Effects of different ecological transition patterns on soil aggregate stability and cementing substances in abandoned jujube forest in the Lvliang Mountain

The ecological transformation of abandoned jujube forests in the Lvliang Mountains is important for the ecological restoration of the Loess Plateau. Soil aggregates are the basic unit of soil structure, and their composition and stability are important indicators for evaluating soil quality. Soil cementing substance is an important material basis for the formation of aggregates. This study revealed the formation mechanism of soil aggregates by investigating the changes in the content of soil aggregates and cementing substance in different ecological transition modes of Ziziphus jujuba economic forests in the Lvliang Mountain. Taking abandonment of jujube forest as control (CK), four different ecological transition modes were selected as the research objects, including replanting Medicago sativa in Ziziphus jujuba orchard (AL), replanting medicinal herbs in Ziziphus jujuba orchard (MM), replanting Pinus tabuliformis in Ziziphus jujuba orchard (CP) and replanting Platycladus orientalis in Ziziphus jujuba orchard (PO). The content and stability characteristics of water-stable aggregates with particle size <0.053, 0.053~0.250, 0.250~2.000, and >2.000 mm were analyzed. The contents of six aggregate organic cementing substances, namely soil organic carbon, fulvic acid, humic acid, polysaccharide, total glomalin related soil protein, and easily extractable glomalin related soil protein, and an inorganic cementing material (calcium carbonate), were determined. The relationships between the content and stability of the aggregates and cementing substances were explored through redundancy analysis. The results showed that: 1) Compared with CK treatment, four ecological transition modes increased the content of aggregates with particle size >2.000 (7.78%−25.56%) and 0.250~2.000 (13.04%−44.71%) mm. In addition, the mean weight diameter and geometric mean diameter of aggregates also increased 0.05−0.16 and 0.02−0.08 mm, respectively, and the CP mode performed best. The aggregate destruction rate decreased 24.85%−66.15%. 2) Compared with the CK treatment, the contents of the six organic cementing substances were increased under the four ecological transition modes, whereas the content of inorganic cementing substance (calcium carbonate) were reduced. 3) The explanation rates of humic acid and easily extractable glomalin related soil protein for the content and stability of aggregates were 59.0% and 14.8%, respectively, reaching a significant level (P<0.05), and the contribution rate of humic acid was the highest (71.0%). In summary, different modes of ecological transformation improved the stability of soil aggregates and the content of organic cementing materials. Among these, the CP mode had the best effect, and humic acid was the most significant cementing substance affecting the content and stability of soil aggregates. The results of this study are important for guiding the ecological transformation of abandoned Ziziphus jujuba forests and promoting ecological restoration in the Loess Plateau.