A Fractal Method of Estimating Soil Structure Changes Under Different Vegetations on Ziwuling Mountains of the Loess Plateau,China

Fractal method is a new method to estimate soil structure. It has been shown to be a useful tool in studies related to physical properties of soil as well as erosion and other hydrological processes. Fractal dimension was used to study the soil structure in soil at different stages of vegetative succession on the Ziwuling Mountains. The land use and vegetation types included cultivated land, abandoned land, grassland, two types of shrub land, and three types of forests. The grassland, shrub land, and forested areas represented a continuum in vegetative succession that had occurred naturally, as the land was abandoned in 1862. Disturbed and undisturbed soil samples were collected from ten vegetation types from depths of 0-10, 10-20, and 20-30 cm on the Ziwuling Mountains, at a site with an elevation of about 1 500 m. Particle size distribution was determined by the pipette method and aggregate size distribution was determined by wet sieving. The results were used to calculate the particle and aggregate fractal dimension. The results showed that particle and aggregate fractal dimensions varied between vegetation types. There was a positive correlation between the particle fractal dimension and the weight of particles with diameter 〈 0.001 mm, but no relationship between particle fractal dimension and the other particle size classes. Particle fractal dimension was lower in vegetated soils compared to cropland and there was no consistent relationship between fractal dimension and vegetation type. Aggregate fractal dimension was positively correlated with the weight of 〉 0.25 mm aggregates. Aggregate fractal dimension was lower in vegetated soils compared with cropland. In contrast to particle fractal dimension, aggregate fractal dimension described changes in soil structure associated with vegetative succession. The results of this study indicate that aggregate fractal dimension is more effective in describing soil structure and function compared with particle fractal dimension.

Response of soil microorganisms to vegetational succession in Ziwuling Forest

This study investigates the changes in soil microbial populations as vegetational succession progress from abandoned farmland to climax forest in the Ziwuling Forest,which is located in the northern part of the Loess Plateau,China.Different organic C and total extractable N between the fumigated and non-fumigated soils were assumed to be released from soil microorganisms.Soil microbial C was calculated using Kec=0.38 and microbial N was calculated using Ken=0.45.The released P was converted to microbial biomass P using Kep=0.40.Soil bacteria,fungi,and actinomycetes increased as the vegetational succession progressed.Microbial C was of the highest amount in farmland.Microbial C,N,and P generally increased from abandoned land to climax community.The results indicated significant(P<0.05)diversities of soil microbial biomass under different vegetation.There was a significant correlation between microbial biomass and soil nutrients.Knowledge about soil microbial populations is important for forecasting vegetational succession and determining the ecological condition of the environment.

RELATIONSHIP OF ECO-ENVIONMENTAL CHANGE WITH NATURAL EROSION AND ARTIFICIALLY ACCELERATED EROSION

The natural landscape of the Loess Plateau was changed by severe soil erosion. The Ziwuling forest area provides research base for tracing back eco environmental change related to natural erosion and artificially accelerated erosion. Using methods of typical region investigations, in situ experimental study and chemical analysis of samples, impact of vegetation destruction and rehabilitation on soil erosion, characteristics of natutal erosion under conditions of natural ecological balance and artificially accelerated erosion resulting from vegetation destruction in forest area, and the processes of artificially accelerated erosion and soil degradation have been analyzed and discussed.