Soil is the basis of the growth and development of terrestrial plants, which elements such as N and P in the soil play an extremely important role in the process of plant growth, material circulation, and energy conversion in terrestrial ecosystem.
4.1 Analysis of soil characteristics of different Pisha sandstones
In this study, we found that the average soil concentrations of TN, AN, Ni, TP, and AP in shallow soil (0–20 cm) of the bare area were 0.51 g/kg, 23.98 mg/kg, 5.14 mg/kg, 4.85 g/kg, and 4.50 mg/kg, respectively. The average concentrations in a soil-covered area were 0.37 g/kg, 16.82 mg/kg, 1.19 mg/kg, 4.64 g/kg, and 7.56 mg/kg, respectively. While these concentrations in the sand-covered area were 0.36 g/kg, 6.23 mg/kg, 0.89 mg/kg, 2.88 g/kg, and 4.39 mg/kg, respectively. According to China's second national soil survey standard 20. All the available nitrogen (AN, Ni) concentrations were at level 6 and all the TP concentrations were at level 1 for the three Pisha sandstone soils (Higher grades indicate lower concentrations and poorer soils). The concentration levels of TN and AP in bare Pisha sandstone soils were both level 5; The TN concentration level for soil covered Pisha sandstone soils was level 6 and the AP concentration level was level 4. The TN concentration level of the sand-covered Pisha sandstone soil was level 6 and the AP concentration level was level 5.
Therefore, it was clear from the above that there was no significant difference in the N and P nutrient profiles of the three Pisha sandstone soils, where the N concentration was extremely deficient and the P concentration was relatively abundant but the available components were lacking. That made it difficult for the local population to grow food crops in the traditional sense (corn, wheat) in the Pisha sandstone region. More people were choosing to grow buckwheat and sorghum, which are more tolerant of barrenness and drought. Based on the overall spatial distribution of N and P nutrients it appears that the soils of all three Pisha sandstones are extremely poor. Pisha sandstone soils have high infiltration rates and saturated hydraulic conductivity as well as mostly alkaline pH values (between 8.89 and 10.02), while natural precipitation has mostly acidic pH values (between 5.2 and 6.8)21. Therefore, the soil is susceptible to chemical reactions while being eroded by surface runoff generated by rainfall. During this erosion process, surface soils and soil nutrients are also continuously lost due to the action of runoff erosion. Local farmers planted economic fruit groves in areas with good soil and water conditions, but that didn't work to reduce surface runoff erosion and improve soil fertility. That' s because most of the planting of economic fruit groves were done on the basis of destroying the native plant communities. The understory vegetation was even more devastatingly destroyed, with the process of surface cooked soil production severely affected, and the pathway for replenishing organic matter and N elements into the soil was basically cut off.
Soil erosion was further exacerbated by the lack of organic matter replenishment, soil consolidation and structural deterioration, making it more difficult for surface vegetation to survive. That is the main reason for the low soil fertility in the Pisha sandstone region 22; 23. The effect of this on the local population was that more fertilizer had to be applied to the land in order to get the normal output in the farmland. About 90% of the fertilizer put into the soil was in the form of chemical fertilizer, although it rapidly replenished the N deficiency in the Pisha sandstone soil. But the consequence of heavy use of chemical fertilizers has been a deterioration of the soil structure and poor permeability of the slab. Erosion is more likely to occur when rainfall generates surface runoff, which causes the loss of N and P elements, resulting in surface source pollution and eventually returning to the vicious cycle of continued fertilizer application 24; 25. By accessing the local farmland, we found that the phenomenon of soil caking was common, and the local farmers could only solve it by manual loosening. This increases the labor cost which is the main reason for the indirect poverty of local farmers.
Large coefficients of variation (CV) of TN and TP of soils in the Pisha sandstone area were approximately 25.02% on average, while the coefficients of variation (CV) of available nutrients AN, Ni, and AP were relatively small averaged about 15.01%. It was mainly due to the fact that the composition of TN and TP in soil nutrients is mainly influenced by the composition of the parent rock minerals that form the soil and most of them are present in the stable condition26. Although the sampling sites of different Pisha sandstone soils all belong to the Pisha sandstone area, the parent rock minerals (rock composition) of the Pisha sandstone forming soils are very complex, consisting of thickly bedded sandstones of Paleozoic Permian, Mesozoic Triassic, Jurassic and Cretaceous, mixed together to form a combined rock structure27. Therefore the degree of variation (CV) of TN and TP in the soil formed after its weathering and decomposition would be very significant (p<0.05). The available nutrients AN, Ni, and AP in the soils of the Pisha Sandstone were closely related to the climatic conditions of the region, land use practices, decomposition of litter, and the forms of nutrient uptake and utilization by plants. More precisely, the available nutrients (AN, Ni and AP) in the soil are more closely related to the agricultural and livestock production practices in the region. The inhabitants in this region are more likely to ensure output by heavy input of fertilizers and pesticides due to lack of knowledge about the nature of Pisha sandstone soil. The effective N and P content of the soil depends more on the residual fertilizer content after erosion by surface runoff (this effect was most pronounced at the top of the slope). That was one of the reasons for its relatively small coefficient of variation28; 29.
4.2 Status of soil TN and TP in different types of area
Due to the concentration of precipitation in the Pisha sandstone region and the special complex geological conditions (combined form of sandstone). It has been formed by the interaction of wind and water erosion over a long period of time, forming a criss-crossing valley21; 30; 31. This leaves rocks of different properties exposed to the surface in turn. As a result, a landscape resembling a ribbon form pattern (different rock layers folded together) was formed on the slope face of the valley. Only at the top and bottom of the slope at the ends of the sub-basin section are there more (mature) soils, while on the slope surface the soils are very sparse that are mostly raw soils formed by direct weathering of the arsenic sandstone (lacking effective N and P for plant use). On slopes it is difficult to retain water and fertilizer due to the large slope(the slope was generally 40° to 60° degrees, with a maximum of 90°). The average concentration of TN and TP were mainly influenced by the parent material (rock composition) that formed the soil. The Pisha Sandstone is also a clastic rock with complex composition and obvious layering26.
Therefore, this results in a relatively large variation in TN and TP concentration despite being in the same Pisha sandstone area. For example, in the same Pisha sandstone area, there were significant differences (p<0.05) in the TN and TP concentrations at different slope locations on the same cross-section. The TN and TP concentration levels at the top and bottom of the slope were significantly higher than those at other slopes. On the contrary, in different Pisha sandstone type areas, due to the same rock properties, the concentrations of TN and TP will tend to be at the same level with a larger number of samples. For example, the average TN concentration of the soil-covered Pisha sandstone was not significantly different from that of the sand-covered Pisha sandstone. There was no significant difference in the mean TP concentration of the soil between the bare Pisha sandstone compared to the soil-covered Pisha sandstone.
4.3 Status of soil AN、Ni and AP in different types of area
Soil N is one of the most important essential elements that can be taken up by plants. AN and Ni are both available N in soil N elements that can be absorbed and used by plants. Although AN is partially affected by TN, 95% of available N in soil comes from the decomposition of organic matter in the soil28; 32. After studying the physicochemical properties of the soil after afforestation, it was found that the reserves of soil organic carbon (SOC), available nitrogen and phosphorus (AN, Ni, AP) in the sand-covered Pisha sandstone area increased significantly, it increased by 47.8-69.1% compared to the level of the soil on the surface without vegetation29. The reason why soil AN and Ni of bare Pisha sandstone were significantly higher than those of soil-covered Pisha sandstone and sand-covered Pisha sandstone was that soil-covered Pisha sandstone and sand-covered Pisha sandstone were subjected to certain anthropogenic disturbance and damage. For example, unreasonable farming, fuelwood cutting and grazing.
Our team's investigators found through extensive field visits and surveys that the current state of affairs was mainly composed of two reasons. Firstly, because the soil conditions of soil-covered Pisha sandstone was better than bare Pisha sandstone (the thickness of soil layer of soil-covered Pisha sandstone was more than 300% higher than bare Pisha sandstone). As a result, most of the production and living activities of local industry, mining and agriculture were concentrated here. Due to population pressure, the excessive intensity of agricultural and livestock production has put a great strain on soil fertility (including AN, Ni, AP). In addition to the irrational use of natural resources by local residents. The effective N (AN, Ni) of the soil cover Pisha sandstone (which originally has the highest soil fertility under natural conditions) becomes lower than that of the partially exposed Pisha sandstone region. The second was due to differences in the way the soil was replenished with water. Since no agricultural production activities would take place in the bare Pisha sandstone region, only drought-resistant native vegetation is present. That requires less water and basically relies on natural precipitation for recharge, which is very similar to sprinkler irrigation in agricultural irrigation. The soil cover Pisha sandstone demanded a great deal of water due to the large amount of agricultural activities to be carried out, especially during the filling period of the crop. When local people replenish their farmland with water, they mostly use diffused irrigation due to economic (lack of funds to purchase sprinkler equipment) and cognitive (no difference between sprinkler and diffuse irrigation) reasons, and only a few areas use sprinkler irrigation. The biggest impact of sprinkler and diffuse irrigation on the soil in the region was found through field visits to be the soil structure. Similarly planted buckwheat farmland, soil porosity using sprinkler irrigation for water replenishment could be more than 50% higher compared with diffuse irrigation. At the same soil moisture level, the increase in soil porosity greatly increased the activity of soil microorganisms and promoted the conversion rate of organic matter to effective N and P (AN, Ni, AP).
Wind and water erosion is more intense in the Pisha Sandstone area, and the accumulation of organic material on the surface is minimal, which severely amplifies the effects caused by anthropogenic damage. Meanwhile the vegetation cover of soil-covered Pisha sandstone and sand-covered Pisha sandstone was declining, exacerbating the rate of loss of available soil N (AN, Ni). Litter and livestock manure can increase the organic matter concentration of the soil, which in turn increases the available N. However, the special climatic (high wind and low rainfall) and geographical conditions (high soil permeability) of the region inhibit the decomposition of litter and livestock manure, which further worsens the loss of available N elements (AN, Ni) from the soil (Gao et al., 2019).
Phosphorus as a relatively stable and poorly migrating sedimentary mineral, where the level of available phosphorus (AP) is mainly influenced by climatic environment, vegetation type and parent material forming the soil33; 34. The AP concentration in the soil-covered Pisha sandstone area was significantly higher than that in the bare Pisha sandstone and sand-covered Pisha sandstone (p<0.05), which is due to the fact that AP is mainly composed of water-soluble phosphorus, partially adsorbed phosphorus and organic phosphorus. Water-soluble phosphorus is present in soil moisture and can be directly absorbed and used by plants, while adsorbed and organic phosphorus needs to be adsorbed in soil aggregates and cannot be free in soil moisture. So this good soil water-holding capacity and loose agglomerate structure is more adsorptive to AP. However, the soil water-holding capacity and agglomerate structure of the soil-covered Pisha sandstone are obviously better than those of the bare Pisha sandstone (less soil) and sand-covered Pisha sandstone (more gravel), which is the main reason for the higher AP concentration of the soil-covered Pisha sandstone.
The distribution patterns of available nutrients (AN, Ni, AP) of the soils were essentially the same characteristics at different slope locations in the three Pisha sandstone areas. The soil concentration levels of available nutrients (AN, Ni, AP) showed an overall decreasing trend from the top of the slope to the middle and bottom of the slope. It is mainly caused by severe soil erosion caused by heavy rainfall during the rainy season (July to September), resulting in serious soil loss and nutrient loss from the middle and lower surfaces of the slope. Due to the concentration of precipitation and the large relative height difference between the top and bottom of the slope (the maximum height difference in the study area exceeds 100 m), it is difficult to form barriers because of the sparse vegetation on the slope near the bottom of the ditch. When runoff from rainfall collects at the bottom of the gully, the flow rate and ability to carry sediment are greatly enhanced, and the nutrient-rich topsoil deposits at the bottom of the slope are more susceptible to erosion. Therefore, in the middle, lower and bottom parts of the slope, there were few nutrient-rich topsoils and mostly immature subsoils. In addition, the soil of Pisha sandstone contains mostly sulfur trioxide and phosphorus pentoxide. These two substances are easily soluble in water and then produce harmful substances such as sulfuric acid and phosphoric acid, which acidify the soil at the bottom of the slope and reduce the biological activity, thus inhibiting the growth of vegetation. This was the main reason for the overall decline of AN, Ni, and AP from the top to the bottom of the slope observed in this study. And this result was consistent with the findings of Wang et al. who studied the slopes of the agro-pastoral transition zone on the Loess Plateau 27. Liu et al studied the nutrient differences between topsoil (0-30 cm) and subsoil (30-60 cm) in the agro-pastoral ecological zone of northwest China and came to similar conclusions29. They found that topsoil was more abundant near the top of the slope, while soils in the middle, lower, and bottom parts of the gully were basically bare subsoil due to erosion.
Therefore, if the soil productivity of Pisha sandstone area is to be improved. On the one hand, it is necessary to increase the protection of existing natural vegetation, reduce negative human interference, prohibit indiscriminate cultivation and grazing, and select infertile-tolerant plants such as Hippophae rhamnoides Linn and Achnatherum splendens (Trin) Nevski according to local conditions to increase vegetation cover and thus reduce soil erosion. On the other hand, certain chemicals should be applied to improve the soil in areas where water tends to accumulate, such as at the bottom of ditches and slope bottoms, so that they can carry more plant growth and thus improve the ecological environment of the Pisha sandstone region. Meanwhile, with the drive of global climate change, the response mechanisms of N and P levels to soil moisture, slope and soil depth in each type of Pisha sandstone area need to be studied more systematically in the future, with a view to providing scientific basis and reference for the scientific evaluation of ecological restoration and management of different types of Pisha sandstone areas.