Moisture control reduces soil salt effect on summer maize

Winter wheat-summer maize double cropping system is the main planting pattern in the Low Plain around Bohai Sea of China, where fresh water is in serious shortage but with sufficient brackish water resources. Rational utilization of salt water resources is great significance for food safety in the area. However, brackish water irrigation of winter wheat caused salt accumulation in the upper soil which affected summer maize seedling emergence. Regulated water management was an effective and feasible way of reducing the negative effects of salinity, which was also beneficial for salt water irrigation in the double cropping system. In this study, a combination of pot and field experiments was conducted. The pot experiment consisting of 3 soil salinities0.8 g·kg^-1(low salt content), 2.3 g·kg^-1(medium salt content) and 3.5 g·kg^-1(high salt content) and 4 (for low salt content, ) or 8 (for medium and high salt water content) water contents (55%-85% of field capacity) were used to test the response of maize seedling emergence to soil water and salt contents. In the field experiment, salt water with different salinities0 (CK), 3 g·L^-1 (SWT1), 4 g·L^-1 (SWT2) and 5 g·L^-1(SWT3) was used to irrigate winter wheat at jointing stage to determine the effect of salt accumulation due to salt water irrigation and desalinization by irrigation/precipitation on maize growth and grain yield. The pot experiment results showed that 60% of field capacity supported normal seedling emergence under low soil salinity (0.8 g·kg^-1). In high soil salinity (3.5 g·kg^-1), seedling emergence was prolonged and the rate of emergence reduced. The effect of soil salinity on seedling emergence became serious with decreasing soil moisture content. Under high salinity conditions, high level of soil moisture alleviated the adverse effects of soil salinity on maize seedling emergence. Field experiment (in 2015 and 2016) results showed that with increasing salt concentration of irrigation water, soil salt content in the 0-20 cm soil layer increased significantly at winter wheat harvest period, with soil salt contents of 1.0 g·kg^-1(CK), 1.3 g·kg^-1 (SWT1), 1.6 g·kg^-1(SWT2) and 2.0 g·kg^-1 (SWT3). After summer maize sowing, an irrigation of 75 mm fresh water kept water content in the plough layer at 70% field capacity, and reduced soil salt content to 1.0 g·kg^-1, which was not significantly affected the growth and yield of summer maize. Yields of summer maize were 9 510.4 kg·hm^-2 (CK), 9 913.6 kg·hm^-2 (SWT1), 9 910.6 kg·hm^-2 (SWT2) and 9 986.0 kg·hm^-2(SWT3) in 2015, and 9 621.8 kg·hm^-2 (CK), 9 455.3 kg·hm^-2 (SWT1), 9 460.2 kg·hm^-2 (SWT2) and 9 221.4 kg·hm^-2(SWT1) in 2016 under salt water irrigation of winter wheat at jointing stage. Considering the temporal distribution of precipitation and the salt leaching of sufficient summer rainfall in the same season of summer maize growth, the effect of soil salt accumulation in winter wheat season on summer maize growth was avoidable. Therefore, irrigation of salt water with less than 5 g·L^-1 salinity at winter wheat jointing stage was safe for the succeeding crop, summer maize. The resonable water managements at key growth stages of winter wheat and summer maize simultaneously stabilized crops annual yield and water-salt balance under brackish water irrigation.