# Analysis of Cultivated Land Productivity in Southern China: Stability and Drivers
## 4 Discussion
### Climate Change and Cropping Systems' Influence on the Stability of Cultivated Land Productivity  
The spatial trend in long-term cultivated land per unit area is key in assessing the stability of cultivated land productivity. The expansion of cultivated land area and an increase in primary crop yield per unit area are two key driving forces for the growth in the NPP of global cultivated land [56]. In the Poyang Lake region, the primary yield per unit area significantly increased in most regions. Notably, the growth in NPP in the southern and northern regions showed obvious spatial heterogeneity, and the increase in NPP in the south was significantly greater than that in the north. Such spatial distribution characteristics emphasize the differences and complexity of cultivated land productivity in different geographical regions. The change in the NPP of cultivated land around Poyang Lake was mainly affected by climate change and lake ecosystems. The lack of obvious changes in the northern region may be related to relatively stable natural conditions, land-use patterns, and agricultural technology levels, while the significant and extremely significant increases in the peripheral region may be related to superior natural conditions, innovation and promotion of agricultural technology, and policy. In addition to natural conditions and the level of agricultural technology, planting patterns, irrigation, and water conservancy projects in agricultural practices have contributed to improving NPP. However, continuous land cultivation and climate change negatively impacted NPP. Extreme weather events, such as droughts, floods, and low temperatures, may lead to crop growth inhibition or death, thus reducing NPP per unit area. Long-term climate change trends, such as temperature increases and precipitation mode changes, may also adversely affect crop growth [20]. In addition, continuous cultivation may lead to soil degradation, such as a decline in soil fertility, structural damage, and soil acidification, which impacts crop growth and yield, leading to a decline in NPP per unit area [66]. In addition to natural factors, the transformation of agricultural planting systems also has an important impact on NPP. A key factor for increasing NPP per unit area is the improvement in the multiple cropping index. Optimizing the crop planting structure, increasing the crop planting density, prolonging the growth period, or adopting rotation, intercropping, and other planting modes can improve the multiple cropping index, crop yield, and NPP per unit area [67,68]. [PERSON] et al. [69] argued that although the total rice yield in southern China decreased owing to a reduction in the double-cropping rice area, the rice planting share in Anhui and Jiangxi and the multiple cropping index in Hunan increased, which contributed to improved NPP in some regions. In recent decades, changes in the rice cropping system in southern China have significantly impacted NPP. Substantial farmland has transitioned from double-cropping to single-cropping rice, which reflects seasonal land abandonment [70]. From 1990 to 2015, the multiple cropping index of rice decreased from 148.3% to 129.3%. The most dramatic change was observed in the middle and lower reaches of the Yangtze River Plain, which includes the Poyang Lake region, with the distribution of rice farming systems in the south changing from north to south, with double-cropping rice decreasing and single-cropping rice increasing [71]. This transformation from double-cropping to single-cropping rice reduced the planting density and multiple cropping indices, affecting the NPP promotion potential. In addition, improved natural conditions in the southern Poyang Lake region, coupled with the north being more directly and significantly affected by the hydrological conditions of the Yangtze River, partially explain why the NPP growth rate in the southern region was higher than that in the northern region.