Effects of irrigation regime and soil clay content and their interaction on the biological yield, nitrogen uptake and nitrogen-use efficiency of rice grown in southern China

Rice is commonly grown in clay soils under flooding or water-saving irrigation. The combination of irrigation regime and soil texture tend to create the distinguished growing environment of crops. However, plant response to this combination environment is seldom studied in rice grown in southern China. To investigate the effects of irrigation regime, soil texture type and their interaction on the biomass production, nitrogen uptake and nitrogen-use efficiency of rice, a shelter experiment was conducted using a randomized complete block design with a factorial arrangement of treatments with four replications. The main treatment was irrigation regime, set in three levels as R ((30mm-100%)), (100% saturation, 30 mm flooded), R ((30mm-90%)), (90% saturation, 30 mm flooded) and R(30mm-70%), (70% saturation, 30 mm flooded). The subtreatment was the soil texture type, set in three levels as 40%, 50% and 60% clay content, respectively. The irrigation regime, soil texture, and their combined interaction had significant effects on the biomass yield and nitrogen-use efficiency of rice. Compared to that of anaerobic-flooded water regime, biomass yield, nitrogen uptake and nitrogen utilization efficiency of the aerobic-flooded water regime were markedly reduced in both seasons of 2016 and 2017. Higher soil expanding clay content improved the biomass production and nitrogen utilization of rice in the both seasons. Maintaining soil water content at saturation and then reflooding was the optimal water management practice for rice cultivation in the soil, with 60% swelling clay content. The optimal combination R S-(30mm-(100)%) ((60%)) presented the greatest nitrogen-use efficiency, which was 34.7% in 2016 and 46.6% in 2017. Soil swelling was dominant in the combination R S-(30mm-(100)%) ((60%)), as it increased the biomass yield, absorption and nitrogen utilization of the rice plant by 64.0%, 81.5% and 76.2% in 2016, and 67.2%, 82.9% and 77.9 in 2017, compared to the combination R ((30mm-70%)) S (40%). Allowing the expansive soil to dry to 30% below the saturation point and then reflooding resulted in a sharp reduction in the nitrogen-use efficiency of rice of 8.2% in 2016 and 10.3% in 2017 due to cracks in the soil that preferentially became the major routes of water and nutrient losses. Soil cracking is dependent on the irrigation regime and soil texture, and cracks must be avoided when yielding rice in vertic clay soil under water saving irrigation. Our results are valuable in deciding what water management option to practice when yielding rice in swell-shrink clay soil. The results are also an important contribution to knowledge of soil, water and rice relationships.