The Environmental Impact Of Gully Vegetation Restoration In The Hilly Loess Plateau

Gullies are an important geomorphic unit in the Loess Plateau. Rehabilitation of gully degraded vegetation system has important practical meaning for controlling soil and water erosion, improving ecological environment, and realizing sustainable development of ecosystem. With gradual implementation of the ecological environment construction in western China, studies of ecological environment effects of vegetation restoration process has become a hot topic of concern researchers. Although researchers and masses of working people have accumulated and created a lot of vegetation restoration measures during the study and practice vegetation restoration and reconstruction in the Loess Plateau; but the environmental impact assessment of gully vegetation restoration has significantly lagged, and the environmental impact mechanism of gully vegetation restoration is not clear. Therefore, the research purpose of this study will related to the environmental impact assessment of gully vegetation restoration in the hilly Loess Plateau. The study has carried out transect survey along different gully positions, analyzed the vegetation community structure, soil moisture and temperature, and soil physical and chemical properties in the vegetation restoration gullies. The main results are as follows:1)In this study, we selected three representative vegetation restoration patterns in gully areas including an artificial Robinia pseudoacacia forest gully(G-Rp), an artificial Caragana korshinskii shrubland gully(G-Ck), and a natural grassland gully(G-Ng).Based on gully vegetation community investigation, 82 species belonging to 32 families were found, where the families of Gramineae(17.1%), Compositae(15.9%),and Leguminoseae(13.4%) accounted to 46.3% of in total species number.After twenty years vegetation restoration and reconstruction, vegetative succession has expressed a better succession trend in three different vegetation restoration patterns on the gully area. The Caragana korshinskii shrub vegetation restoration pattern and the Robinia pseudoacacia forest vegetation restoration pattern had a good function effect in plant community coverage of herb layer and coverage and biomass of community litter; Natural restoration patterns had a good function effect in biomass of plant community herb layer. The species diversity index of artificial vegetation restoration pattern was more effective than natural vegetation restoration pattern in the gully area. There was the Caragana korshinskii shrub vegetation restoration pattern better than the Robinia pseudoacacia forest vegetation restoration pattern. The species similarity of the Caragana korshinskii shrub vegetation restoration pattern was higher than the Robinia pseudoacacia forest vegetation restoration pattern, and natural restoration pattern on the gully area in different slope position. The species similarity index of the gully area showed that the upper of slope was higher than the lower of slope and the bottom of gully, and there the east slope’s was higher than the west slope’s. The construction of artificial vegetation can promote to the process of vegetation restoration on gully area in the hilly loess region. We suggested that the process of artificial vegetation construction need to intensify the introduction of native species, to enhance species diversity, to rationalize spatial configuration of tree species, to attention the proportion of shrubby and arboreal tree species in the gully area.2)Soil moisture and temperature are important indicators of soil environment, which have significant impact on a variety of biochemical processes in soil. Through investigating on soil moisture and temperature along 0-500 cm profile and spatial distribution of different vegetation restoration patterns and rainfall gradient gully, the main results and conclusions are as follows:(1) Soil moisture and temperature among different vegetation restoration pattern gullies have significant differences. Thereinto, soil water content(SWC) of the different vegetation restoration pattern gullies expressed as grass gully > shrub gully > arbor gully, and the coefficient of variation of SWC showed as arbor gully > shrub gully > grass gully. Soil temperature of the different vegetation restoration pattern gullies expressed as arbor gully > grass gully > shrub gully, and the coefficient of variation of soil temperature showed as grass gully > shrub gully > arbor gully. Results showed that different vegetation restoration patterns could differentiation of gully SWC and temperature, which may be closely related to the biological characteristics of the dominant species.(2) Soil moisture and temperature among different rainfall gradient gullies have significant differences. Thereinto, Gully SWC and precipitation has a positively correlation. Gully average SWC along 0-500 cm profile showed as Xihekou >Yanhewan >Liandaowan, and the coefficient of variation of SWC expressed as Liandaowan >Yanhewan >Xihekou. Average temperature along 0-500 cm profile in gully expressed as Liandaowan >Xihekou >Yanhewan, and the coefficient of variation of soil temperature showed as Yanhewan >Xihekou >Liandaowan. The results show that rainfall differences can have a significant impact on the gully soil moisture and temperature. Therefore, during the process of gully vegetation restoration, local rainfall condition and spatial configuration of vegetation types should be attention for achieving the improvement of ecological environment.3)After two decades of re-vegetation in the gully area, overall levels of C, N, P(except between G-Rp and G-Ck), and available K in the accumulated soil were similar between revegetation types in the gully areas. However, there were significant differences in levels of nitrate, ammonium, available P, and soil p H between the re-vegetated gullies. Soil properties mainly increased in quality from the top to the bottom of gully areas, except at the bottom of the G-Rp. Non-metric multi-dimensional scaling(NMS) ordinations and multi-response permutation procedure(MRPP) analyses indicated that soil properties significantly differed depending on re-vegetation types and gully positions. Our results demonstrate that soil properties could be improved by different re-vegetation types in the gully areas, and artificial plantations could significantly better improve available soil nutrients than natural grasslands in the gully areas. Thus, artificial re-vegetation could be a valuable measure for controlling soil erosion, and improving eco-environmental conditions in the gully areas of the Loess Plateau.4)The study evaluated the effects of three re-vegetation types on soil carbon, nitrogen and phosphorus in the gully areas. Results indicated that, three re-vegetation types in the gully areas could significantly improve soil organic carbon and soil total nitrogen contents and storages, and could slightly reduce soil P contents and storages, in which that G-Ck had significantly reduced soil P contents and storages compared with CK(slope cropland)(P<0.05); the spatial distributions of soil C, N, P contents and storages were significant different in the gully areas(P<0.05), they mainly increasing in quantity from the top to the bottom of gully areas(except at the bottom of the G-Rp); there is a significantly and positively linear correlation between soil C and N(soil C and N were significantly and positively linear correlation relationship)(P<0.0001). The implementation of re-vegetation in gully area, which as one of important geomorphic unit, can significantly increase carbon sequestration and N stocks, and it has an important and positive meaning for regional ecosystem restoration and environmental improvement.5)Coupling coordination between vegetation and soil is a virtuous important symbol of a positive succession during gully vegetation restoration processes. Based on the investigation and analysis of vegetation and soil in three different vegetation restoration pattern gullies, a two-level target system of nine vegetation factors and eleven soil factors was conducted, and the weight of factors was determined by analytic hierarchy process(AHP), and models of vegetation and soil coupling degree and coupling coordinative degree were constructed. The results showed that the development tendency of vegetation-soil system in different vegetation restoration pattern gullies have been basically in the coordinated development of the intermediate level after twenty years of vegetation restoration. Robinia pseudoacacia gully was an intermediate level of development model, and vegetation and soil have a synchronous style. Caragana korshinskii gully and natural grassland gully were an intermediate level of development model, and soil expressed a lagged style. This revealed that different vegetation restoration patterns in gullies have different impacted on the vegetation- soil systems, but in general to promote the coordinated and healthy development of gully ecosystems. By the further correlation analysis and linear regression analysis, results showed that: between vegetation factors and soil physical and chemical properties, there had a variety of interrelated relationships, and meet a good linear regression relationship. Overall, vegetation succession process in gully areas was the interaction and interplay process between vegetation and soil; collaborative succession relationship between vegetation and soil was an important foundation for the healthy development of gully re-vegetation.