Studies On The Ecological Effect During The Process Of Vegetation Restoration In Abandoned Farmland Of Loess Hilly-hully Region

Grain for green project is an effective measure of vegetation restoration, which plays an important role in the restoration and reconstruction of degradation and fragile ecosystem. The Loess Plateau, as a typical area of soil and water loss and ecological degradation in China, has become the focus site for the implementation of ecological restoration engineering in this country. Revealing and interpretation of the ecological effect laws as well as its driving factors during the process of ecological restoration has important scientific value and practical guiding significance for re-construction of the regional vegetation, repair fragile ecosystem function and improve the ecological environment. Based on the restoration ecology theory, and also using ecosystem positioning method, the present study designed to investigate the influence of restoration approach(artificial or natural),conversion type(forestland or grassland), plantation type(woodland or shrubland) as well as conversion period(time since conversion) on community biodiversity, soil quality, carbon sequestration, nitrogen cycle, soil water content and microclimate in Zhifanggou watershed of Ansai country, a classic deeply incised hilly-gully place in loess plateau region. We aimed to provide theoretical basis for controlling and interferencing the ecological effect of ‘Grain for Green' project, and also provide direction for policy-making. The results are as following:1. Effect of vegetation restoration on community structure and biodiversity.Through quadrate investigation and static analysis, we found that there is abundant species that belong to 23 families, 63 genera and 73 species. Among the understory communities, the dominant families include Gramineae, Compositate and Leguminosae; there is an obvious floristic characteristics, in which the world distribution has an absolute advantage. The mix forest has a relative great species richness and diversity index which increased the heterogeneity inside community, whereas the community structure of pure thrubland is simple and also with a lower biodiversity. Among the communities with various conversion times from cropland, the young plantations have a higher biodiversity while the elder plantation has a lower one because of their stable community structure derived by long term competition. During the community succession, the understory grass plants, including Heteropappus altaicus, Barnyard grass, Patrinia scaniosaefolia, Artemisia vestita, Salsola and Lespedeza daurica, have a bigger important value and species riche breadth, as well as a higher overlap index. This is resulted from their acclimated to local water deficiency conditions, and which guarantee them advantage than others in competition for resource, so become the dominant species.2. Effect of grain for green project on soil quality.Compared with cropland, vegetation restoration significantly increased the content of >0.25 mm aggregate(macro-aggregate), and also increased soil aggregate stability. Vegetation restoration decreased bulk density of surface soil, increased soil porosity and gas permeability, and at last improved soil physical quality.Vegetation restoration also significantly increased the content of soil organic matter, total nitrogen, available nitrogen and available potassium, so enhanced soil fertility and improved soil chemical quality.Compared with cropland, vegetation restoration greatly increased the activity of soil urease and sucrase, that as the well indicators of soil biological quality. There is present a season specificity for those enzymes activity. For urease, the ensyme activity is pixed in spring, followed by summer, and with the smallest activity in autumn. For sucrase, there is also a pix in spring, followed by the smallest value in summer, and then rised in autumn,but still lower than the spring' value. These ensyems activity decreased along with soil depth, but we have not find an obvious difference among the community with a various restoration time.3. Vegetation restoration on soil carbon sequestration and carbon- nitrogen relationship.Vegetation restoration not only significantly increased soil organic C(SOC) and soil total N(STN), but also greatly improved labile organic C(LOC) and soil labile N(SLN). In 0-60 cm soil profile, compared with both 20~40 cm depth and 40~60 cm soil depth, there is a significant change of the organic matter(C and N) in 0~20 cm soil depth, which indicated that the shallow soil is more sensitive to land use change. Whereas both SOC and LOC present a nonlinear accumulation characteristics across time,both STN and SLN increased linearly with the restoration time increasing. We also found the increase of both SOC and STN in 0~20cm soil depth is mainly relevant with restoration time, however, both in 20~40 cm and 40~60 cm soil depth, it is significantly related with re-vegetation mode. As for SOC increasing and activation among different vegetation type, there is a trends of woodland, then shrubland, followed by abandoned land. Whereas abandoned land is benefit of STN restoration,artificial highwood and shrubbery accelerate SLN, means available N restoration in degraded and barren soil. Finally, compared with field, vegetation restoration significantly increased the correlation between SOC and STN. Among the same vegetation type, the related coefficient(r2) between SOC and STN increased with restoration time below 26 yr, while above 26 yr, it maintain constant. As for the similar restoration time but different vegetation type, the positive effect of woodland is bigger than shrubland, while the latter is bigger than abandoned land. In a word, vegetation restoration on degenerated farmland significantly increased CO2 sequestration, restored soil nutrition availability, and improved the relationship between soil C and N. In a short revegetation time, artificial restoration is superior to natural restoration.Afforestation of degraded cropland significantly promoted CO2 sequestration and restored soil N availability, which consequently reduced N limitation and sustained C sequestration. Above a vegetation age threshold, i.e. 26 yr, afforestation significantly improved the linear correlation relationship between soil organic C(SOC) and total N(TN) stocks in the 0–60 cm soil profile. This linear SOC-TN stock nexus decreased with increasing soil depth, and the potential of having improved SOC stock, TN stock and the ratio between them in deeper soil layers increased with increasing time after afforestation. Managed plantations provide superior CO2 sequestration and improvement in the SOC-TN correlation only for a short period after afforestation when compared to natural restoration where trees establish by natural colonization.4.Effect of vegetation restoration on soil water content and carbon-water relationship.Disregard to artificial or natural approach, vegetation restoration commonly make soil water content decreased in the initial stage, but after 26 yr, soil water content start rise though with a low rate. In correspondence, there is an inverse relationship between SOC stocks and soil water content disregard to season, soil depth and restoration approach, which means carbon sequestration derived by vegetation restoration is at the expense of the consumption of soil moisture.Through soil profile, the relationship between SOC stocks and water content is significant, at 0-20 cm soil depth, then with the soil depth deeper, the relationship gradually become insignificant. There is a significant relationship between SOC storks and water content in summer(July) than in spring(April). With the restoration time increasing, SOC stocks and water content gradually become mutual benefit, and after restoration 26 yr, the reverse relationship changed to positive relationship.The SOC stocks-water content relationship has an important application in field, that is when the SOC accumulated over 30 mg?ha-1, the water content decreased close to wilt coefficient of 6%. So in practice, people should maintain the SOC sequestration rate below this value through some disturbance method, such as controlling community density, planting more thrub and grass rather than forest etc., thus make the carbon and water balance inside the plantation soil.5.Effect of Grain for Green project on microclimate.Compared with cropland, the average light intensity within a day for mixed forest, pure shrubbery and abandoned land has decreased by 3.9×104lx, 3.4×104 lx, and 0.9×104lx, respectively, and the mean air temperatures of them decreased by 5.3?, 1.3 ?and 0.5?, respectively. Our results also revealed that the highest temperatures on soil surface of them have decreased by 14.9?, 16.8 and 10.7? ?, respectively, but the corresponding air relative humidity has increased by 8.4%, 7.3% and 2.0 %, respectively. It is evident that vegetation restoration is capable of producing significant microclimate effects on temperature-reduction, air humidification, light-harvesting and wind-resistance. In addition, in a short-term scale, artificial-assistant restoration is more favorable than natural restoration in microclimate improving. And with the community architecture more complex, the positive effects on micro-climate become more significant. However, these effects were obtained at the expense of soil moisture, especially for the artificial forest. Although mixed forest relative to pure forest has better soil moisture, it still can not escape soil water conditions deterioration. On practice, adjusting the critical point with the minimum water consumption to obtain the biggest local microclimate effect, is privotal to the success of vegetation construction in arid and semi-arid areas of Loess Plateau.