Extracted rapeseed meal biochar combined with digestate as a soil amendment: Effect on lettuce (Lactuca sativa L.) biomass yield and concentration of bioavailable element fraction in the soil

Biochar and digestate are widely used as soil amendments as they improve its properties and composition. This work deals with the utilization of two types of locally (Central Europe) produced biowaste, namely extracted rapeseed meal and digestate. Biochar was produced from rapeseed meal, the mixing of which with liquid digestate resulted in an innovative type of soil amendment that has not been yet studied. The addition of biochar to digestate is expected to stabilize and simplify its safe application and storage. Several variants of soil amendments were tested in a pot experiment carried out on lettuce (Lactuca sativa L.) for few growing seasons. During the experiment, emphasis was placed on determining the bioavailable fraction of elements in the soil. This determination is often missing in current studies, although it brings essential information, since plants receive nutrients only in certain bioavailable forms. Furthermore, the effect of soil amendments on the biomass yield biomass, properties and elemental composition of the soil was monitored. The harvested biomass was divided into roots and shoots in order to monitor the distribution, translocation and accumulation of the elements in the plant. The obtained results demonstrated that rapeseed meal biochar itself was the most effective soil amendment, as the highest yield of harvested biomass (tenfold higher that the control) was archived. In this tested variant, significant increases in the concentrations of macro and micro nutrients as well as CEC values in the soil were obtained. The bioavailable fraction of elements did not coincide with the increases in the total concentration of elements. The potassium case was an exception, in which its soil bioavailable fraction boosted, and was directly correlated with the higher yield of harvested biomass (R2=0.9223 for shoots, R2=0.9272 for roots). Furthermore, biochar stimulated the accumulation of N, P, K and, conversely, inhibited the transport of Fe, Zn, Na towards the leaves. These outcomes are in harmony with our previous work dealing with the evaluation of rapeseed meal biochar as a suitable soil amendment. However, further investigation by applying the biochar in real soil/field conditions is required to confirm the obtained conclusions. Finally, the application of biochar to soil supports circular economy by the utilization of biomass waste in sustainable agriculture, improves soil quality, increases crop yield, and reduces the need for industrially produced fertilizers and sorbents.