| Little leaf Peashrub (Caragana microphylla Lam.), a species of the genus Caragana Fabr. of Legume Family, is a deciduous and drought-resistant shrub. It is the main species cultivated in the Loess Plateau in northwestern Shanxi, China. Since1960s, this shrub has been widely planted and plays important roles in ecological environment management, such as windbreak, sand fixation, soil and water conservation, and as valuable forage in northwestern Shanxi. However, few studies have been carried out to study the dynamic changes of nutrient contents in the shrub-soil system which provides foundation information for the ecological recovery program (Grain-for-Green) in the Loess Plateau in northwestern Shanxi. The objectives of this study are (1) to characterize the dynamics of biomass and nutrient contents of the shrub;(2) to characterize the dynamics of nutrient contents at the rhizospheres of the shrub;(3) to determine the spatial and temporal changes of nutrient contents in soils under different land uses; and (4) the dynamics of soil enzyme activities in the soil-shrub system under different land uses.1. The content of nitrogen in the roots of the shrub was higher than the leaves because of the nitrogen fixing by the legumes in the roots. The contents of other mineral nutrients in the leaves of the shrub were higher than stems and roots.The fresh above-ground biomass of the shrub and nutrient contents of the shrub organs showed increase followed by a decrease with the increase of growth age and the peaks occurred at its age of30years old. As the increase of growth age, the contents of raw protein and raw fat of the shrub decreased and the contents of raw cellulose increased.2. The contents of soil organic matter and total nitrogen at rhizosphere of the shrub were higher than the bulk soil. This result may be interpreted by the two processes at root zone, nitrogen fixing by the legumes in the large root system and exudation of low molecular weight organic acids from roots.The content of soil available phosphorus at rhizosphere of the shrub was higher than the bulk soil. This result may be interpreted by the replacement of adsorbed phosphorus to CaCO3by the low molecular weight organic acids released into root zone. The biomass of the shrub increased when external addition of phosphorus fertilizer was applied, and the possible reason is that the shrub need a large amount of phosphorus to meet their growth requirement and phosphorus is a limiting factor in calcareous soil.The content of soil available potassium at rhizosphere of the shrub was higher than the bulk soil.3. The soil physical and chemical properties changed as the establishment of the shrub vegetation. As the growth of the shrub, there were increases in soil bulk density in the depth of0-20cm, soil porosity, field water holding capability, and soil aerating condition, and the content of sand with diameter>0.05mm decreased while the content of silt with diameter between0.002and0.05mm increased.As the growth of the shrub, the contents of soil total nitrogen, organic matter, and nitrate showed increases followed by decreases with the peaks at the shrub age of30years old. The contents of soil total phosphorus and potassium showed non-significant change, but the contents of soil available nutrient, especially phosphorus and potassium, decreased with the increase of the age of the shrub. The contents of soil total nitrogen, organic matter, available phosphorus and available potassium showed to be higher at the layer0～20cm than those at the layer20～40cm.Soil physical properties showed significant difference between land uses in this study. The order of soil bulk density in depth of0～20cm was shrub plantation<forest<farmland<Poplar plantation<fallow, while the soil porosity showed to be an opposite order. Soil chemical properties showed significant difference between land uses in this study. The contents of soil organic matter and total nitrogen at the vegetation of shrub mixed with Poplar were higher than those at the shrub or Poplar. The order of the contents of soil available phosphorus was farmland> shrub, shrub mixed with Poplar> Poplar. The highest content of soil available phosphorus in farmland was due to the large amount of phosphorus fertilizer applied.There was an obvious spatial distribution of soil chemical properties. The contents of soil organic matter and available nutrients decreased with the increase of distance from the center of the shrub, and this phenomenon was coincident with the fertile island effect in literature.4. As the increase of the age of the shrub, the soil enzyme activities in the shrub showed gradual increases followed by decreases with peaks at the shrub age of20years old. During the growth of the shrub, the soil enzyme activities decreased with the increase of soil depth and the horizontal distance from the center of the shrub.There was no significant difference of catalase activities between land uses. The order of soil alkaline phosphatase activities in the soil depth0～20cm was shrub> forest> Poplar> fallow> farmland.The soil microbial populations gradually decreased with soil depth in the shrub. The abundance of bacteria was dominant (>79%) of the total organisms, and it was as high as90%at the center of the shrub. The abundance of organisms decreased with the increase of distance from the center of the shrub. The community of soil organisms showed spatial distribution, i.e. decreases of the percentages of bacteria and fungi and increases of the percentage of actinomycetes with the increase of the distance from the center of the shrub.This study of the dynamics of nutrient contents in the shrub-soil system can be implemented in the practice follows.(1) Based on the observed temporal change of the biomass and nutrient contents in the shrub-soil system in this study, it is feasible to cut back the shrub with age older than30years in order to restore the shrub vegetation. New roots and shoots will sprout and the biomass of the new vegetation will be equal to or greater than the old one. Thus, mowing and cutting back of the shrub with age older than30years, such as cutting bars, is an effective means of rejuvenation. According to the comprehensive criteria in this study including above-ground biomass and the contents of nutrients, raw protein, raw fat and cellulose, the optimum time is recommended at the age of three years old for the shrub to be used as forage.(2) Proper grazing in the old shrub. Since animal husbandry especially herding in the region is well developed, sheep’s eating young leaves and flowers of the shrub might have partial effect of mowing and cutting back old shrub mentioned above. Additionally, the manure of the sheep will improve soil physical and chemical condition for shrub growth. This grazing is not allowed in young shrubs because the young shrub will be simply destroyed or the establishment of its crown will be delayed.(3) The plantation of the shrub mixed with Poplar is recommended as an alternative way of land use because it showed improvements in soil physical and chemical properties and enhancement of soil enzyme activities in this study. |
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