## 3 Results
### Spatial and Temporal Comparison of Added and Lost GPP  
As depicted in Figure 9, from 1991 to 2020, Guangdong Province lost 55.6 million tons of GPP and gained 40 million tons, resulting in a deficit of 15.5 million tons. Most prefecture-level cities failed to achieve a balance in GPP. Cities that maintained a basic GPP balance were mainly in the PRD, including Zhongshan, Zhuhai, Foshan, Dongguan, and Shenzhen. Zhanjiang experienced a serious imbalance, losing up to 261 thousand tons in 2010. From 1991 to 1995, 15 prefecture-level cities faced serious imbalances, driven by the reform policies that spurred significant economic and social development, resulting in extensive cropland being encroached upon or abandoned. Since the early 21 st century, China has implemented the strictest cropland protection measures. During the 2000-2005 period, GPP was significantly supplemented, with 11 prefecture-level cities achieving basic equilibrium or even perfect balance. These well-compensated areas were mainly located in the western Guangdong region. After this period, the GPP balance improved compared to the 1990s. According to China's policy of the requisition-compensation balance of cropland, Guangdong Province has generally achieved a dynamic balance in cropland area and grain production. However, considering the impact of cropland abandonment on GPP, the newly added GPP has not completely offset the lost GPP.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multicolumn{1}{c}{**Districts**} & \multicolumn{3}{c}{**Area (km\({}^{2}\))**} & \multicolumn{3}{c}{**GPP (10\({}^{4}\)t)**} & \multicolumn{3}{c}{**Area (km\({}^{2}\))**} & \multicolumn{1}{c}{**GPP (10\({}^{4}\)t)**} \\ \cline{3-10} \multicolumn{1}{c}{} & **1991–2000** & **2001–2010** & **2011–2020** & **1991–2000** & **2001–2010** & **2011–2020** & **1991–2020** & **1991–2020** \\ \hline \multirow{5}{*}{PRD} & Dongguan & 666.75 & 449.26 & 386.02 & 29.59 & 22.45 & 23.19 & \multirow{5}{*}{} & \multirow{5}{*}{} \\  & Foshan & 832.73 & 780.58 & 696.76 & 45.40 & 43.14 & 49.49 & & \\  & Guangzhou & 1500.35 & 1400.05 & 1307.57 & 64.09 & 62.65 & 65.98 & & \\  & Shenzhen & 438.76 & 343.76 & 319.11 & 15.47 & 12.92 & 13.49 & & \\  & Zhongshan & 413.48 & 386.75 & 364.47 & 17.59 & 16.84 & 17.72 & 27,646.26 & 1212.26 \\  & Zhuhai & 308.98 & 276.86 & 332.72 & 4.15 & 3.94 & 4.96 & & \\  & Huizhou & 2065.61 & 1837.81 & 1759.69 & 89.06 & 83.49 & 93.92 & & \\  & Jiangmen & 1959.56 & 2148.80 & 1755.18 & 77.42 & 84.47 & 88.62 & & \\  & Zhaoqing & 1816.27 & 1722.22 & 1376.17 & 65.61 & 53.55 & 63.07 & & \\ \hline \multirow{5}{*}{EG} & Shantou & 644.49 & 535.57 & 512.59 & 26.15 & 21.86 & 24.73 & \multirow{5}{*}{} & \multirow{5}{*}{} \\  & Shanwei & 1157.29 & 913.34 & 1027.29 & 44.88 & 36.52 & 46.38 & & \\  & Jieyang & 1564.35 & 1249.34 & 1073.70 & 70.22 & 56.56 & 59.38 & & \\  & Chaozhou & 649.11 & 515.66 & 422.58 & 24.54 & 19.46 & 19.56 & & \\ \hline \multirow{5}{*}{WG} & Zhanjiang & 4828.42 & 4440.01 & 4087.52 & 296.44 & 282.80 & 316.81 & \multirow{5}{*}{} & \multirow{5}{*}{} \\  & Yangjiang & 2059.52 & 1909.23 & 1250.46 & 82.04 & 77.35 & 60.27 & 26,817.51 & 1520.07 \\ \cline{1-1}  & Maoming & 3171.66 & 3217.11 & 1853.58 & 141.26 & 147.28 & 115.83 & & \\ \hline \multirow{5}{*}{NG} & Meizhou & 1693.05 & 1351.37 & 1425.61 & 54.02 & 39.81 & 58.91 & \multirow{5}{*}{} & \multirow{5}{*}{} \\  & Yunfu & 1490.85 & 1411.38 & 1027.13 & 50.55 & 40.33 & 44.87 & & \\ \cline{1-1}  & Shaoguan & 1925.13 & 1602.81 & 1257.45 & 100.03 & 66.32 & 74.64 & 24,991.77 & 1006.29 \\ \cline{1-1}  & Qingyuan & 2864.81 & 2491.39 & 1913.23 & 121.56 & 90.52 & 101.41 & & \\ \cline{1-1}  & Heyuan & 1854.51 & 1231.11 & 1451.94 & 64.67 & 40.12 & 58.55 & & \\ \hline \hline \end{tabular}
\end{table}
Table 5: Statistics on the area and GPP of complemented cropland.