退耕还林与还草对土壤理化性质及分形特征的影响-以宁夏荒漠草原为例

Yanchi County of Ningxia lies in an agro-pasture transitional zone in northern China. The objectives of this study were to investigate the changes in soil properties and related fractal features associated with conversion of cropland to agroforestry or grassland. We selected four categories of site: cropland, cropland under conversion into grassland, cropland under conversion into agroforestry, and natural grassland; the latter taken as the control. The results showed: 1) Soil moisture content, soil pH and soil electrical conductivity values for both cropland under conversion into grassland and cropland under conversion into agroforestry differed from those in natural grassland. 2) After completing conversion of cropland into grassland, the soil nitrogen and organic carbon contents (0.60% and 5.93%, respectively) were the highest among the four sites studied. 3) Soil texture data for the site categories cropland, cropland under conversion into grassland, cropland under conversion into agroforestry, and natural grassland, respectively, were: clay and silt fraction, 10.87%, 7.47%, 8.73% and 6.53%; very fine sand fraction 39.07%, 35.20%, 38.87% and 31.00%; fine sand fraction, 29.73%, 34.93%, 32.80% and 36.73%; and the coarse sand fraction, 20.27%, 21.93%, 19.60% and 25.73%. Compared with natural grassland, the soil fractions in each particle size category differed significantly (P<0.05) at the cropland site under conversion into agroforestry, and there was also a significant difference (P<0.05) in the contents of very fine sand and coarse sand at the site under conversion from cropland into grassland. 4) The soil fractal dimensions of the four sites ranked from high to low: cropland (2.08)>cropland under conversion into agroforestry (2.01)>cropland under conversion into grassland (1.94)>natural grassland (1.87). 5) There was a positive (P<0.05) correlation between soil clay and silt and electrical conductivity, and a negative (P<0.01) correlation with C:N ratio. There was a negative (P<0.01) correlation between soil fine sand content and soil pH. There was a negative (P<0.01) correlation with soil electrical conductivity, and a positive (P<0.01) correlation with C:N ratio. There was a significant positive correlation between soil fractal dimension and soil electrical conductivity, and a negative correlation with C:N ratio (P<0.01). There were no correlations between soil coarse sand, soil very fine sand and the various other soil data (P>0.05). 6) There was a linear negative correlation between the fractal dimension of soil particle size distribution and fine sand content (P<0.01), and a linear positive correlation with the content of very fine sand (P<0.01), and logarithmic correlation with the content of clay (P<0.01), but no correlation with coarse sand content (P>0.05). Overall, the data show that the conversion of cropland into agroforestry or grassland in the desert steppe region of Ningxia has a profound impact on soil physical and chemical properties, soil particle size distribution and fractal dimension properties. Compared with natural grassland, at the cropland site under conversion into grassland the levels of soil organic C and total N were significantly increased; while at the cropland site under conversion into agroforestry, the proportions of soil clay and fine sand, and the soil fractal dimension were significantly increased. These results show that the conversion of cropland into agroforestry or grassland is conducive to improvement of soil texture and ecosystem restoration in areas where there is soil degradation linked to over-cropping.