Effects of nitrogen application levels on soybean photosynthetic performance and yield: Insights from canopy nitrogen allocation studies

Abstract

Context

Increasing photosynthetic nitrogen use efficiency(PNUE) at the canopy level can increase seed yield with reduced nitrogen inputs. An appropriate increase in nitrogen application can maximize canopy optimal nitrogen allocation and improve nitrogen use efficiency.

Objective

The objectives of this study were to employ a plot comparison approach to study the vertical distribution of light and nitrogen attenuation in crop canopy, the variations in nitrogen allocation within the photosynthetic system, and the limiting factors of PNUE based on the soybean(Glycine max L.) leaves at different positions during the filling stage.

Methods

A two-year field experiment was conducted in Heshan Farm, Heilongjiang Province(China), to evaluate the effects of two soybean varieties and four nitrogen applications.

Results and conclusions

The results showed that an appropriate nitrogen increases photosynthetic rate(Pn) in the middle and lower leaves and slows down the aging process and chlorophyll degradation in the lower leaves. As the canopy height decreases, the limiting factors that restrict PNUE transition from carboxylation and electron transport system to light-harvesting and electron transport system. Increasing nitrogen applications can significantly improve PNUE in the lower leaves. Two years of soybean yield showed that the N1 averagely increased by 38.1 % for Jinyuan55 and N1.5 increased by 47.1 % for Keshan1 compared to N0. Nitrogen application significantly improves the allocation of photosynthetic nitrogen in the middle and upper leaf positions, promoting the hundred-grain weight in the middle and upper leaf positions. In summary, optimized nitrogen allocation accounted for the improvement in canopy PNUE while maintaining a high grain yield.