Effects of combined application of different organic materials on soil quality and plant growth in urban green spaces

To improve the soil quality of urban green spaces and promote plant growth, a 36-month field experiment was conducted to explore the effects of different combined application schemes of different organic materials (green waste compost, biochar, food waste compost, and organic fertilizer) on soil fertility, physical properties, and plant growth. Five treatments were set up: CK (control); T1, 42 t·hm⁻² green waste compost + 10 t·hm⁻² organic fertilizer; T2, 42 t·hm⁻² biochar + 10 t·hm⁻² organic fertilizer; T3, 21 t·hm⁻² green waste compost + 21 t·hm⁻² biochar + 10 t·hm⁻² organic fertilizer; T4, 42 t·hm⁻² green waste compost + 10 t·hm⁻² food waste compost. The results showed that: (1) All organic material treatments significantly increased soil organic matter, available NPK contents, and cation exchange capacity (CEC), improved soil bulk density and porosity, enhanced soil quality index, and promoted plant growth. Among them, T1 had the best effect on increasing soil organic matter, hydrolyzable nitrogen, and available phosphorus; T2 was most effective in improving soil available potassium; T4 showed the optimal performance in enhancing soil quality index and plant growth indicators (plant height, crown width, aboveground biomass, underground biomass, and total biomass). (2) Comparison of T1, T2, and T3 revealed that T1 had the highest increase amplitude in soil N, P, and CEC, while T2 was superior in promoting available potassium. During the first 24 months of treatment, T1 exhibited the best effect on increasing soil organic matter (P<0.05); by the 36th month, T2 showed the most prominent promotion on organic matter, and T3 was optimal in improving soil quality index as well as plant height, ground diameter, crown width, and biomass (P<0.05). (3) Comparison of T1 and T4 indicated that T4 was significantly superior to T1 in improving soil available potassium, CEC, physical structure, soil quality index, and plant growth performance, but the opposite was true for the promotion of hydrolyzable nitrogen and available phosphorus. In the first 24 months, T1 was better than T4 in increasing organic matter, while the trend reversed at the 36th month. The combined application of different organic materials in this study can effectively achieve the synergistic effect of "soil quality improvement - plant growth promotion". For short-term improvement in enhancing organic matter accumulation and N-P supply, T1 is preferred; for long-term maintenance of soil fertility, soil quality improvement, and plant growth promotion, T4 is recommended. The findings provide precise technical support for soil improvement of landscape green spaces and resource utilization of organic wastes.