Soybean phenology and leaf gas exchange parameters
Soybean phenology (e.g. flowering time) and all leaf gas exchange parameters were significantly affected by drought and salinity stress, while no significant effect was observed as consequence of biochar addition, except for photosynthetic rate and stomatal conductance (Table 1). Drought stress at low and high intensity significantly decreased leaf photosynthetic rate by 26.3% and 37.9%, stomatal conductance by 38.9% and 55.0%, intercellular CO2 concentration by 15.8% and 17.1% and transpiration rate by 49.6% and 71.2%. On the contrary, drought stress significantly increased WUE-leaf by 45.4% and 102.4% at D-L and D-H treatments, respectively. Soybean flowering time was delayed by almost one day under salinity stress. In addition, salinity stress significantly decreased photosynthetic rate, stomatal conductance, intercellular CO2 concentration and transpiration rate (–20.7%, –26.3%, –10.5% and –27.2%, respectively) relative to the non-salinity treatment.
The present study showed few interactive effects of treatments on leaf gas exchange parameters and no effect on soybean flowering time (Table 1, Figure 1). Photosynthetic rate and stomatal conductance were significantly influenced by interactions both drought × salinity and drought × salinity × biochar. Intercellular CO2 concentration was significantly affected only by salinity × biochar addition interaction. WUE-leaf showed significant changes considering drought × salinity and salinity × biochar addition interaction.
Drought and salinity stress significantly affected soybean biomass productivity and root growth (Table 2). With drought stress increasing, shoot biomass (–28.9% and –48.3% at D-L and D-H, respectively), root biomass (–4.7% and –34.3% at D-L and D-H, respectively) and total biomass (–25.5% and –46.3% at D-L and D-H, respectively) were depressed significantly compared with the control. On the contrary, drought stress significantly increased root length (21.7% and 10.6% at D-L and D-H, respectively) compared with the control and the longest root length occurred in D-L treatment.
Salinity stress significantly decreased root biomass (–24.5%) and total biomass (–13.2%) relative to control treatment. In accordance with root biomass, salinity stress significantly decreased root length by 21.7% compared with control.
Biochar addition showed significantly effects on shoot biomass, root biomass and total biomass, but had no effect on the ratio of shoot/root and root length (Table 2). With biochar addition rate increasing, higher shoot biomass (14.3% and 43.6% at B1 and B2, respectively), root biomass (15.8% and 31.5% at B1 and B2, respectively) and total biomass (14.6% and 41.6% at B1 and B2, respectively) than control were observed.
Generally, biomass production was partially affected by the interactive effects of drought stress, salinity stress and biochar addition (Table 2). Specifically, drought × salinity stress interaction significantly affected root length, root biomass and the total biomass production (Figure 2). It is worth mentioning that root length showed no difference among drought stress when salinity was added, but without salinity addition root length was enhanced by 35.5% under D-L and 28.1% under D-H compared to D-C treatment (Figure 2j). In addition, the drought stress × biochar addition interaction significantly affected shoot biomass, total biomass, and root length but not root biomass. With biochar addition increasing, drought stress depressed shoot biomass (averaged from –19.0% to –53.8%) and total biomass (averaged from –14.8% to –51.7%) stronger compared with control. Particularly, drought stress significantly increased root length (55.2% and 50.6% at low and high drought stress, respectively) only in B1 treatment.
Soybean gained the highest grain yield (10.46 g pot–1) at the D-C treatment with well irrigation. Drought stress significantly reduced the grain yield of soybean by 17.7% and 42.6% under low and high drought, respectively (Table 2). Similarly, salinity stress significantly lowered the grain yield by 21.1% compared with the treatment with no salinity addition. While, biochar addition significantly enhanced grain yield by 3.1–14.8% compared with the control.
Soybean grain yield was partially affected by the interactive effects of studied treatments (Table 2). As expected, drought × salinity stress interaction significantly affected grain yield with worse performance when salinity was added together with drought stress (Figure 3). Besides, drought stress interaction with biochar addition also significantly affected soybean grain yield. No significant effect on soybean yield was observed considering the interaction of drought stress × salinity stress × biochar addition.
Drought stress showed a positive effects on WUE-yield while salinity stress showed a negative effects on WUE-yield in this study (Table 2). Under water deficit, WUE-yield was increased by 27.5% and 25.5% under low and high drought stress, respectively. On the contrary, salinity stress significantly decreased WUE-yield by 24.2% compared with the non-salinity addition soils. Biochar addition significantly enhanced WUE-yield 15.6% at high addition rate while showed no effect at low addition rate.
WUE-yield was significantly affected by the studied treatments interaction (Table 2, Figure 3). Drought stress significantly increased WUE-yield but salinity addition significantly decreased WUE-yield under control treatment, however, biochar addition relieved the effects.