## 3 Results  
### Soil PSD and Ds under Different PSC Systems  
Soil PSD of surveyed plots under six PSC systems were classified (as shown in Figures 3-5). In the surveyed plots, the prevailing soil PSC was the size of sand particles (>70.000%) followed by silt and clay (<4.000%) contents. Soil of this nature is classified as quartisamment (U.S. Soil Taxonomy), which is identified in semiarid regions of China. Amount of variability as observed in P1 and other plots. From P1 (2700 trees-ha\({}^{-1}\)) to P1X (900 trees-ha\({}^{-1}\)), the content of clay and silt decreased, while sand content gradually increased with forest density. Compared with CK (0.000 \(\pm\) 0.000%, 11.000 \(\pm\) 0.000%, and 89.000 \(\pm\) 0.000% for clay, silt, and sand contents under USDA (1993) PSC system), from P1 (4.667 \(\pm\) 1.155%, 20.000 \(\pm\) 1.732%, and 75.333 \(\pm\) 2.887%) to P1X (0.000 \(\pm\) 0.000%, 11.333 \(\pm\) 0.577%, and 88.667 \(\pm\) 0.577%), clay contents were increased by 466.700% and 0.000%. silt contents were increased by 81.819% and 3.0273%. Sand contents were decreased by 15.356% and 0.374%. Clay and silt content differed greatly between MPPs and CK. Figure 3: Clay contents of surveyed plots under six PSC systems. Vertical bars represent standard errors (\(n=3\)). ANOVA with Duncan test was used with different letter in the same row are significantly different at the 0.05 level. Among clay, silt, and sand contents calculated by using six PSC systems, China (1987), USDA (1993), ISSS (1929), and ISO14688 (2002) PSC systems had the widest range of clay (\(I<2.0\) um), while Katschinski (1957) and Blott & Pye (2012) had a relatively narrow range of clay (\(I<1.0\) um and \(\phi>9.0\)). As shown in Figure 3, clay contents of six PSC systems varied greatly, especially in plots of P\({}_{\rm II}\), P\({}_{\rm III}\), P\({}_{\rm IV}\), P\({}_{\rm V}\), and P\({}_{\rm VI}\) (\(p<0.05\)). The ISO14688 (2002) and Blott & Pye (2012) PSC systems had the widest range of silt (\(2\) um \(<I<63\) um, \(4.0<\phi<9.0\)), while ISSS (2002) PSC system had a relatively narrow range of silt (\(2\) um \(<I<20\) um) (Figure 4). There were significant differences between Blott & Pye (2012) and ISSS (1929) PSC systems in calculating silt contents of MPPs and CK plots (\(p<0.05\)). Additionally, there were significant differences between China (1987), USDA (1993), and ISSS (1929) PSC systems in calculating silt contents in plots of P\({}_{\rm V}\) and P\({}_{\rm VI}\) (\(p<0.05\)) (Figure 5). Accordingly, there were significant differences between ISSS (2002) and the other five PSC systems in calculating the content of sand fractions in surveyed plots (\(p<0.05\)). The ISO14688 (2002) and Blott & Pye (2012) PSC systems had a relatively narrower range of sand. Moreover, there were obviously differences between these two PSC systems and other four PSC systems in calculating the content of sand fractions in plots of P\({}_{\rm II}\), P\({}_{\rm III}\), P\({}_{\rm V}\), P\({}_{\rm VI}\), P\({}_{\rm VII}\), and CK (\(p<0.05\)). Based on the results of soil PSD under different PSC systems, then to explore \(D\)s calculated by different PSC systems. \(D\)s calculated by China (1987), Katschinski (1957), USDA (1993), ISSS (1929), ISO14688 (2002), and Blott & Pye (2012) PSC systems covered a range from 1.897 to 2.519, 1.897 to 2.507, 2.0372 to 2.575, 2.00140 to 2.545, 1.895 to 2.554, and 1.918 to 2.504, respectively. The highest \(D\) was found in P\({}_{\rm I}\) with clay and silt contents of 4.667 \(\pm\) 1.155% and 20.000 \(\pm\) 1.732% and sand content of 75.333 \(\pm\) 2.887% (USDA (1993) PSC system). The lowest \(D\) value corresponded to CK with clay and silt contents of 0.000 \(\pm\) 0.000% and 11.000 \(\pm\) 0.000% and higher sand content (89.000 \(\pm\) 0.000%) (Figures 3-5). In addition, the general sequence of \(D\)s is the following: USDA (1993)? ISO14688 (2002)? ISSS (1929)? Katschinski (1957)? China (1987)? Blott & Pye (2012) PSC systems (Figure 6). Figure 4: Silt contents of surveyed plots under six PSC systems.