Impact of organic acids cultivation time and types on composition of debris and available nutrient in coal gangue

Abstract: Organic acids are ubiquitous in litters and rhizosphere in terrestrial ecosystem, and low molecular weight organic acids and humic acids are two common acids among them. They play a significant role in soil genesis, soil fertility and soil decontamination. However, the efficacy of organic acids in restoring vegetation in coal gangue is poorly understood. We took the coal gangue at Panji coal mining in Huainan city as an example. Low molecular weight organic acids and humic acid solution each at concentration of 10 mmol/L were added into the coal gangue in a 120-day incubation. The change in physical and chemical properties of the gangue was measured periodically. The results showed that the organic acids, especially malic acid and humic acid, promoted refinement of the gangue mineral particles and particle agglomerations, and helped improving aggregation of the mineral particles in the gangue. The incubation increased the clay particles by an average 81.9% and reduced the proportion of silt and very coarse sands by an average 21.05% and 62.34%, respectively. Many coarse sands were broken down to silt and clay which in turn formed aggregates. The pH, electrical conductivity (EC) and bioavailable nutrients in the coal gangue debris responded differently to the organic acids and incubation time. The pH of the studied gangue was 6.72 prior to incubation, and amending it with tartaric acid, oxalic acid and humic acid increased the pH to 7.69, 7.75 and 7.71 respectively. The pH changed with time in all the treatments; in the malic acid treatment, it changed in an "N" shape. In all treatments, EC changed with time in an opposite direction to the change in pH. The EC of the gangue was 416.67μs/cm prior to the incubation; the incubation increased the EC in CK and the treatments with malic acid and citric but reduced it in other treatments. Specifically, incubating with malic acid increased the EC by 146.78%, the highest among all treatments. Incubation with organic acids promoted the release of alkali-hydro nitrogen (AN) and available phosphorus (AP) from the mineral particles, while incubation with malic acid improved the release of AN more than others. In contrast, the citric was most effective in releasing AP. In all treatments, prolonging the incubation enhanced the release of alkali nitrogen and available phosphorus. Water infiltration and addition of the organic acids inhibited the release of available potassium(AK) from the mineral particles in the early stage, but promoted the release of the potassium. However, there was no significant difference between the organic acids. The results presented in this paper shed insight into the efficacy of different organic acids in remediating coal gangue.