Commercial lignite coal-derived amendments for improved pasture growth and soil health

Commercial lignite-derived products for use in agriculture are produced and used globally and a number of these products are manufactured from Victorian lignite coal. In marketing these products, manufacturers make claims of plant benefits including increased crop yield, enhanced root development and increased nutrient uptake and improvements in soil health by chelation of cations, pH buffering and promotion of beneficial soil bacteria and fungi. There is little evidence of published scientific studies that support these claims and of those that are available, few focus on those derived from Victorian lignite coal. To investigate these claims, eight commercial products based on either raw-lignite or humate were sourced from three manufacturers. Raw lignite coal was also obtained directly from the mine. Chemical characterization of these products showed considerable variability in the humic acid (HA) concentration, ranging from 4 to 75% and in the inorganic components, particularly potassium, which ranged from 0.1 to 17%. In a glasshouse study six lignite-derived products were applied at the manufacturer’s recommended rate to two soil types. Shoot and root growth of two pasture species, lucerne (Medicago sativa L.) and ryegrass (Lolium multiflorum Lam.), shoot nutrient concentrations, soil pH, microbial biomass and mycorrhizal colonization analyses showed inconsistent results between the two soil types and the two pasture species. There was no clear evidence of a link between the HA and nutrient concentrations of the products, and pasture growth and soil health responses. In a second glasshouse study, three lignite-derived products were applied at and in excess of the manufacturer’s recommended rate to a mildly acidic, sandy soil. Raw lignite coal and a lignite-mineral blend did not promote a lucerne growth response however application of soluble humate granules (SHG) at rates up to 20 kg/ha promoted shoot and root growth, with maximum benefit attained at 20 kg/ha; five-times the manufacturer’s recommended rate. Application rates beyond 20 kg/ha did not provide additional growth benefits but importantly, resulted in a significant decrease in shoot nitrogen concentration with no concurrent loss in lucerne biomass. Application of SHG at 20 kg/ha also coincided with a delay in the appearance of chlorotic symptoms in lucerne shoots, and an increase in soil pH. A subsequent incubation study indicated that soil pH increases were driven by the pasture rather than product application. To further investigate the effect on lucerne and ryegrass growth and to assess interactions with soil nitrogen species, SHG was applied at 20kg/ha to a sandy, low organic matter soil in outdoor mesocosms. Significantly, the product promoted early-stage shoot and root growth of ryegrass. Enhanced root growth was observed particularly at a lower soil depth which could enable the pasture to access moisture in drought conditions. The promotion of ryegrass growth coincided with the earlier provision and higher overall concentration of soil ammonium. At this application rate, in these conditions, a growth benefit was not identified for lucerne. To investigate the manufacturer claims that commercial lignite-derived products promote beneficial soil bacteria and to further investigate the effect on soil nitrogen dynamics, the response of the soil bacterial community to the addition of SHG at 20 and 300 kg/ha was assessed by high-throughput 16S rRNA amplicon sequencing. The abundance of the nitrogen-cycling families Rhodospirillaceae, Hyphomicrobiaceae, Bradyrhizobiacea, Beijerinckiaceae, Burkholderiaceae and Nitrosomonadaceae increased while Nitrospiraceae remained unchanged. Other bacterial families whose abundance increased significantly were Xanthomonadaceae, Sinobacteraceae and Chitinophagaceae while the abundance of Gemmataceae, Streptomycetaceae and Ktedonobacteraceae decreased. These results indicate that the addition of SHG changed the soil bacterial community structure. The work presented in this study demonstrates that pasture growth and soil health benefits can be gained by the soil application of Victorian-lignite derived products. These benefits are highly dependent on the pasture species, soil type and product application rate, highlighting the need for farmers to evaluate these products within their individual environmental constraints and management systems. This work also indicates the need for manufacturers to reassess the product application rates that are recommended to farmers.