The effect of lignin application on plant growth and soil biological quality

There is a wide variety of agricultural waste co- and by-products that could potentially be valorised in high-value applications. One of such products is lignin, the second most abundant organic biopolymer after cellulose. Because of the large amounts of pruning wastes in the Mediterranean regions, lignin extraction can be one of the possibilities for valorisation and sustainable management of agricultural wastes. Research on the application of lignin, particularly lignosulfonates, is limited to its use as a biostimulant for root growth under controlled laboratory conditions and as a complexing agent in micronutrient foliar fertilizer formulations. Little is known about the impacts of lignin extracted from various feedstock on plant growth and soil quality. We investigated the potential of lignin as plant biostimulator and soil conditioner in a pot experiment with fresh soil and lignin extracts obtained from three types of pruning wastes (urban trees, fruit and forest trees) using ryegrass as a test plant, under laboratory conditions. Two doses of lignin extracts (equivalent to 5 and 20 kg lignin-C ha-1) were applied to assess whether the effect on plant growth and soil quality depends on the rate of application. Soil and plant parameters were determined seven weeks after the grass was planted at 17 °C and 16 h photoperiod. Root biomass significantly increased (62-152%) in treatments with lignin addition, particularly lignin from urban and forest pruning wastes compared to the control. However, the increase in root biomass did not result in a simultaneous increase in shoot biomass or N uptake showing the need to apply additional plant nutrient. The microbial biomass C did not significantly respond to the application of lignin. A significantly higher dehydrogenase enzyme activity was recorded in samples with the high dose of lignin extracted from the urban wastes compared to the lower dose. Urban waste lignin extract contains 15-18 times more total N compared to the lignin extracts from forest and fruit trees, which could explain its significant effect on enzymatic activities and root biomass. The findings show that differences in feedstock properties may influence the plant growth stimulating activity of the lignin. Further research is needed to improve the plant growth-stimulating effect of lignin, to investigate the simultaneous application of the major plant nutrients and the response of the microbial community to lignin application.