Soil Nitrogen Application Regulates Photosynthetic Limitations in Plants: Insights from a Controlled Experiment

Soil nitrogen (N) availability modulates the relative magnitudes and dominant roles of stomatal, mesophyll, and biochemical limitations to photosynthesis (denoted as lsc, lm, and lb, respectively); however, the dynamic changes in these factors and their relative contributions under varying N application levels remain controversial. In this study, we quantified the responses of photosynthetic limitations in soybeans subjected to four N application levels (0, 7.5, 15.0, and 22.5 g urea m−2). Moderate N addition (≤15.0 g urea m−2) increased lsc and lb but decreased lm, whereas excessive N addition (22.5 g urea m−2) caused a rapid increase in lm, exceeding both lsc and lb. lm was negatively correlated with mesophyll conductance (gm) and net photosynthetic rate (An), whereas lb was positively correlated with An. Changes in photosynthetic limitation were accompanied by increased chlorophyll and leaf N contents and increased activities of aquaporins (AQPs), carbonic anhydrase (CA), and Rubisco under moderate N addition. These results indicated that the relative contribution of lm to the total limitation (denoted as MCL) decreased under moderate N addition but increased sharply under high N addition, exceeding the contributions of lsc and lb (denoted as SCL and BL, respectively). Under an excessive N supply of 22.5 g urea m−2, MCL was the highest among the three components, although this ranking should be interpreted considering the reference-state dependence of the limitation framework. This study provides a basis for rational N fertilization in soybean production.