Calcium alginate-biochar composite as a novel amendment for the retention and slow-release of nutrients in karst soil

Soil erosion is a prominent environmental problem in karst areas. To reduce the loss of nutrients in karst soil, two kinds of calcium alginate-biochar composites (CA-MBs) were synthesized. Soil column leaching and pot experiments under different treatments were carried out to investigate the retention and slow-release of CA-MBs on nutrients. The results showed that the retention effects of calcium alginate-rice straw biochar composite (CA-MRB300) and calcium alginate-Eupatorium adenophorum biochar composite (CA-MEB600) on nutrients under fertilization were better than that under non-fertilization. The addition of 0.5 wt% CA-MRB300 and CA-MEB600 reduced the loss of nutrients after fertilization, and 0.5 wt% CA-MRB300 reduced the cumulative leaching amount of PO43-, NH4+, NO3-, and K+ by 48.05%, 28.76%, 36.63%, and 9.08%, respectively. Surface precipitation, ion exchange, and physical adsorption were considered to be the main retention mechanisms of different nutrients based on analyzing the physicochemical properties of CA-MBs and their effects on soil physicochemical properties. The addition of 1.5 wt% CA-MRB300 had the best promotion effect on the root length, stem length, and dry weight of soybean under non-fertilization, indicating that CA-MRB300 could be used as a slow-release fertilizer to promote the growth of soybean. This study develops a novel soil amendment to reduce the loss of nutrients in karst soil. [Display omitted] •CA-MRB300 is prepared as a novel amendment to retain nutrients in karst soil.•Retention effects on nutrients under fertilization are better than non-fertilization.•The addition of 0.5 wt% CA-MRB300 has good nutrients retention performance.•CA-MRB300 promotes the growth of soybean roots and stems.