# Urbanization and Grain Production Pattern of China: Dynamic Effect and Mediating Mechanism
## 2 Theoretical Analysis and Hypothesis  
Urbanization is a process of changes in social structure, spatial structure, and economic structure, which is intuitively manifested by the movement of population to cities and towns, the transformation of agricultural land into construction land, and the resulting changes in industrial structure. Under the dualistic urban-rural institutional structure, the migration and agglomeration of rural population to urban areas constitutes the main driving force for urbanization in China [25]. However, the limited urban land area inevitably limits the population carrying capacity. As a result, the urban land boundaries extend continuously from the suburban areas to the rural hinterland. In academic circles, the GPP is often characterized by the grain production concentration index (GPCI), which is the share of grain production in a region [11]. As the material basis for grain production, cropland resources can be reallocated and adjusted due to urbanization and converted to industrial or urban construction land with the expansion of cities and towns [26], resulting in the withdrawal of cropland resources and fluctuations in grain supply and changes in the GPP. Despite the policy of balancing land occupation and compensation in urbanization, which aimed to protect cropland, the quality of cropland is often reduced due to insufficient or neglected compensation for occupied cropland. Changes in the quantity and quality of cropland in different regions inevitably lead to differential fluctuations in grain production and trigger changes in the GPP. The consequent scarcity of cropland also drives up the cost of grain production, which reduces the incentives of farmers to engage in grain production. These rigid constraints on cropland and labor may be detrimental to grain production [27]. Urbanization also has an impact on the ecological environment, and this environmental impact on agriculture is mainly manifested in pollution and destruction to water bodies, soil, and other ecosystems of farmland, and these negative environmental externalities may have a negative impact on local grain production [28]. In addition, urbanization also comes with industrial structure upgrades that promote the flow of factors and resources to secondary and tertiary industries. As a result, the factor inputs in the agricultural sector are restructured, and farmers are less likely to plant grains in cropland, which is also detrimental to regional grain production. In conclusion, the constraining effect of urbanization on grain production inevitably affects the GPP and decreases the GPCI. Therefore, the following hypothesis can be formulated. **Hypothesis 1 (H1). **_Urhanization can directly and negatively affect GPP--i.e., urbanization decreases the GPCI._  
The reduced cropland due to urbanization, on the one hand, leads to changes in the cropland utilization manifested as the reallocation of input intensity and the structure of production factors, triggering fluctuations in the multiple-crop index (MCI). On the other hand, facing a rise in production cost, farmers must choose to continue planting grain, planting non-grain crops, or abandoning farming for land circulation, thus triggering a change in planting structure (PS). In addition to the changes in agricultural production methods, urbanization also contributes to the advancement of agricultural technology (ATECH). At the macro level, urbanization affects the GPP mainly through the reallocation effect of cropland utilization, the structural effect of planting choices, and the progress effect of agricultural technology. Specifically, the indirect effect mechanism of urbanization on GPP is mainly reflected in the following transmission paths (Figure 1). The purpose of cropland utilization is to improve the output and economic efficiency of cropland based on its stock by redistributing production factor inputs and tapping the potential of cropland utilization [29].