## 4 Discussion
### Factors Influencing the Atmospheric Oxygen Balance  
As shown in Table 5, oxygen emission was strongly related to the ecological land area (\(r=0.978\)) and the proportion of urban population (\(r=-0.927\)). Significant correlation also existed between oxygen emission and the proportion of rural population (\(r=0.927\)), the GDP of primary industry (\(r=0.851\)), rural population (\(r=0.815\)), the proportion of ecological land area (\(r=0.786\)), and the proportion of construction land area (\(r=-0.780\)). Therefore, ecological land, with the strongest correlation to oxygen emission, was the primary factor that directly affected oxygen emission. As we know, agricultural and forest land was an important part of ecological land in a region. Thus, the more ecological land that was available in relationship to the proportion of rural population and primary industry would result in a greater release of oxygen. Oxygen consumption was strongly related to primary energy consumption (\(r=1.000\)) and GDP (\(r=0.990\)). Energy consumption would directly lead to oxygen consumption, with 96.14% of oxygen \begin{table}
\begin{tabular}{c c c c} \hline
**Study Area** & **Vegetation NPP Density** & **Vegetation NEP** & **Vegetation Oxygen Emission** \\  & **(g C m\({}^{-2}\) a\({}^{-1}\))** & **Density(g C m\({}^{-2}\) a\({}^{-1}\))** & **Density (g O m\({}^{-2}\) a\({}^{-1}\))** \\ \hline Guangzhou [17] & - & - & 2326 \\ PRD [27] & 1002 & - & 1263 \\ Subtropical China [68] & 1000 & - & - \\ Russia [58] & - & 255 & - \\ Germany [58] & - & 598 & - \\ Italy [58] & - & 879 & - \\ Loblolly pine forest in California [67] & - & 602 & - \\ This study & 937 & 505 & 1346 \\ \hline \end{tabular}
\end{table} 
Table 4: _Cont._ \begin{table}
\begin{tabular}{c c c c} \hline
**Influencing Factor** & **Oxygen Emission** & **Oxygen Consumption** & **Oxygen Balance** \\ \hline Total population & \(-0.192\) & **0.971** & 0.238 \\ Urban population & \(-0.382\) & **0.977** & 0.393 \\ Proportion of urban population & \(-0.927\)** & 0.528 & **0.770** \\ Rural population & **0.815** & \(-0.060\) & \(-0.666\) * \\ Proportion of rural population & **0.927** & \(-0.528\) & \(-0.770\)** \\ Construction land area & \(0.151\) & **0.703** & \(-0.102\) \\ Proportion of construction land area & \(-0.780\)** & 0.374 & **0.950** \\ Ecological land area & **0.978** & \(-0.203\) & \(-0.842\)** \\ Proportion of ecological land area & **0.786** & \(-0.383\) & \(-0.950\)** \\ GDP & \(-0.323\) & **0.990** & 0.240 \\ GDP of primary industry & **0.851** & \(-0.525\) & \(-0.704\) * \\ GDP of secondary industry & \(-0.164\) & \(-0.531\) & 0.083 \\ GDP of tertiary industry & \(-0.365\) & **0.785** & 0.348 \\ GDP of industry & \(-0.212\) & \(-0.503\) & 0.143 \\ Primary energy consumption & \(-0.291\) & **1.000** & 0.244 \\ \hline \end{tabular} Note: ** Correlation is significant at the 0.01 level; * Correlation is significant at the 0.05 level.  
\end{table} 
Table 5: Correlation coefficients among the influencing factors and atmospheric oxygen emission, consumption, and balance. consumption in the PRD stemming from the burning of fossil fuels. Furthermore, energy consumption was highly correlated to GDP. Generally speaking, the larger the GDP, the more fossil fuels combusted and the more oxygen consumed. The oxygen balance was very strongly correlated with the proportion of construction land area (\(r=0.950\)) and the proportion of ecological land area (\(r=-0.950\)), as well as having a significant correlation with the proportion of urban population (\(r=0.770\)) and the proportion of rural population (\(r=-0.770\)). The proportion of different land use types and population types could mirror the process of urbanization. The extent of urbanization was obviously an important factor affecting the urban oxygen balance. The greater the extent of urbanization, the larger the energy consumption was. Therefore, together with a smaller ecological land area due to urban expansion, there would be a higher oxygen balance index, _i.e._, serious oxygen deficit.