anthesis accumulated substances in plant affected the grain filling process of wheat, thereby affecting final grain weight [12; 13]. Therefore, it was crucial to ensure sufficient photosynthesis and dry matter transport at the grain filling stage. An appropriate N application rate could optimize the grain filling rate and duration, while excessive N application reduced the grain filling rate, decreased the amount of assimilates transported from nutrient organs to grains, and led to a decrease in grain weight [14]. Moreover, a larger canopy consumed more nutrients, which was not conducive to the accumulation of dry matter at filling stage [15]. In addition, N application methods also affected grain filling and dry matter accumulation processes. Studies in some regions showed that water and fertilizer integration could match water and N supply with plant demand, improve grain filling duration and biomass, and ultimately increase grain yield [16; 17]. However, differences in regional environment, soil fertility, and variety resulted in inconsistent suitable N application rates for winter wheat. In the NCP, the amount of N fertilizer applied in winter wheat season was 60-385 kg N ha\({}^{-1}\)[18]. However, most of the fields in the NCP were managed by smallholder farmers, resulting in uneven soil fertility and significant differences in yield. It seemed difficult to promote and apply a single recommended amount of N fertilizer as it was difficult to match the water and fertilizer structure in the soil. Under the conditions of fully tapping into the production potential of wheat varieties, grain yields in the region exceeded 12,000 kg ha\({}^{-1}\)[19].