Physiological Effects Of Different Nitrogen Forms On The Iron Nutrition Of Drip-irrigated Rice Under Calcareous Soils

?Object?Drip-irrigated?DI?rice is a high-water efficiency cultivation system,but DI rice often exhibits iron?Fe?deficiency in the field compared to flood-irrigated rice in agricultural practice at low temperature in the seedling stage.Since rice is an ammonium-producing and prefer-warming crop and suitable for flooding cultivation,it is speculated that the candidate reasons of physiological Fe-deficiency could be:?1?low soil temperature,?2?soil water regime changed,?3?soil redox potential?Eh?increased and?4?soil inorganic nitrogen?N?changed to nitrate due to enhanced nitratfication.In this study,by simulating the growth conditions of rice under drip irrigation,the mechanism of physiological Fe deficiency in rice under different soil temperature conditions was studied.Based on this,the effects of different N forms on the availability of Fe in the rhizosphere of rice,the reuse and the transportation of Fe in rice were studied.These results can clarify the physiological effects of different N forms on rice Fe nutrition on calcareous soils.At the same time,this study can also provide a theoretical basis for optimizing the rational fertilization of drip-irrigated rice.?Methods?From 2017 to 2018,a series of pot experiments were conducted in the glass greenhouse and mesh room at the Agricultural Experiment Station of Shihezi University,and a series of hydroponic simulation experiments were conducted at the Key Lab of Oasis Ecology Agriculture of Xinjiang Production and Construction Group,respectively.The tested rice varieties were screened for?paddy?rice varieties suitable for drip irrigation,namely Fe-efficient varieties T43?Oryza sativa L.cv.T43?and Fe-inefficient varieties?Oryza sativa L.cv.T04?,respectively.In this study,three aspects research:?i?the effects of soil Eh on Fe availability of drip-irrigated rice,?ii?the effects of different N forms?NH₄⁺-N,NO₃^--N?on the availability of Fe in rhizosphere,and the absorption and the reuse of Fe in rice,?iii?the effects of different N forms?NH₄⁺-N,NO₃^--N?on the development of root aerenchyma and Fe transport of drip-irrigated rice were studied..?Results?1.Elevated soil Eh reduced soil DTPA-Fe and Fe?II?content.Compared with flooding irrigation,the active Fe concentration of rice leaf,Fe absorption and biomass of rice were significantly reduced by forced aeration of soil in flooding irrigation.Soil DTPA-Fe content,rice Fe uptake and rice biomass of drip-irrigated treatments were the lowest in these three soil conditions.2.Compared with NO₃^--N,application of NH₄⁺-N to DI rice can significantly reduce rhizosphere pH and rice seedling death rate,increased DTPA-Fe content in rhizosphere soil,SPAD value of rice leaves,shoot dry weight,root dry weight and Fe content of rice.3.Compared with the optimal soil temperature treatments,low soil temperature significantly reduced rice biomass,leaf SPAD value,rice root surface area,root activity and Fe uptake.Rhizosphere acidification and soil Fe activation effect caused by absorption of NH₄⁺-N were weakened by the soil low temperature.Under low soil temperature conditions,application of NH₄⁺-N to rice compared with NO₃^--N can alleviate the decrease of biomass,leaf SPAD value,root surface area,root activity and Fe uptake of DI rice.4.Compared with NH₄⁺-N,the leaves of rice?T43 and T04?treated with NO₃^--N showed severe Fe-chlorosis symptoms and chlorophyll content decreased significantly under the 12^th day of Fe deficiency.The xylem sap pH and cell wall Fe fraction both of the shoot and the root in NH₄⁺-N were significantly lower than that in NO₃^--N.However,the Fe concentration in xylem sap,the Fe concentration of soluble and organelle components in leaves and roots,and the shoot Fe content and shoot/root Fe ratio were all significantly higher in NH₄⁺-N than those in NO₃^--N treatments.5.Under low Fe culture conditions,higher aerenchyma ratio and porosity were observed in the roots of rice?T43 and T04?supplied with NO₃^--N compared with NH₄⁺-N.Compared with NH₄⁺-N,rice root water absorption capacity,root hydraulic conductivity,xylem sap flow rate,rice leaf water potential,Fe concentration in xylem sap,and shoot Fe concentration and Fe content in NO₃^--N were decreased significantly.However,root Fe concentration and Fe content in NO₃^--N treatments were significantly higher than that in NH₄⁺-N treatments.?Conclusions?1.The reason why DI rice seedlings are susceptible to Fe deficiency in the low soil temperature are:?1?the increased soil Eh leads to the decreased soil DTPA-Fe content,the soil moisture condition of drip irrigation is not conducive to Fe absorption of DI rice;?2?the low soil temperature limits the growth of roots and reduced root activity of DI rice,and the NO₃^--N in soil increased Fe deficiency of DI rice.2.The application of NH₄⁺-N to rice can alleviate Fe deficiency by:?1?acidifying rhizosphere and improving DTPA-Fe content in the rhizosphere;?2?increasing the root surface area and root activity which ensuring the strong Fe absorption capacity of rice roots;?3?reducing the pH value of rice xylem sap,which mobilizing Fe from the cell wall to cell soluble and organelle both in roots and in leaves and in return increases the efficiency of Fe utilization in rice;?4?reducing root aerenchyma fraction and root porosity,thereby improving the water absorption capacity of the roots and promoting more Fe transport from roots to shoots.