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
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摘要
以宁夏荒漠草原区为研究对象,分别选取农田、退耕还草地、退耕还林地和天然草地为研究样地,研究了退耕还林与还草对土壤理化性质及分形维数变化特征的影响。结果显示:1)土壤含水量、pH和电导率既受到退耕还林与还草措施的影响,亦受到季节变化的影响。2)农田实施退耕还林与还草工程后,退耕还草地的土壤全氮和有机碳含量最高,其平均含量分别为0.60%和5.93%。3)农田、退耕还草地、退耕还林地和天然草地的土壤黏粉粒含量分别为10.87%、7.47%、8.73%和6.53%;极细沙含量分别为39.07%、35.20%、38.87%和31.00%;细沙含量分别为29.73%、34.93%、32.80%和36.73%;粗沙粒含量分别为20.27%、21.93%、19.60%和25.73%。退耕还林地与天然草地相比土壤粒径在各粒级中均存在显著差异(P<0.05),退耕还草地与天然草地相比仅极细沙和粗沙粒含量存在显著差异(P<0.05)。4)分形维数介于1.87～2.08。土壤分形维数(D)值从高到低表现为农田(2.08)>退耕还林地(2.01)>退耕还草地(1.94)>天然草地(1.87)。5)土壤黏粉粒与土壤电导率达到正相关水平(P<0.05),与土壤碳氮比为显著负相关关系(P<0.01);土壤细沙粒与土壤pH为负相关关系(P<0.05),与电导率为显著负相关关系(P<0.01),而与碳氮比达到显著正相关水平(P<0.01);土壤分形维数与电导率达到显著正相关水平(P<0.01),与碳氮比呈显著负相关关系(P<0.01);而土壤极细沙粒和粗沙粒与各指标之间均无相关性(P>0.05)。6)土壤粒径分布分形维数与细沙粒含量呈线性负相关关系(P<0.01),而与极细沙粒含量呈线性正相关关系(P<0.01),与黏粉粒含量呈对数相关关系(P<0.01),但与粗沙粒含量未表现出相关性(P>0.05)。研究表明,在宁夏荒漠草原区实施退耕还林与还草工程,对土壤理化性质、土壤粒径分布和分形维数特征均产生一定的影响。与天然草地相比,退耕还草地中土壤有机碳和全氮含量显著提高;而退耕还林地中土壤黏粉粒、极细沙粒的分布及分形维数显著上升。说明实施退耕还林与还草工程,有利于农田退耕导致的退化区域土壤质地的改善以及退化生态系统的有效恢复。
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.
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.
引文
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