Plant-growth promotion by biochar-organic amendments mixtures explained by selective chemicals adsorption of inhibitory compounds

Biochar is known to promote plant growth and is marketed worldwide as a soil amendment. However, benefits of pure biochar on crop yields are, in some cases, limited, while its mixture with non-pyrogenic organic amendments (NPOAs) has a more consistent stimulatory effect. Here, we studied the biochar effect together with three NPOAs (fishmeal, Medicago, and maize straw) used alone and in all possible combinations on the growth of lettuce (Lactuca sativa), soybean (Glycine max), maize (Zea mays), and rice (Oryza sativa). The bulk fraction of organic materials was characterized by 13 C-CPMAS NMR spectroscopy and the liquid fraction by liquid chromatography-mass spectrometry (LC-MS). Biochar and NPOAs showed different effects, ranging from inhibition to strong stimulation of crops. For NPOAs, the major inhibitory effects were found in undecomposed materials, especially fishmeal with a low C/N ratio and a high nitrogen. The effect of the mixture depended on the chemical quality of the NPOAs and the target species, with fishmeal having a common synergistic effect on soybean and lentils. 13C-CPMAS NMR showed major changes in the bulk fraction of biochar-NPOA mixtures enriched in carbonyl C, alkyl C, and O-alkyl C types. Moreover, LC-MS showed a large effect of biochar in the liquid fraction by selective adsorption of many putative chemical compounds, especially when mixed with fishmeal and Medicago straw. This approach clarifies for the first time that biochar extensively adsorbs a number of organic compounds from the NPOA feedstock, which not only causes significant detoxification but also triggers remarkable biostimulation of plant growth.