Chitosan-coated compound fertilizer application and crop rotation alleviate continuous cotton cropping obstacles by modulating root exudates

Crop rotation combined with chitosan-coated compound fertilizer (CCCF) application could effectively solve continuous cropping obstacles (CtO), but how it affect crop root exudates to solve CtO remains unclear. In this study, soils (0–20 cm in depth) subjected to 20-years continuous cotton cropping and 20-year cotton-maize-wheat rotation were collected and used for the four treatments including continuous cropping (Ct), continuous cropping combined with CCCF application (CtC), crop rotation (Rt), and crop rotation combined with CCCF application (RtC) in a pot experiment. The soil bacterial community structure was analyzed by sequencing technology, and the root exudates, soil nutrient contents, and enzyme activities were also measured. The results showed that soil nutrient availability and soil enzyme activities in the CtC, Rt, and RtC treatments significantly increased compared with those in the Ct treatment (p < 0.05). The root exudates except for proline, salicylic acid, and coumaric acid increased in the Ct treatment, while all root exudates reduced in the CtC, Rt, and RtC treatments. Specially, coumaric acid was only detected in the Rt and RtC treatments. The contents of proline and salicylic acid in the CtC treatment increased compared with those in Ct treatment. Further, the relative abundance of Actinobacteria (Actin-), Acidobacteria (Acido-), and Gemmatimonadetes (Gemma-) all decreased in the CtC, Rt, and RtC treatment, while that of Proteobacteria (Prote-), Chloroflexi (Chlor-), and Bacteroidetes (Bacte-) all increased compared with those in Ct treatment. Correlation analysis between root exudates and bacterial phyla showed that coumaric acid, proline, and salicylic acid were negatively correlated with Actin-, and Myxococcota (Myxoc-), Methylomirabilota (Methy-). This may be due to that coumaric acid, the root exudate of the previous crop (wheat), can inhibit the propagation of Actin-, Acido-, and Gemma-, there by improving soil nutrient availability and enzyme activity. Besides, CCCF can inhibit Actin-, Methyl-, and Others by increasing the contents of proline and salicylic acid and the relative abundances of Prote-, Chlor-, and Bacte- and regulating the relationship between root exudates and rhizosphere soil environment (nutrients, enzyme activity, and bacterial diversity), thus solving CtO. This study will help us better understand the mechanism by which crop rotation combined with CCCF application solves CtO.