Combined effects of biochar addition and nitrogen fertilizer reduction on the rhizosphere metabolomics of maize (Zea mays L.) seedlings

Aims Rhizosphere metabolomics can potentially help us to better understand belowground root-environment interactions that are mediated by root exudation in the rhizosphere. The main goals of the present work were to characterize the pattern of maize root exudation in response to straw-derived biochar (BC) addition and chemical nitrogen (N) fertilizer reduction and to explore the underlying mechanisms. Methods Two sets of pot experiments were performed independently that involved planting maize in aquic brown soil to which BC was added at dosages of 0 or 5% (w/w, equivalent to 112.5tha(-1)) combined with urea N application at rates of 150kgha(-1) (100%) or 105kgha(-1) (70%) for a total of four treatments. Samples containing root exudates were analyzed using nuclear magnetic resonance, and quantitative real-time reverse transcription-PCR (qRT-PCR) was performed to analyze gene expression in maize roots. Results The 5% BC addition significantly influenced the global rhizosphere metabolome of the maize seedlings regardless of the N application level, but without BC addition, the rhizosphere metabolome was not significantly affected by a 30% reduction in N. The effects of N reduction on the metabolite profiling of the native root exudates were stronger than those of BC addition and an obvious interaction was observed between BC addition and N reduction. BC addition combined with N reduction significantly changed the levels of some amino acids (e.g., causing a 1.75-fold increase in isoleucine) and organic acids (e.g., causing a 2.16-fold increase in malonate and a 2.15-fold increase in acetate) in the root exudates. Soil environmental factors, including the size of NH4+-N pool and total P content, had a strong positive correlation with rhizosphere metabolome. The decrease in root biomass caused by N reduction was partially mitigated by BC addition, and the expression of ZmMATE1 (for multi-drug and toxic compound extrusion transporter) in root was significantly up-regulated (P<0.05) by BC addition and N reduction. Conclusions Maize roots can reshape their rhizosphere metabolome under BC addition combined with N reduction and its underlying mechanism may involve the synergistic effects of soil environmental factors, root growth, and the expression of transport-associated genes.