3.1 Nutrient content of the leaves and litter layer for the three plantations
The content of organic carbon (OC), TN, TP and TK were different in leaves and litter layer for the different tree species (Fig. 2). The organic carbon content in the leaves and litter of P. tabuliformis was clearly higher than that of L. principis-rupprechtii and P. crassifolia (Fig. 2A). The content of N, P and K in the leaves and litter of L. principis-rupprechtii and P. crassifolia, however, was higher than that of P. tabuliformis (Fig. 2B, C). The C:N ratio, C:P ratio and N:P ratio of leaves of P. tabuliformis was the highest, followed by L. principis-rupprechtii and then P. crassifolia (Fig. 2E, F, G). The C:N ratio and C:P ratio of litter for P. tabuliformis was the highest, followed by P. crassifolia, and the lowest for L. principis-rupprechtii (Fig. 2E, F). The N:P ratio of litter of P. tabuliformis was higher than those of L. principis-rupprechtii and P. crassifolia (Fig. 2G).
3.2 Soil nutrient content for the three plantation stands
Overall, SOC, TN and TP showed a gradually decreasing trend from the litter layer to deep soil layers for the three plantation stands (Fig. 3). The only exception was for the TP in the L. principis-rupprechtii stand, where there were no significant differences between different soil layers (Fig. 3A, B, C). This trend was because C, N and P released by litter decomposition were mainly concentrated in the topsoil layer, with only a small percentage of nutrients reaching the deeper soil layers. However, there was no significant difference in soil TK in the different soil layers for the three plantation species (Fig. 3D). Furthermore, the C:N, C:P and N:P ratio exhibited a gradually decreasing trend from surface soil layers to deep soil layers; the exceptions were for the C:N ratio of the P. crassifolia stand and the C:P and N:P ratio of the P. tabuliformis stand, where there were no significant differences between the different soil layers (Fig. 3E, F, G). On the whole, the SOC, TN, TP, C:N ratio, C:P ratio and N:P ratio of the L. principis-rupprechtii stand were higher than in the P. crassifolia and P. tabuliformis stands; except for individual nutrient indexes (such as TN and N:P ratio), for which there was no significant difference between the surface and the deep soil layer.
Available nutrients ( -N, -N, AP and AK) in the soil also exhibited a gradually decreasing trend from the topsoil to deep soil layers for the three forest stands (Fig. 4). The available nutrients were highest in the L. principis-rupprechtii stand, followed by the P. crassifolia stand, and lowest in the P. tabuliformis stand. The differences declined with depth in the soil profile for the three forest stands, so there was no significant difference in AP and AK in the deepest layer.
3.3 Soil physical properties for the three plantation stands
The different tree species also had different effects on the soil physical properties of the different soil layers (Fig. 5). There was no significant difference in soil bulk density, soil capacity, soil total porosity and soil capillary porosity in soil layers down to 30cm under the P. tabuliformis stand (Fig. 5A, B, C, D). The soil capacity, soil total porosity and soil capillary porosity exhibited a gradually decreasing trend from the topsoil to deep soil layers for L. principis-rupprechtii and P. crassifolia stands, while soil bulk density showed the opposite trend. The soil bulk density of P. crassifolia and P. tabuliformis stands was higher than that of the L. principis-rupprechtii stand (Fig. 5A), while the soil capacity, soil total porosity and soil capillary porosity of the L. principis-rupprechtii stand were higher than those of the P. crassifolia and P. tabuliformis stands (Fig. 5B, C, D), except that there was no significant difference in the soil capillary porosity in the 20-30cm layer (Fig. 5D).
3.4 The correlation between soil nutrient content and physical properties
Pearson correlation analysis was performed to evaluate the correlation between soil nutrient content and physical properties (Table 1). The results reveal that the SOC, TN, -N, -N, AP and AK in the soil were significantly positively correlated with soil capacity, total porosity and capillary porosity (P<0.05 or P<0.01), with the exceptions that there was no significant correlation between soil total porosity and -N, TP and TK. However, there was significant negative correlation between soil bulk density and SOC, TN, -N, -N, AP and AK (P<0.05 or P<0.01). In addition, the SOC, TN, -N, -N, AP and AK contents of soil were significantly positively correlated with each other (P<0.05 or P<0.01), and the soil capacity, total porosity and capillary porosity were also significantly positively correlated with each other (P<0.05 or P<0.01). These results indicate that the water permeability and water storage capacity of the soil markedly improved with increasing soil organic matter and available nutrients.
Table 1. Pearson correlation coefficients between soil nutrient contents and physical properties.
|
SOC
|
TN
|
TP
|
TK
|
-N
|
-N
|
AP
|
AK
|
Soil bulk density
|
Soil capacity
|
Total porosity
|
TN
|
0.98**
|
|
|
|
|
|
|
|
|
|
|
TP
|
0.34
|
0.45
|
|
|
|
|
|
|
|
|
|
TK
|
-0.50
|
-0.44
|
-0.24
|
|
|
|
|
|
|
|
|
-N
|
0.81**
|
0.81**
|
0.18
|
-0.24
|
|
|
|
|
|
|
|
-N
|
0.72*
|
0.73*
|
0.28
|
-0.20
|
0.95**
|
|
|
|
|
|
|
AP
|
0.92**
|
0.88**
|
0.17
|
-0.28
|
0.87**
|
0.81**
|
|
|
|
|
|
AK
|
0.79*
|
0.83**
|
0.30
|
0.03
|
0.91**
|
0.88**
|
0.88**
|
|
|
|
|
Soil bulk density
|
-0.88**
|
-0.91**
|
-0.31
|
0.47
|
-0.82**
|
-0.69*
|
-0.71*
|
-0.73*
|
|
|
|
Soil capacity
|
0.95**
|
0.96**
|
0.27
|
-0.46
|
0.84**
|
0.73*
|
0.82**
|
0.78*
|
-0.97**
|
|
|
Total porosity
|
0.80**
|
0.87**
|
0.29
|
-0.25
|
0.69*
|
0.55
|
0.61
|
0.70*
|
-0.95**
|
0.92**
|
|
Capillary porosity
|
0.85**
|
0.84**
|
0.11
|
-0.20
|
0.95**
|
0.90**
|
0.91**
|
0.91**
|
-0.79*
|
0.87**
|
0.71*
|
** indicates P<0.01, * indicates P<0.05.
3.5 Soil particle-size distribution for the three plantation stands
The percentage of clay and silt in the topsoil of the three plantation stands are higher than those in the deep soil, which is gradually decreasing from topsoil to subsoil (Table 2). The percentage of sand in the topsoil of the L. principis-rupprechtii and P. crassifolia plantation stands is lower than subsoil, which is gradually increasing from topsoil to subsoil. However, the distribution of sand in P. tabuliformis plantation stand has high heterogeneity.
The soil texture triangle showed the texture class of three different soil layer for three different plantation stands (Fig. 6A). The different soil layers of Larix principis-rupprechtii and Red-leaf plum plantations had similar soil particle size composition. the sand content of subsoil was generally higher than topsoil, and the clay content of topsoil was generally higher than subsoil (Fig. 6B). Moreover, the distribution of sand and clay contents of different layers soil had high heterogeneity for Pinus tabuliformis plantation stand (Fig. 6B).
3.6 The relationship between soil particle-size distribution and soil physical and chemical properties
Path analysis showed that soil organic carbon (SOC) had direct effect on clay (0.76), silt (0.66), and sand (-0.94), while total potassium (TK) had an indirect effect on clay (0.75), silt (0.85), sand (-0.79). The SOC and TK had a negative effect on sand, while SOC and TK had a positive effect on clay and silt (Fig. 7). In addition, we can also find that SOC has a negative effect on TN, while SOC has a positive effect on soil bulk density (BD). This is also confirms the results in Table 1.