# Diagnosing Phosphorus Limitation in Subtropical Forests in China under Climate Warming
## 4 Discussion
### Factors Affecting Phosphorus Limitation in Subtropical China  
Though the additional P demand over 1951-2010 varied among forest sites and forest types, the P limitations are largely dependent on current ASP. For example, the AAPD for EBF in Mt. Gongga is higher (231 mg\(\cdot\)m\({}^{-2}\)), but the AAPD/ASP there is the lowest (6.7%) among the EBFs in the four sites (Table 2), which indicates the importance of the ASP in determining the intensity of P limitation, and hence, it is necessary to investigate the spatial distribution of ASP in subtropical China.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Scenario** & **Shennongjia** & **Qian Yanzhou** & **Mt. Gongga** & **Dinghushan** \\ \hline RCP4.5 (S\({}_{1}\)) & 32.1 & 24.7 & 12.8 & 27 \\ RCP8.5 (S\({}_{1}\)) & 62.2 & 68.4 & 32.2 & 47 \\ RCP4.5 (S\({}_{2}\)) & 3.2 & 2.5 & 1.3 & 2.7 \\ RCP8.5 (S\({}_{2}\)) & 6.3 & 6.9 & 3.3 & 4.7 \\ \hline \hline \end{tabular} S\({}_{1}\) and S\({}_{2}\) are the scenarios for the low and high mineralization rates, respectively. The accumulated additional phosphorus demand (AAPD) is for the period of 2018–2099, and ASP is the available soil phosphorus observed in 2018.  
\end{table}
Table 4: Comparison of AADP/ASP (%) for RCP8.5 and RCP4.5 among forest sites for the low and high mineralization rate scenarios.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Forest Site** & \(\Delta\)**T (\({}^{\circ}\)C)** & \(\Delta\)**NPP (gC\(\cdot\)m\({}^{-2}\)-a\({}^{-1}\))** & \multicolumn{2}{c}{**AAPD (mg\(\cdot\)m\({}^{-2}\))**} \\ \cline{3-5}  & & & **S\({}_{1}\)** & **S\({}_{2}\)** \\ \hline Shennongjia & 3.99 & 327 & 568 & 57 \\ Qian Yanzhou & 3.41 & 368 & 644 & 65 \\ Mt. Gongga & 2.91 & 371 & 651 & 65 \\ Dinghushan & 2.32 & 381 & 668 & 67 \\ \hline \hline \end{tabular} \(\Delta\)T and \(\Delta\)NPP represent temperature and NPP difference between 2099 and 2018. The accumulated additional phosphorus demand (AAPD) is for the period of 2018–2099, and ASP is the available soil phosphorus observed in 2018. S\({}_{1}\) and S\({}_{2}\) are the scenarios for the low and high mineralization rates, respectively.  
\end{table}
Table 3: Predicted temperature and NPP difference as well as P limitation in forest sites in subtropical China under RCP8.5.  
Our estimates of ASP density show a large variety among sites and forest types. The ASP density is controlled by many factors, such as the parent material, soil weathering regime, biota, and the relative importance of these controls during soil development [51]. Therefore, the large variety in observed ASP density is understandable. [PERSON] et al. (2014) reported that the observed ASP densities in the central subtropical region (109\({}^{\circ}\)45\({}^{\prime}\) E, 26\({}^{\circ}\)50\({}^{\prime}\) N) for ENF are in the range of 620-840 mg\(\cdot\)m\({}^{-2}\)[55], which are lower than our estimate of the ASP density for ENF in Qian Yanzhou. [PERSON] et al. (2005) demonstrated that the average ASP in tropical and subtropical China is 4800 mg\(\cdot\)m\({}^{-3}\) in the top 50 cm soil [51], which could be changed to a density of 2400 g\(\cdot\)m\({}^{-2}\) for comparison in this study, and is close to our estimates of ASP density in Mt. Gongga.  
The AAPD/ASP for both EBF and ENF in Qian Yanzhou are the highest among those in the forest sites, and are due to higher AAPD and lower ASP. The forests in Qian Yanzhou are plantations, which implies human activities may have decreased the ASP there. According to the eighth national inventory of forest resources, there are 41.06 million hectares of plantations in subtropical China, which account for 19.8% of China's total forest area. [PERSON] et al. (2014) reported an even lower (less than 840 mg\(\cdot\)m\({}^{-2}\)) ASP in plantations in the central subtropical region near Qian Yanzhou [55], which implies lower ASP may widely exist in the subtropical plantations.