# Sloping Farmlands Conversion to Mixed Forest Improves Soil Carbon Pool on the Loess Plateau
## 4 Discussion  
### Vegetation Restoration Types Affect SOC and Soil N and P Properties  
Land-use change following farmland conversion can cause a change in soil C [11]. The mixed forests had the largest positive effects on SOC, followed by the grasslands (Figure 2), because the mixed forests and natural grassland consume less soil water than single shrubs and trees in arid and semiarid regions [32], leading to a higher soil water content or unchanged soil water following vegetation restoration [32]. Higher soil moisture will promote plant growth and thus produce more plant biomass and litter input into the soils, consequently improving the accumulation of SOC [2]. In addition, the mixed forests had the largest positive effects on soil N and P compared with the sloping farmland (Figure 3). The direct possible reason is that mixed forests resulted in the greatest increase in SOM among the four vegetation restoration types in the study area. Usually, different vegetation types provide different surface residues and root distributions [33], leading to varied soil N content. For example, the patterns of soil N dynamics differed greatly among different tree species used in afforestation and depended on the transfer of soil organic matter (SOM) into soil via the roots of ground vegetation and litter decomposition [9]. Soils with different vegetation undergo different litter decomposition processes and rates, meaning that the release of N and P in soil differs [34]. Generally, all vegetation restoration types significantly  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Factors** & **Soil Properties** & \begin{tabular}{c} **Pearson Correlation** \\ **Coefficient** \\ \end{tabular} & **Relative Contribution Rate (\%)** & \(p\) \\ \hline \
\begin{tabular}{c} Total \\ Error \\ \end{tabular} & & &
\begin{tabular}{c} 81.6 ** \\ 18.4 * \\ \end{tabular} & & \\ \hline C fractions & LOC & 0.976 ** & 6.0 & \textless{}0.01 \\ \hline \multirow{4}{*}{Soil nutrients} & TN & 0.937 ** & 5.8 & \textless{}0.01 \\  & AN & 0.969 ** & 6.0 & \textless{}0.01 \\  & TP & 0.833 ** & 5.2 & \textless{}0.01 \\  & AP & 0.818 ** & 5.1 & \textless{}0.01 \\ \hline \multirow{4}{*}{Soil physical properties} & pH & \(-\)0.477 & 4.2 & \textgreater{}0.05 \\  & BD & \(-\)0.097 & 0.6 & \textgreater{}0.05 \\  & Sand & 0.039 & 0.2 & \textgreater{}0.05 \\  & Silt & 0.370 & 2.3 & \textgreater{}0.05 \\  & Clay & 0.180 & 1.1 & \textgreater{}0.05 \\ \hline \multirow{4}{*}{Soil microbial activities} & Urease & \(-\)0.089 & 0.6 & \textgreater{}0.05 \\  & Amylase & \(-\)0.213 & 1.3 & \textgreater{}0.05 \\  & Phosphatase & 0.916 ** & 5.7 & \textless{}0.01 \\  & Saccharse & 0.615 * & 3.8 & \textless{}0.05 \\  & Cellulase & 0.693 ** & 4.3 & \textless{}0.01 \\  & Polyphenol oxidase & \(-\)0.872 ** & 5.4 & \textless{}0.01 \\  & Catalase & 0.587 * & 3.6 & \textless{}0.05 \\  & MBC & 0.841 ** & 5.2 & \textless{}0.01 \\  & MBN & 0.353 & 2.2 & \textgreater{}0.05 \\  & MBP & 0.813 ** & 5.0 & \textless{}0.01 \\  & BR & 0.894 ** & 5.5 & \textless{}0.01 \\  & qCO\({}_{2}\) & \(-\)0.411 & 2.5 & \textgreater{}0.05 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Pearson correlation coefficient between soil organic carbon and other soil properties. Note: * Correlation is significant at the 0.05 level (\(p\) \(<\) 0.05) (2 tailed) and ** Correlation is significant at the 0.01 level (\(p\) \(<\) 0.01) (2 tailed). N = 15. ** indicates the value was the explain rates based on the coefficient of determination (\(R^{2}\)) of the multivariable linear regression analysis; * indicate the residual contribution rate of the multivariable linear regression, which indicates other factor’s contribution to SOC that were not determined in this study.  
increased TN, TP, AN and AP content (Figure 3), mainly because vegetation cover, plant species and biomass increased markedly after farmland abandonment [6]. Ground litter decomposition by microbes and root extension may contribute the most to soil N and P accumulation [34,35].