Fractal approach to estimating changes in soil properties following the establishment of Caragana korshinskii shelterbelts in Ningxia, NW China

To identify the effects of shelterbelt establishment on soil recovery, we compared soil properties at depths of 0–5, 5–10, and 10–20 cm as affected by Caragana korshinskii shelterbelts across a chronosequence of soil undergoing restoration for 7, 11, and 26 years, as well as reference grasslands in Ningxia, NW China. Fractal dimension of soil particle size distribution (PSD) was also integrated to describe variations in soil properties. The results indicated that (1) fine particles, total porosity, total nitrogen, phosphorous, and available nitrogen, phosphorous, and potassium increased significantly with plantation age (p < 0.05). Sand particles tended to decrease with restoration time, especially in the topsoil layers (0–5 cm). (2) In most cases, selected physicochemical properties of soil in shelterbelts recovered more than in the reference grasslands; moreover, planting Medicago sativa had greatly positive effects on soil nitrogen accumulation due to azotification. (3) Fractal dimensions of soil PSD ranged from 2.3946 to 2.6351. Regression analyses showed fractal dimensions had significant linear correlations with soil physicochemical properties (R2 = 0.511–0.870, p < 0.01), except total potassium (R2 = 0.248). Therefore, we suggest D as a considerable and reliable parameter to reflect variations in soil properties affecting vegetation restoration efforts, and additionally as an implication of desertification. This improved information will contribute to a better understanding of vegetation solutions for anti-desertification efforts.