Soil improvement benefit analysis
The soil texture is one of the important physical properties of soil, which is also an important index. The survival and growth of the Artificial Tamarix Chinensis forests greatly depend on the powder content(Deng et al. 2016; Dexter et al. 2004). As seen from the vertical distribution of the soil grain size (Fig. 1), the grain size composition was changed as follows: the mass percentage of sand decreased with the increase of the soil depth, and the mass percentage of powder and clay increased with the increase of the soil depth. The proportion of the powder in the soil texture at each test site was slightly higher than that of each control site. It indicates that the growth of the Artificial Tamarix Chinensis forests could improve the soil texture and contribute, to a certain extent, to the growth of the herbaceous plants within the forest, which is further beneficial to improve the soil texture. However, it takes a long time before significant change can be observed other than the short period of this project. The soil fertility generally depends on the soil organic matter as a key material basis.
Figure 1 Vertical distribution of soil particle size in different experimental areas
The soil organic matter content is an important indicator of soil fertility(Six et al. 2000; Yin et al. 2010). In this project, the content of the organic matter in each soil layer at each test site was higher than that at each control site respectively (Fig. 2). For the distribution in soil, the organic matter in the layer between 0–20 cm was of highest and gradually decreased in layers from 20 to 60 cm, but not significant. It is speculated that Tamaxix Chinensis was inoculated with the Cistanche and greatly affected by human activities, such as annual ploughing, inoculation and Cistanche harvesting, causing a large amount of organic matter buried in the lower layers. Therefore, little difference in organic matter content was observed among different soil layers.
Figure 2 Vertical distribution of soil organic matter in different experimental areas
The soil organic carbon is a key component of the arable soil, and plays a very important role in soil fertility, environmental protection and sustainable development of farmlands(Sartori et al. 2007; Su et al. 2018; Wang et al. 2010; Zhang et al. 2018). In this project, higher content of organic carbon in each layer at each test site (except Moyu County) was observed than that at the corresponding control site (Fig. 3). Since the organic carbon comes from the soil organic matter, the same trend can be observed in terms of organic carbon and organic matter, i.e. decreasing from top to bottom.
Figure 3 Vertical distribution of soil organic carbon in different experimental areas
Same as the organic matter, the three necessary nutrients for plant growth, N, P, and K, are mainly derived from the accumulation of biological organisms (Zuo et al. 2010). In this project, the distribution of soil total N, total P and total K at each test site were basically the same as that of the organic matter, and their contents were higher than those at the control sites (Fig. 4). Therefore, it can be seen that the growth of the Artificial Tamarix Chinensis forests could enhance the supply of soil N, P and K. And the individual difference may depend on the different soil parent material and soil organic matter. In addition, the annual harvesting of Cistanche could also take away certain amount of N, P and K, an unignorable reason accounting for such difference.
Figure 4 Vertical distribution of soil total N, P and K in different experimental areas
To clarify the correlation among the physical and chemical properties of the soil at the ecology restoration sites, the correlation analysis of the average values for the different indicators of each soil layer was conducted. Let X1: organic matter (g/kg), X2: organic carbon (g/kg), X3: total N (g/kg), X4: total P (g/kg), X5: total K (mg/kg), and X6: grain size < powder (%), and the relevant analysis results are shown in Table 8.
Table 8
Correlation degree of soil physical and chemical properties factors
| X1 | X2 | X3 | X4 | X5 | X6 |
X1 | 1.00 | 0.84 | 0.51 | 0.55 | 0.52 | 0.50 |
X2 | | 1.00 | 0.12 | 0.13 | 0.63 | -0.46 |
X3 | | | 1.00 | 0.25 | -0.43 | 0.34 |
X4 | | | | 1.00 | -0.59 | 0.74 |
X5 | | | | | 1.00 | 0.03 |
X6 | | | | | | 1.00 |
It can be seen from the above table that there is close correlation among the soil physical and chemical factors. Significant positive correlation among soil organic matter, organic carbon, total N, total P and total K was observed to be consistent with the theory. Secondly, significant positive correlation was also observed between the soil organic matter content and soil grain size < powder content, indicating that with the increase of the organic matter content in soil, there were more frequent microbial activities, faster decomposition rate of the sand and better optimization and improvement in soil texture.At the same time, there is close correlation between the composition of soil particles and the content of N and P in soil. In general, higher proportion of fine particles generates finer texture, and it is more favorable for the absorption and storage of the nutrients. The increased nutrient contents could be, in turn, conducive to the formation of the soil aggregate structure and to the improvement of the soil stability(Yang et al. 2016; Yi et al. 2007).
Regional microclimate improvement benefit analysis
The regional microclimate refers to that, within the limited range of the Artificial Tamarix Chinensis forests in the ecology restoration area, the local meteorological factors, such as light, temperature and humidity, are significantly different from those outside of the range. Its formation is due to the radiation characteristics of the underlying surface and the different exchange process with the atmosphere(Dale et al. 1999).
In this project, there was consistency in the daily temperature ranges of the artificial Tamarix Chinensis forests at all test sites (Fig. 5). The daily trend was to increase and then gradually decrease, with a parabolic shape. The highest temperature was observed at about 14:00 in local time. In general, the regulation of the air temperature with the windbreak forest in August is more obvious than that in April. This is due to the hot temperature in summer, the lush canopy, the reduced net radiation, the lowered solar radiation and long-wave radiation at the arrival zone, and absorption of much heat by the transpiration of the trees. In general, the regional microclimate improvement of temperature by the Artificial Tamarix Chinensis forest is mainly reflected in the stabilization of the temperature on both low and high ends of the temperature range.
Figure 5 Diurnal temperature variation during April and August in different experimental areas
There was consistency in the daily relative humidity ranges of the artificial Tamarix Chinensis forests at all test sites. The relative humidity at the test sites was higher than that at the control sites both in April and August (Fig. 6). The effectively increased relative humidity within the forests was mainly due to the occlusion of the canopy, reduced wind speed, weakened turbulent exchange, hindered diffusion of water vapor, and prolonged detention of the water vapor from the canopy transpiration and soil evaporation. The daily trend was exactly opposite to the temperature. It was decreased and then increased with an inverted parabolic shape. The lowest relative humidity was observed at about the time of the highest temperature (14:00–16:00) when there were calm wind and fastest transpiration of the leaves and crops. In addition, the regulation of the air relative humidity with the windbreak forest in August is more obvious than that in April. This is due to the lush canopy blocking the exchange between inside and outside of the forest and to the powerful root system absorbing enough soil moisture for the consumption of the transpiration and supplying the moisture in the air(Freedman et al. 2014; Yin et al. 2007).
Figure 6 Diurnal relative humidity variation during April and August in different experimental areas
Reduced wind speed is the most basic benefit of Artificial Tamarix Chinensis forests. In this project, significantly reduced wind speed by the Artificial Tamarix Chinensis forests was observed (Fig. 7). The wind speed reduction in August was significantly better than that in April, due to the lush canopy in summer. The leaves were less in April and the wind blocking was largely achieved by the branches of the trees. The windproof performance was elevated in August due to the growth of branches and leaves, the friction of which, together with the trunks, consumed more kinetic energy of the wind (Liu et al. 1996; Ma et al. 2009; Okin et al. 2006).
Figure 7 Change of relative wind speed during April and August in different experimental areas