The complex and volatile international landscape has significantly impacted global grain supply security.  This study uses a complex network analysis model to examine the evolution and trends of the global major grain trade from 1990 to 2020, focusing on network topology, centrality ranking, and community structure.  There are three major findings.  First, the global major grain trade network has expanded in scale, with a growing emphasis on diversification and balance.  During the study period, the United States, Canada, China, and Brazil were the core nodes of the network.  Grain-exporting countries were mainly situated in Asia, the Americas, and Europe, and importing countries in Asia, Africa, and Europe.  Second, a significant increase in the number of high centrality countries with high export capacity occurred, benefiting from natural advantages such as fertile land and favorable climates.  Third, the main global grain trade network is divided into four communities, with the Americas-Europe community being the largest and most widespread.  The formation of the community pattern was influenced by geographic proximity, driven by the core exporting countries.  Therefore, the world needs to enhance the existing trade model, promote the multi-polarization of the grain trade network, and establish a global vision for the future community.  Countries and regions should participate actively in global grain trade security governance and institutional reform, expand trade links with other countries, and optimize import and export policies to reduce trade risks.

The application of organic fertilizers has become an increasingly popular substitution in maize production to reduce gaseous nitrogen (N) loss and soil degradation caused by inorganic fertilizers.  Organic fertilizer plays a key role in improving soil quality and stabilizing maize yields, but studies that refine different substitution rates remain poorly documented.  A field study was carried out in 2021 and 2022 based on a long-term trial initiated in 2016.  The experiment included five organic fertilizer N substitution rates with equal input of 200 kg N ha^–1: 0% organic fertilizer (T1, 100% inorganic fertilizer), 50.0% organic+50.0% inorganic fertilizer (T2), 37.5% organic+62.5% inorganic fertilizer (T3), 25.0% organic+75.0% inorganic fertilizer (T4), 12.5% organic+87.5% inorganic fertilizer (T5), and no fertilizer control (T6).  The average result of two years showed that T3 and T1 had the highest grain yield and biomass, respectively, and there was no significant difference between T1 and T3.  Compared with T1, 12.5, 25.0, 37.5, and 50.0% substitution rates (T5, T4, T3, and T2) significantly reduced total nitrogen loss (NH₃、N₂O) by 8.3, 16.1, 18.7, and 27.0%, respectively.  Nitrogen use efficiency (NUE) was higher in T5, T3, and T1, and there was no significant difference among them.  The organic fertilizer substitution directly reduced NH₃ volatilization and N₂O emission from farmland by lowering ammonium nitrogen and alkali-dissolved N content and by increasing soil moisture.  These substitution treatments reduced N₂O emissions indirectly by regulating the abundance of AOB and nirK-harboring genes by promoting soil moisture.  The 37.5% of organic fertilizer substitution reduces NH₃ volatilization and N₂O emission from farmland by decreasing ammonium nitrogen and alkali-dissolved N content and increasing moisture which negatively regulate the abundance of AOB and nirK-harboring genes to reduce N₂O emissions indirectly in rainfed maize fields on the Loess Plateau of China.