Biochar application rate does not improve plant water availability in soybean under drought stress

Biochar, a form of porous pyrogenic carbon, has been shown to influence soil physicochemical properties and increase crop yield. However, it is unclear whether biochar application at different rates will improve crop performance in a clay soil under drought conditions through increasing plant water uptake. Here, we examined performance of soybean after amendment with biochar (pyrolysed at ~400 °C for 5 h) at four application rates (0, 25, 50 and 100 t ha−1) and grown under three moisture regimes (40%, 60% and 80% of field capacity). The instantaneous plant water content was determined periodically by recording relative water content, water retention capacity, and water uptake capacity in leaves while the cumulative water stress and water use efficiency (WUE) were determined from aboveground δ13C signatures and amount of water applied, respectively. The yield and yield attributes were also recorded. Results showed that across all drought treatments, biochar application significantly enhanced crop growth rate and increased total biomass production with a doubling in yield in the 100 t ha−1 biochar application. However, the seed yield did not increase with biochar application. Although biochar application improved WUE (biomass produced per unit of water applied), it most likely did not improve plant water uptake, since none of the measured plant water status indicators were affected by biochar. Instead, the enhanced biomass production may have been caused by an improvement in plant nutrition. Possibly, biochar application increased N acquisition through biological nitrogen fixation as there was a significant relationship between nitrogen (N) concentration in aboveground biomass and total biomass production. Moreover, application of biochar increased soil available potassium (K), and enhanced K uptake may have increased stress tolerance in soybean. Therefore, our findings show that biochar could improve plant performance in a clay soil by improving nutrient supply rather than by increasing water uptake.