Biomass accumulation and water use efficiency of faba bean-ryegrass intercropping system on sandy soil amended with biochar under reduced irrigation regimes

Intercropping is advantageous for optimizing crop productivity and resources utilization efficiency compared to monocultures. Yet, the effects of water/fertilizer management on crop growth and water use efficiency (WUE) in intercropping systems composed of legumes and non-legumes remain largely elusive. Biochar, as a soil amendment, could increase soil water storage and crop yield, but its interaction with soil water deficit and response of WUE are ambiguous. We investigated the growth, biomass and WUE in faba bean-ryegrass intercropping system amended with 550 ℃-pyrolyzed wheat straw (WSBC) and 800 ℃-pyrolyzed cleaning residues biochar (CRBC) under different irrigation treatments. Compared to the no biochar controls, CRBC decreased aboveground biomass (FDM) and seed yield (FGY) of intercropped faba bean, despite improved soil water-holding capacity, leaf water potential (LWP) and leaf hydraulic conductance (Kl) of faba bean. In contrast, CRBC significantly increased ryegrass aboveground biomass (GDM), although reduced the total aboveground biomass (TDM) and WUE (WUEf+g) of faba bean-ryegrass mixtures. These effects were not evident under WSBC. Compared to full irrigation, deficit (DI) and partial root-zone drying (PRD) irrigation enhanced GDM and WUEf+g, but lowered LWP and Kl, FDM and FGY, and PWU (plant water consumption) and TDM. Despite lower maximum quantum yield of photosystem II, PRD outperformed DI in improving biomass and WUE. The principal component analysis showed that PWU rather than biomass contributed significantly to the enhanced WUEf+g under DI and particularly PRD. Moreover, reduced irrigation regimes limited photosynthetic capacity of faba bean by depressing apparent quantum yield and increasing light compensation point, yet biochar addition did not. It was concluded that biochar produced at high temperatures might not be conducive in improving WUE of faba ban-ryegrass intercropping. Instead, PRD could be a prospective option to offset the adverse effect of biochar on plant under water-scarcity conditions.