Effects of biochar, compost and straw input on root exudation of maize (Zea mays L.): From function to morphology

Returning crop straw, such as biochar, into the field is increasingly recognized as a valid, environmentally-friendly agricultural strategy to improve soil quality, increase crop yields, and reduce combustion-induced air pollution. Root exudation has potential importance for plant performance, rhizosphere function and plant-soil interactions; however, the effects of crop straw input on maize root exudation remain unclear. After three seasons of a field experiment, a mesh bag experiment was conducted in situ in the fourth season to study the effects of three straw incorporation treatments (chopped maize straw, SD; compost produced by maize straw, SC; biochar produced by maize straw, BC) and control (no straw, CK) on root exudation and their underlying mechanisms in a maize cropping system. Discrimination in the metabolic profiling of the root exudates between the CK and all the straw-incorporated samples was clear, but the separation between the samples from the three different straw strategies was limited. Among the 32 assigned components of maize root exudates, the fluctuations in organic acids (up to 16-fold) were stronger than those in sugars and amino acids when straw was incorporated. The response of the organic acid exudates was linked to the transcript levels of the ZmMATE1 and ZmMATE2 genes, which are associated with root exudation (Spearman correlations, P < 0.05). Straw incorporation influenced the root phenotypic appearance and morphology, which are characterized by longer and thinner root, larger root surface area, and more root tips in BC treatment, whereas shorter and thicker root, and fewer root tips in SD treatment. The changes in the root morphology and metabolomes were responsible for the responses of the maize root exudates to straw incorporation. Regulation of genes (ZmCycD2; 2a, ZmEXPB8, ZmXTH1, and ZmARF1) that associated with root growth and development, and changes in the viability and anatomical structure of root tips were involved in the modifications of root morphology. This information will be useful for modulating rhizosphere processes and soil fertility, and for guiding and recommending residue management practices in crop production with relatively larger yields.