4.1. The Response of Forest Characteristics to TN and TP
Previous studies have focused relatively more on explaining the composition of watershed land cover to explain changes in nutrients [
23], but there are not many studies on the impact mechanisms of specific land cover internal characteristics. However, the absorption and interception of nutrients by forests can effectively reduce the output of watershed pollutants [
42]. This study deeply explored the characteristics of forestland to explore the mechanism of forestland characteristics on the nitrogen and phosphorus output intensity of the basin.
The results of this study indicated that the output intensity of TN and TP in the basin were consistent, but the output intensity of total phosphorus was lower than that of total nitrogen. The rainy season output intensity of nitrogen and phosphorus pollutants was much higher than that in the dry season, mainly due to the severe mineralization process of litter in the rainy season. Therefore, in the rainy season, the polymorphism of nitrogen and phosphorus in the litter is more easily washed into the river by rainfall [
43,
44], and the rainy season also has more abundant precipitation than the dry season, resulting in more surface runoff [
45]. Gu, et al. [
46] and Wilson [
47] also proved that the phosphorus dynamics in soil was strongly influenced by climatic factors.
• How the hydrological characteristics of different FLTs and the degree of litter mineralization of affect the nutrient output intensity.
Compared with FRSE and FRSD (
Figure 5), FRST had smaller nitrogen and phosphorus output intensity, indicating that mixed forests have more effects especially the prominent absorption and interception on nitrogen and phosphorus nutrients. This result is consistent with the conclusion of Sprenger et al., that the FRST obtained in the study performed better at preventing nutrient loss [
48]. As an important carrier of nutrient salt outputs such as nitrogen and phosphorus, water directly affects the migration and transfer process of nutrients such as nitrogen and phosphorus [
21,
23]. Liu et al., found a significant positive correlation between runoff and nitrogen and phosphorus loss, and the degree of feedback from different forest types to forest hydrology also led to differences in nitrogen and phosphorus output intensity [
49]. Mixed forests have the characteristics of multiple forests. The canopy can effectively intercept precipitation and reduce the kinetic energy of raindrops [
34]. The lower litter layers increase the surface coverage [
50], buffer the splashing kinetic energy of raindrops that penetrate the forest, increase soil water infiltration and reduce surface runoff. The amount of nitrogen and phosphorus transported by runoff is limited, which directly reduces the output intensity of nitrogen and phosphorus. Compared with mixed forests, evergreen and deciduous forests represent single forest types. Although the evergreen forest had obvious advantages in intercepting the canopy, the surface cover was lower. When rainfall reach the canopy to and eventually the ground, certain degree of surface runoff results [
51]. As shown in
Figure 4, the nitrogen and phosphorus output of the FRSD was similar to the FRSE pattern during the rainy season, but there were significant differences between March and April. Drought mainly occurs throughout winter, and when rain occurs in March, the litter layer of the deciduous forest can intercept and block the occurrence of surface runoff, however, due to the polymorphism of nitrogen and phosphorus compounds in the litter, nitrogen and phosphorus mineralization occurs [
43]. The mineralized nitrogen and phosphorus are easily washed away by rainwater and directly enter the river in runoff, increasing the nitrogen and phosphorus output intensity of the basin.
• Forests can control the production of NSP pollutants and simultaneously intercept the migration of NSP pollutants.
An increase in forest area contributes to water, infiltration, reducing the possibility of soil nutrient loss. The results of this study also prove this point.
Figure 6 shows that when the forest area was greater than 75%, the nitrogen and phosphorus output intensity was the lowest. The hydrological process is an important mechanism to explain the changes in NSP pollutants in the basin [
14]. The increase in forests reduces the total flow at the basin scale. The large number of NPS in the soil lacks transport carriers [
39], reducing the risk of pollutant output. Cecílio et al., pointed out that in areas with high forest cover, the water flow is relatively more stable and sustainable [
35], which also explains why the annual output intensity of nitrogen and phosphorus was stable and in the sub-basin with 75% forest coverage in this study. However, in this study, when the forest coverage rate was between 50–75%, the nitrogen and phosphorus output intensity was relatively higher intensity.
Figure 1 shows that this part of the sub-basin was mainly located in the transition zone from mountain to plain, and the area not covered by forest was reclaimed as sloping land. Studies have proven that sloping farmland is an important source of nitrogen and phosphorus output [
26,
42], which directly increases the nitrogen and phosphorus output intensity of these sub-basins. This finding also proves the research results of Cecilio et al. [
35]. The impact of forest cover on the nitrogen and phosphorus output of the basin was mainly shown in the large watersheds, and in the smaller watershed, the impact of forest cover was uncertain [
52].
• Forest buffers could trap, infiltrate, adsorb, and convert contaminants.
NPS pollution is mainly divided into process and migration factors [
53]. The extensive and deep roots and highly permeable soils in forest areas provide nutrient flow protection, forming a sink. Contaminants are trapped, infiltrated, adsorbed and converted as they flow through forest buffers via surface runoff [
53]. However, few scholars have quantitatively studied the influence of the distance between forest and river on the output intensity of nitrogen and phosphorus. This study explored the influence of the position of forestland and river on the nitrogen and phosphorus output in a single independent catchment unit. The forestland around the river had a significant reduction effect on the amount of nitrogen and phosphorus in the river. The nitrogen and phosphorus output intensity of the forestland within 1000 m of the river reduced the TN output by 55.22% and the TP output by 53.48% compared with the forestland that was 1000 m away. However, there is little doubt here why the nitrogen and phosphorus output intensity when the forest distance was less than 500 m was not lowest. There are three main reasons. First, in the hot and humid environment during the rainy season, the mineralization process of the litter in the forest is intense. When rainfall occurs during the rainy season (March–September), polymorphic nitrogen and phosphorus in the litter are more easily washed by rain and enter the river [
43,
54], as shown in
Figure 8, and the difference in the nitrogen and phosphorus output intensity is large. Moreover, the second reason is related to the location. The runoff on steep terrain is relatively more serious in terms of soil erosion, resulting in more soil entering the river. Vilmin et al. [
55] pointed that slow runoff increased the accumulation of phosphorus, and polymorphic nitrogen and phosphorus are lost and eventually enter the river with water and sediment. Third, the balance of forest infiltration and evaporation has a direct effect on the nitrogen and phosphorus output transport capacity of the basin [
30].
4.2. Optimized Allocation of Forestland
At present, most studies have adjusted the land use layout to obtain the best management model for NPS pollution prevention and control, but there has been less consideration regarding the internal characteristics of forestland. The existing research can only determine the number of areas of forestland in the best management mode, and the location relationship with other land types; how the characteristics of the forest interior should be configured is not clear. This study tentatively proposes an optimal allocation model based on three types of FLTs, WFC and DFR. At the same time, considering the differences in the ecological and economic functions between mountainous and plain areas, a differentiated allocation model based on the three elements is proposed based on the topography.
The results of the study showed that FRST had a relatively lower nitrogen and phosphorus output intensity while the other characteristics remained unchanged (
Table 5). Therefore, this configuration used FRST as the main FLTs. Based on clarifying the type of forest stand, we further explored the allocations of WFC and DFR in different terrain conditions. HB was divided into mountainous areas and plain areas. The sub-basin in each area was divided into low-intensity, medium-intensity and high-intensity output according to the natural break method. The optimal allocation of forestland under different terrains was investigated by studying the allocation of forestland in low-intensity sub-basins.
Table 7 shows the status of forestland allocation in the low-intensity sub-basins with the nitrogen and phosphorus outputs in the mountainous and plain areas.
Mountain areas are recommended for high-coverage forests; they should be densely planted and in close proximity to river areas. In the sub-basins with the lowest nitrogen and phosphorus output intensity (
Table 7), most of the sub-basin forest coverage exceeded 75%, and the distance from the river was also within 500 m. Bonnesoeur et al., found that the soil erosion intensity of large forests on steep slopes was relatively lower [
27]. Therefore, when forestland was used in the mountainous area, it tended to maintain the water and soil conservation water source function, which requires high coverage, especially in areas with steep slope, to reduce the nitrogen and phosphorus output caused by soil erosion via runoff [
56].
It is not necessary to plant high-coverage forestland in the plain area, but afforestation should be carried out near the water outlet of the cultivated land. It is also recommended that a forest buffer zone be placed at different distances from the river. Cultivated land in the plain area of the study area was the main type of land cover, and the distribution of forestland was lower and scattered. Therefore, for this reason, the forest coverage of the sub-basin with the lowest nitrogen and phosphorus loss in the plain area was less than 25%;
Table 7 shows that more than 50% of the sub-basin forestland with a low-intensity nitrogen and phosphorus output was within 500 m of the river. Although the coverage of forestland was low, the distance between the forestland and river was relatively close, and the forestland was similar to the existence of the buffer zone. Zhang et al., found that the existence of a river vegetation buffer could reduce the TN (by 13.94%) and TP (by 9.86%) entering a river, reducing the risk of nitrogen and phosphorus output in the basin [
57]. This result suggests that the allocation of forestland in the plain should focus on the buffer function to intercept and absorb the nitrogen and phosphorus flowing through the forest [
27]. The results in
Table 6 also show that increasing the forest coverage had a positive effect on reducing nitrogen and phosphorus. Appropriately increasing the forest coverage to reduce the nitrogen and phosphorus output intensity also needs to be considered. However, due to the developed agriculture in the plain area, the soil fertility is also good. Large-scale afforestation is obviously inappropriate, and therefore, large-scale afforestation along the river is a good choice. Riverside shelterbelts should be constructed, riverside soil should be consolidated, and surface runoff should be intercepted.
Therefore, comprehensively considering the layout pattern of mountains and plains, in the HB, our proposed model in the mountainous area is FRST + WFC (>75%) + DFR (<500 m). the plain area is a little different: FRST + WFC (>25%) + DFR (<500 m). The results of the optimized pattern of forestland layout were shown in
Figure 9. We optimized the configuration mode from mountain and plain terrain.
4.3. Research Issues and Prospects
Our results showed that the specific characteristics of forestland in the watershed have a certain effect on the regulation of NPS pollution in the watershed. The research of some scholars in other regions also proves our point of view. Such as Cecílio et al. [
35] proved that the location of forestland could influence the streamflow. Gu et al. [
46] pointed out that lithological and bioclimatic had impact on soil phosphatase activities in California temperate forests. These studies all revealed that the characteristics of land use have a significant impact on nitrogen and phosphorus output.
When studying the impact of coverage and forest location, disturbances in terrain conditions were found. Topographical characteristics influence nutrient transport pathways [
58], and relatively higher slope variability leads to higher flow and erosion intensity, increasing the amount of nutrient outputs in the basin [
59]. The feedback of nitrogen and phosphorus output in forestland under different topographic conditions deserves further study.
Under the same topographic conditions, the nitrogen and phosphorus output of the watershed with the same forestland characteristics, forest coverage and geographical location still had large differences. Studies have shown that the landscape pattern of watershed cover has an important impact on the hydrological cycle and nutrient pollution process of a basin [
21,
60,
61]. Lee et al. (2009) and Shi et al., found that large unbroken forests showed greater water purification potential and improved the water quality [
8,
22]. In this study, affected by topographic conditions, economic development and agricultural farming, the distribution of forestland in the transition zone of hills and plains was not concentrated, and the size of forest plaques also differed. These factors may have resulted in the output intensity of nitrogen and phosphorus in the basin having the same characteristics as forestland. Different dominant factors and the actual impact capacity of the forest landscape pattern will be the focus of the next phase. Of course, the focus of this study is the output characteristics of NPS pollution load in each season of the year. To more accurately show the influence of forestland characteristics on the nitrogen and phosphorus output of the basin, long-term research can be carried out in subsequent research. The next study will compare and analyze the impact characteristics of forestland features on watershed nutrients at different time scales.