Assessing the difference of biochar and aged biochar to improve soil fertility and cabbage ( Brassica oleracea var. capitata ) productivity

Purpose Biochar plays an active role in increasing crop yield and improving soil quality due to its unique properties and structure. However, the physical and chemical properties of biochar also change over time after applying to the soil, which is referred to as aging. The distinction between modifying soil fertility and soil nutrient status is uncertain (especially in soil potassium levels). Materials and methods We used three approaches to simulate aging progress of biochar, including acidification (AB), dry–wet cycle (DWB), and freezing–thawing cycling (FB). We used element analyzer, BET-N adsorption method, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscope (SEM) to observe the difference in physical and chemical properties between original biochar (OB) and aged biochar (AB, DWB, and FB). In addition, we undertook pot experiment to assess the impact of original biochar and aged biochar on soil fertility status, soil enzyme activities, and the growth of cabbage, especially in the difference in promoting soil potassium (K) level. Results and discussion The main results were as follows: Original biochar and aged biochar improved soil fertility and cabbage growth, but the improvement effect of aged biochar on soil environment was weakened. Among all-aged biochars, the AB had the worst effect on the soil environment. Compared to without biochar treatment (CK), the water-soluble K, available K, exchangeable K, and non-exchangeable K were increased by 43.60%, 45.56%, 46.49%, and 44.30%, respectively, under original biochar treatment. However, the promotion effect of soil potassium level was significantly decreased under the AB treatment. Additionally, the C and N content of biochar increased with aged biochar treatment, and the increasing trend was further obvious after applying it to the soil. Moreover, aged biochar treatment affected the surface of biochar, and was more susceptible to erosion in the soil by long-term water leaching. Conclusions Overall, the impacts of aged biochar on cabbage growth and soil fertility were inhibited compared to original biochar treatment, further providing a basis and reference for the proper application of biochar in agriculture production.