Nitrogen utilization in mechanical transplanted rice with long seedling age and its correlation with environmental temperatures

Abstract: As an important element determining grain yield, optimum seedling age plays a pivotal role in achieving high grain yield for mechanically transplanted rice. Rice seedlings which grow for a longer time are not suitable for transplanting by machine, and are called "long age seedlings." However, in wheat-rice rotation regions, rice seedlings are transplanted by machine at long seedling age due to the occurrence of unfavorable factors, such as a late harvest for the previous crop, the limitation of the number of transplanting machines, and the adverse weather, which has severely threatened the machine-transplanting rice yield. The objectives of this paper were to clarify the law of nitrogen demanding and allocation in mechanical transplanted rice with a long seedling age, providing references for rice cultivation under a mechanical transplanting pattern. With conventional artificial transplanting rice as a control, the field experiment was designed to investigate the changeable law for nitrogen requirement, transportation, and allocation in mechanical transplanted rice with a long seedling age. The growth duration of mechanical transplanting rice with a long seedling age was postponed by 4-13 d as compared with artificial transplanted rice. In the case of mid-early maturing cultivars, the amount of nitrogen requirement in mechanical transplanting rice with a long seedling age was less than that in artificial transplanting rice. However, mid-late maturing cultivars had the converse performance. When compared with artificial transplanted rice plants, the nitrogen export amount and rate of leaves and stems tended to dwindle from the booting to the heading stage as well as the nitrogen absorption amount of panicles at the maturity stage, whereas it was the contrary for the nitrogen export amount and rate and conversion rate of leaves from the heading to the maturity stage in mechanically transplanted rice. Canonical correlation analysis indicated that the nitrogen requirement amount of rice plants and nitrogen exportation amount and rate of stems and sheaths were closely related with effective accumulative air temperature, and the nitrogen conversion rate of leaves was intimately correlated with the daily mean air temperature. The characteristics of nitrogen requirement and utilization in mechanically transplanted rice plants with a long seedling age were dominated not only by genotypes but also by environmental temperature. Based on a conventional nitrogen fertilizer application for mechanically transplanted rice, the application ratio of earing fertilizer should be decreased appropriately for mid-early rice cultivars with a long seedling age while it should be enhanced for mid-late cultivars with a long seedling age on the premise of normal maturity. In addition, effective agronomic practices should be adopted to enhance nitrogen usage efficiency of mechanically transplanted rice with a long seedling age, which may gain compensation for the yield loss which resulted from mechanical transplanting at an over-optimum seedling age.