Closing yield (YG) and nitrogen use efficiency (NUE) gaps amongst smallholder farmers are critical to ensuring food security and environmental sustainability. Here we report a comprehensive analysis of a data set derived from farm-surveys and previously published studies in a typical high N input area of China. Using scenario analysis, farmers from both sources were divided into four rice production levels to assess the YG and NUE of smallholder farmers. Farm surveys showed that rice yield and partial factor productivity of applied N (PFP_N) averaged 8273 kg ha⁻¹ and 23.1 kg kg⁻¹ at regional-scale, with a potential increase of 21–43% and 33–52%, respectively. A wide variation in yield and PFP_N across different types of farmer was observed. Optimized N management significantly narrowed YGs by 3.7% and PFP_N gap by 63.5% on average based on the published literature. Multiple factors, including excessive N rates, small rice planting area (0.33 ha), the aging (51 years old) and low education level (6 years of education) of the major labor force, can partly explain causes of yield and NUE gaps. These findings provide farm-based evidence that centralized management, good agronomy plus advances in knowledge and technologies are essential for future agricultural development.

Accurately understanding the critical threshold of leaf magnesium (Mg) concentration is crucial for rapid diagnosis of crop Mg status; however, little information is available on critical Mg concentration for different physiological processes in dicots and monocots. Here, we investigated the sensitivity of biomass, chlorophyll (Chl) at different leaf positions/ages, leaf area (LA), and photosynthesis (P_n) to Mg deficiency between rice (Oryza sativa L.) and cucumber (Cucumis sativus L.). Plants were grown hydroponically under twelve Mg concentration gradients. Results showed reducing the external Mg supply to a certain level resulted in significant decline in biomass, Chl, LA, and P_n in both plants. A leaf Mg threshold of 0.97 mg g⁻¹ DM (dry matter) for total biomass was found in rice, which was not identified in cucumber. Critical Mg thresholds for Chl a, b, and carotenoids (Car) showed a decreasing trend with leaf age, suggesting Chl in upper young leaves are more sensitive to Mg deficiency; however, visible Mg-deficiency symptoms were predominantly in mid-aged leaves with a higher rate of Mg remobilization, especially in cucumber. Leaf critical Mg concentrations for Chl a+b, P_n, and LA were 1.22, 1.05, and 1.00 mg g⁻¹ DM in rice, respectively, which were lower than those of cucumber, 4.23, 4.09, and 3.55 mg g⁻¹ DM, implying that cucumber was more susceptible to low Mg stress; Chl a+b was the most sensitive indicator of Mg deficiency. Overall, Chl a+b of upper young mature leaves can be used as an early diagnostic index of Mg nutrition in crops, especially Mg-insensitive crops.