Can biochar improve pumpkin productivity and its physiological characteristics under reduced irrigation regimes?

Water stress is one of the main constraints which could limits crop productivity, especially in the arid regions. This study hypothesized that the combination of biochar amendments could be a part of sustainable agricultural strategy in semi-arid environment, therefore the objective was to investigate the effects of biochar rates under different irrigation regimes on soil properties, physiology and yield of pumpkin (Cucurbita pepo L.). The experiment was conducted in Gorgan (Iran) in two successive years (2014 and 2015) and consisted of a factorial combination of four irrigation regimes [45%, 60%, 75% and 90% (called I1, I2, I3 and I4, respectively) of maximum allowable depletion (MAD) of available soil water (ASW)] with four rates of biochar derived from maize-straw feedstock [0, 5, 10 and 20 t biochar ha−1 (called B0 B5, B10 and B20, respectively)]. The results indicated that the incorporation of biochar into the soil increased both soil permeability (low bulk density and high pore volume) and the water holding capacity, particularly when biochar was applied at the rate of 20 t ha−1. Moreover, after pumpkin cultivation, the B20 showed high soil microbial biomass carbon. The reduction of irrigation water increased the activity of antioxidant enzymes, the content of malondialdehyde (MDA), oxygen radicals (O2−), and hydrogen peroxide (H2O2) in pumpkin leaves, while all the antioxidant activities and proline contents declined on pumpkin plant treated with biochar, especially at the highest rate (B20). The adoption of biochar as amendment increased chlorophyll content and the uptake of nutrients in leaves regardless of irrigation regimes. The seed yield showed a decrease of 6%, 46% and 58% in I2, I3 and I4, respectively, in comparison with I1 regardless of biochar rate. Response rate to biochar was ceased in lower biochar rates by prolonging irrigation, but I2 response to biochar ceased in higher biochar rate as Bcritical (14 t biochar ha−1). This implies that I2 irrigation regime improved response of pumpkin to biochar, which was accompanied by its high WUE. These results indicate that biochar amendment could provide a suitable strategy for improving the physical status of agricultural soils, water use efficiency and vegetable crop yield.