Effects of organic fertilizer application on Morus alba growth characteristics and root morphology in abandoned mining areas

In this study, we aimed at investigating how organic fertilizer application could impact the root morphology of Morus alba in abandoned mining areas and examining the correlation between root morphology and growth. We used M. alba ‘Yuesang No.11’ as experimental material and established six treatment (i.e., a control and five experimental) groups. We treated the control group with chemical fertilizer (CK), while the experimental groups with biogas slurry (T₁), cattle manure (T₂), cattle manure + biogas slurry (T₃), earthworm casts (T₄), and earthworm casts + biogas slurry (T₅). We measured M. alba yield, plant height relative growth rate, and root growth index in each group. We obtained the following results. First, compared to CK, the M. alba yield significantly increased in each organic fertilizer-treated group (P < 0.05). The annual yield was the highest (21.49 t·ha⁻¹) in the T₅ group, and the plant height relative growth rate significantly increased in all the organic fertilizer-treated groups except for but T₁ (P < 0.05). Second, compared to CK, the root tip lengths and numbers as well as the root numbers with 0～0.5 mm diameter, significantly increased in the organic fertilizer group (P < 0.05). We observed the highest root length and number with diameters of 0～0.5 mm in the T₅ group, being 5.2 and 14.5 times higher than those in the fertilizer treatment group, respectively. The M. alba fork and cross numbers as well as root areas and volumes increased significantly (P < 0.05) in all organic fertilizer-treated groups. Third, compared to CK, the specific root length of the organic fertilizer-treated groups increased by 5.7%～115.9%, and the specific root areas in the T₂, T₃, T₄, and T₅ groups increased by 4.1%, 7.0%, 38.7%, and 49.6%, respectively. Furthermore, the root topological indices of the organic fertilizer-treated groups were closer to 1 than that of the CK group. Finally, our correlation analysis unraveled a positive correlation between fresh grass yield and specific root length, specific root area (P < 0.01), root length, root tip, topological index, and soil ammonium nitrogen content (P < 0.05). In conclusion, organic fertilizer application in abandoned mining areas could enhance M. alba root growth and morphological development. Root morphology and soil ammonium nitrogen content affected the growth of above-ground parts. Finally, M. alba growth was the most enhanced by earthworm casts and biogas slurry application in abandoned mining areas.