禁牧对黄土高原半干旱草地土壤粒径多重分形特征的影响

Effects of Grazing Exclusion on the Fractal Characteristics of Soil Particle Size in Semi-arid Grassland on the Loess Plateau

研究半干旱地区草地禁牧后土壤粒径分布非均匀性和异质性的变化特征,揭示禁牧对草地土壤粒径分布的影响,为退化草地的恢复治理及科学合理利用提供理论支撑。以宁夏云雾山不同禁牧草地(0,10,15,27,35年)为研究对象,运用多重分形理论,探讨禁牧对草地不同土层(0—10,10—20,20—40,40—60 cm)土壤颗粒分布的影响,明确粒径分布变化的主要影响因素。结果表明:草地禁牧降低土壤容重,增加地下生物量、土壤养分(包括土壤有机碳及全氮)及土壤含水量,在禁牧27年达到峰值。多重分形谱函数Δf>0,表明土壤结构特征由占比偏大的粒级主导,禁牧后期(35年)砂粒含量达到69.29%,显著高于禁牧前期(10年);容量维数D_(0)变化幅度为0.866~0.891,并随禁牧年限递增;信息熵维数D_(1)和关联维数D_(2)随禁牧年限均呈先增加后减小的趋势,且在禁牧10年和15年时随土层深度递增,但在35年时随土层深度递减;D_(1)/D_(0)范围为1.012~0.904,随土层深度递减。土壤粒径分布与土壤有机碳呈显著正相关,表明土壤有机碳是影响颗粒分布的主要因素。结果表明,禁牧可以改善土壤养分及结构,但长期禁牧不利于半干旱区退化草地的恢复。

This study investigated the characteristics of heterogeneity and non-uniformity of soil particle size distribution in semi-arid grasslands after grazing exclusion,to evaluate the influence of grazing exclusion on the distribution of soil particle size of grassland.Our results are expected to provide support for the rational restoration and management of degraded grasslands.We examined the changes in soil particle size distribution using the multi-fractal theory at different layers(0—10 cm,10—20 cm,20—40 cm,40—60 cm)along a 35-year chronosequence following grazing exclusion of grassland in Yunwu Mountain,Ningxia autonomous region,and revealed the dominant drivers for the variation of soil particle size.The results showed that grazing exclusion resulted in a reduction of soil bulk density,while an increase in underground biomass,soil nutrients contents such as organic carbon,total nitrogen,and water content,and these values peaked at 27 years after exclusion.Changes of multifractal spectral functions(Δf>0)showed that the soil was dominated by particles with larger size,and the sand content accounted for 69.29%of total particles at the late period of exclusion(35 years),which was significantly higher than that in the early period(10 years).The capacity dimension(D_(0))increased with the exclusion years,ranging from 0.866 to 0.891,The information entropy dimension(D_(1))and the correlation dimension(D_(2))showed a trend of first increasing and then decreasing,and increased with the soil depth at the 10-year and 15-year sites,while decreased with the soil depth at the 35-year site.D_(1)/D_(0) decreased with soil depth,ranging from 1.012 to 0.904.There was a significant positive correlation between soil particle size distribution and soil organic carbon,indicating that soil organic carbon was the main factor affecting the particle size distribution.Our results indicated that 27-year exclusion could improve the soil nutrient levels and soil structure status,but a long-term exclusion was not conducive to the restoration of degraded grassland in the semi-arid region.