Effects Of Legume Species Introduction On Plant Communities And Soil Propenies On Abandoned Croplands Under Different Landscape Positions On The Semi-arid Loess Plateau,China

Due to the natural and man-made causes, the semi-arid Loess Plateau has become the most serious soil erosion region in China even in the world. Restoration and revegetation are importation methods to control soil erosion and improve the ecological environment in this region. Legumes forage plays an important role in the sustainable development of the ecosystem on the Loess Plateau, have been widely used in the revegetation process due to their capacities in providing high-quality feed, nitrogen fixation and improving soil quality. Revegetation by converting abandoned croplands to artificial grassland by introducing forage legumes can rapidly increase vegetation and provide high-quality forage. It is not only benefit to the soil erosion controlling but also the livestock development. However, it is still lack of effective evaluates for the dynamics of vegetation and soil in the revegetation process of abandoned croplands after forage legumes introducing. In order to evaluate the long-term effects of revegetation on vegetation and soil properties, we conducted an experiment for 11 years on the semi-arid Loess Plateau by introducing legume forage on abandoned croplands. Our objectives were to evaluate the effects of revegetation on aboveground biomass, vegetation cover, species composition, functional groups and life lifestyles in plant communities, and soil organic carbon, light fraction organic carbon, heavy fraction organic carbon, nitrogen, phosphorus and soil water dynamics in soil on abandoned croplands after forage legumes introducing. Three landscape positions were selected, i.e., Field Ⅰ, north-east facing, upper slope; Field Ⅱ, south-east facing, middle slope, and Field Ⅲ, south-east facing, top slope. Three treatments were established, i.e., i) cessation of cultivation and natural revegetation, ii) introduction of Medicago sativa L. at a seed density of 22.5 kg-ha-1, and iii) introduction of Melilotus suaveolens L. at a seed density of 11.3 kg-ha-1. Achieved the following results:In the treatment of natural revegetation, aboveground biomass was low in all the fields and had no significantly increase trend with time. The soil water content at the soil layer of 1.4-5 m had a decline trend with time, it was more obvious in Fields Ⅱ and III, and however, it was still higher than the soil permanent wilting point of 4.5% in all fields. Soil organic carbon and total nitrogen contents at the soil layer of 0-20 cm had slight increase trends with time, but total P and available P had no significantly changes with time in all the fields.In Medicago sativa treatment, aboveground biomass had an increase trend in the first six years and than declined gradually at all the fields; it was greater in Field I than in Fields II and III, and the highest value of aboverground biomass in Field I was 8956 kg/ha and appeared in the sixth year. Soil water content at the soil layer of 0-1.4 m had a lager annual fluctuation indicating that it was the soil layer that precipitation can be replenished. Soil water content at the soil layer of 1.4-5 m had a significantly decline trend over year; and in the 11th year, it was near to the soil permanent wilting point in Field I and it had been reached the soil permanent wilting point in Fields II and III. Soil organic carbon and total nitrogen contents at the soil layer of 0-20 cm had significantly increase trends with time in all the fields, and it had a greater improving in Field I than Fields II and III, however, they had gradually decline trend after Medicago sativa had been planted for seven years. Total P content had a slight increasing trend and available P had a first increase and then decline trend with time in all the fields.In Melilotus suaveolens treatment, aboveground biomass was greater in the first two years, and afterwards it declined gradually in all the fields. Soil water content at the soil layer of 1.4-5 m had a significantly decline trend over year, but it was still higher than the soil permanent wilting point in all fields, and it was more obvious in Fields II and III. Soil organic carbon and total nitrogen contents at the soil layer of 0-20 cm had significant increase trends with time, but total P and available P had no significantly changes with time in all the fields.When compared to the other two treatments, Medicago sativa treatment had greater aboveground biomass, vegetation cover, soil organic carbon, light fraction organic carbon, heavy fraction organic carbon, total N and minal N, but lower plant density and species richness, and consume much more soil available P and soil water in the soil layer of 1.4-5 m during the whole experimental period of 11 years. Revegetation by introducing Medicago on degraded arable land can potentially promote SOC accumulation by enhancing the transformation from the less stable fraction of light fraction to the more stable and high-density faction of heavy fraction.Multivariate analyses were used to evaluate the relationships between plant communities and environmental variables. The introduction of Medicago and Melilotus to abandoned fields had different effects on later-successional species, and changed the successional trajectory of vegetation in the abandoned fields studied. Time since restoration was the most important factor influencing the composition of vegetation. Slope position, soil moisture content, annual precipitation, and slope/aspect were also key factors driving the composition of the plant community.We concluded that alfalfa should be used as the preferred plant species for the revegetation on the semi-arid Loess Plateau. Natural revegetation and sweet clover introducing are not effective ways for the revegetation in this area.