## 4 Discussion  
Despite the country's overall increasing trend of cropland GPP, a spatial pattern of \"increasing in the north and decreasing in the south\" was identified, which is consistent with changes in grain yield in China [50; 51]. The increase in grain yield per unit area was the direct reason for the rise in cropland productivity in the north of China [52; 53], which increased from 168.5 kg mu\({}^{-1}\) (about 252.8 g m\({}^{-2}\)) in the 1980s to 316.5 kg mu\({}^{-1}\) (about 474.8 g m\({}^{-2}\)) in 2007, primarily due to agriculture policy support and technological progress [53; 20; 54; 55]. Since 2004, China has implemented agricultural subsidy programs, such as \"Quality seed subsidy\", \"Direct grain subsidy\", and \"Machinery subsidy\". When domestic chemical fertilizer prices began to rise with international prices, China extended the subsidy program to include the \"Aggregate input subsidy\", and the total growth of the four major subsidies continued [7]. In addition, we noted that the low- and medium-yield cropland in China was about two-thirds of the total cropland, and the grain yield was unstable and had large fluctuations between years [56]. To promote grain yield, China has launched construction projects for well-maintained farmland, which aimed to improve low- and middle-efficiency cropland and lead to the construction of 400 million mu high-standard farmland by 2020, of which about 70% is located in the main production areas of China (i.e., HHH, NE, and MLY) [57]. Researchers have further evaluated the effectiveness of the constructed high-standard farmland. The results showed that productivity in the project area has generally improved, especially in the north of China. In 35% of the project area, productivity increased by more than 100 kg mu\({}^{-1}\) (about 150 g m\({}^{-2}\)) until 2017, and approximately 97% of it was located in NE, IM, and HHH [58], which is consistent with the regionally averaged results shown in Figure 3. In addition to human activities, climate change has also affected cropland productivity; because of the strengthening of the East Asian summer monsoon, precipitation increased in northern China [59], and the rising precipitation promoted an increase in cropland GPP in NE and IM, which have relatively dry climates and widely distributed rainfed farming. Although the irrigation area, consumption of fertilizers, and quantity of agricultural machinery also significantly increased, the significant decrease in cropping intensity was a predominant factor in the decline in cropland GPP in southern China, covering MLY, SW, and SC. These regions experienced an increase in single-cropping area and a decrease in multiple-cropping area [20]. Urban expansion and the growth of out-migrant agricultural workers will lead to extensive farming [60]. In seeking maximum economic benefits at the household level, increases in the out-migrant population have caused declines in available farmworkers and continuous growth in labor costs for major crops [20]. The percentage of households in rural areas without farming activities markedly increased from less than 5% to 23% between 2000 and 2013 [61]. More provinces in southern China have become the main sources of migrant workers, which is consistent with the distribution of hotspots of declining cropping intensity [20]. Although there have been fewer farmworkers, landowners have preferred to retain their land for family farming [60], which accounted for 59% of the land transfer area [62], and this can lead to decreased cropping intensity [63]. Further contributing to the cost of productivity are rising rural labor wages, which annually increased by more than 8% after the mid-2000s [64; 65], and increasing productivity costs can reduce enthusiasm for farming [66]. Although the cropland productivity in southern China has decreased, the food security of China is essentially stable because of the productivity increase in northern China [62]. The continuous increase in cropland productivity in northern China faces a series of challenges.