Biochar modifies the content of primary metabolites in the rhizosphere of well-watered and drought-stressed <i>Zea mays</i> L. (maize)

As root exudation may be altered by drought stress, we investigated if biochar amendment could moderate these effects. In a pot experiment with maize, treatments amended with straw or wood biochar were exposed to different irrigation regimes: well-watered (irrigated to 90% of water holding capacity (WHC)), drought (no irrigation for 4 days), and recovery (irrigated to 90% of WHC for 4 days after drought). Photosynthesis was measured during plant growth. At harvest, the leaf water potential (LWP), plant biomass, rhizosphere and bulk soil NH4+ content, pH, multiple substrate-induced respiration (MSIR), and rhizosphere content of selected primary metabolites (as an indication of root exudation) were determined. While the plant biomass was unaffected by biochar amendment, biochar had positive effects on the LWP and photosynthetic parameters in the initial drought and recovery phases. Furthermore, soil pH and NH4+ content were affected by biochar and the C-substrate utilization (MSIR) increased in the biochar treatments, independently of irrigation. Both biochars significantly altered the rhizosphere content of primary metabolites, especially under full irrigation and drought, and it is suggested that direct and indirect effects of biochar application on soil properties are the cause of these changes. Biochar specifically increased the content of organic acids, and drought even had an additive effect on the content of succinic acid in the wood biochar treatment.