As the number of landfills increases worldwide, methane emissions from these sites have become a pressing environmental issue. Methane is a potent greenhouse gas with a significant impact on global warming potential. Methane emissions from landfills exacerbate the greenhouse effect and are detrimental to the recovery and restoration of vegetation in the cover layers. Vegetation is an essential component of landfill cover systems and affects methane oxidation efficiency and rate of the cover layer. However, methane limits the growth of plants in the cover layer, resulting in low methane oxidation capacity. How to improve the growth of plants under methane conditions and the effects of plants on methane emissions requires further exploration, and these are the main factors affecting the performance of the cover system.

A series of pot tests were conducted to systematically investigate the effects of sludge-amended soils on plant growth, rainfall infiltration, and methane oxidation capacity. In total, three experimental groups with sludge amendments of 5%, 10%, and 15% (w/w, dry weight ratio), labeled S5, S10, and S15, respectively, were set up. A control group (labeled S0) without sludge amendments was set up for comparison. All tests were conducted at a methane flux of 6 mL/min to simulate landfill conditions. During the tests, plant characteristics (e.g., growth rate, average plant height, and foliage coverage), soil suction, water content, and gas composition were systematically monitored. In addition, incubation flask tests were conducted after the pot tests to determine the methane oxidation rates at different soil depths, providing insight into the methane oxidation capacity of the sludge-amended soils.

The test results revealed the following significant effects of sludge amendments on soil and vegetation: (1) Sludge amendments boosted plant growth: The plant characteristics, such as growth rate, average plant height, and foliage coverage, of the three experimental groups were significantly improved compared with those of the control group. (2) Enhanced water storage capacity: The addition of sludge increased the water retention and infiltration capacities of the soil, particularly under different water contents. During the dry periods, soil suction increased with a higher sludge content, indicating improved water-holding capability. During the rainfall periods, the water storage capacity of the S15 group was 1.2 times higher than that of the S0 group, indicating the effectiveness of sludge in enhancing the soil water capacity. (3) Improved methane oxidation capacity: Sludge significantly enhanced the methane oxidation capacity of the soil, especially in the root zone. The methane oxidation efficiency of the S15 group in the root zone was 1.4 times that of the S0 group. Meanwhile, the methane oxidation rate of the S15 group in the root zone was 1.6 times that of the S0 group. These findings highlight the role of sludge in enhancing the methane oxidation of the landfill cover.

Overall, sludge amendments improved plant growth and enhanced the tolerance of vegetation under methane-stressed conditions. Plant growth further contributed to improved methane oxidation capacity, thereby reducing methane emissions from the landfill cover. The addition of sludge also increased rainfall infiltration and soil water retention capacity. These findings provide valuable scientific evidence to support the development of biologically enhanced landfill cover systems, providing a sustainable and effective approach to methane mitigation and landfill management.