Mechanism Of Action Of Concentrated Nitrogen Supply In Primary Root Zone To Promote Efficient Nitrogen Utilization In Rice

Nitrogen（N）fertilizer application method is one of the main reasons affecting rice yield and nitrogen utilization,and one-time root zone concentrated N supply can significantly improve rice yield and N fertilizer utilization,but there are few studies on the mechanism based on one-time root zone concentrated N supply to promote rice yield and N utilization.A one-year pot experiment was conducted to investigate the effects of root zone fertilization on root growth and distribution in rice seedlings.Two N fertilizer varieties（urea U;controlled release urea CRU）and three N application rates（low N application rate L:0.97 g N/pot;medium N application rate M:1.22 g N/pot;high N application rate H:1.51 g N/pot）were used.Meanwhile,a two-year field trial was set up with treatments including no N application（CK）,farmer spread urea（FFP）and four partial nitrogen supply（LNS）treatments,including mechanical side deep fertilization with urea（UM）,root-zone fertilization with urea（UR）,mechanical side deep fertilization with CRU（CRUM）,root-zone fertilization with CRU（CRUR）,The aim of this study was to investigate the mechanism of rice yield formation under the combination of concentrated root zone N supply（M and R）and N fertilizer varieties（U and CRU）.The experiment was conducted with the local widely used medium rice Yang Liangyou 6 to elucidate the mechanism of efficient N fertilizer utilization under different N application methods coupled with N fertilizer varieties in terms of rice root growth,yield,N uptake and leaching loss,and to provide theoretical and technical support for lighter fertilization and efficient N fertilizer utilization in rice.1.The pot test showed that the root biomass,total root surface area,total root volume,mean diameter,total absorption area and active absorption area of the common urea root zone fertilization treatment increased and then decreased with the increase of N application after 20 and 30 d of transplanting,while the corresponding root morphological indexes of the controlled-release urea root zone fertilization treatment gradually increased or remained stable with the increase of nitrogen application.Compared with CK,the root biomass and active absorption area of UH treatment decreased significantly by 28.08%and 42.49%,respectively,and the root crown ratio of UL treatment increased significantly by 33.33%;the order of root biomass of each treatment was CRUH>CRUM=CRUL>UM=UL>UH=CK in both 0-5cm and 5-10 cm soil layers,while the root biomass of 10-15 cm soil layer The root biomass of CRUH treatment was higher than the other treatments and significantly increased by 81.28%compared with CK.2.The pot experiment showed that the antioxidant enzyme activity increased with the increase of nitrogen application in the root zone of common urea at 30 d after transplanting,with the highest MDA content in the UH treatment,which was significantly higher than CK by 10.22%-20.80%;while there was no significant difference in the antioxidant enzyme activity and MDA content in the controlled-release urea treatment.3.The field trial showed that aboveground nitrogen uptake,yield,N fertilizer uptake utilization,agronomic efficiency and bias productivity increased by 20.47%,16.35%,65.96%,86.17%and 16.32%,respectively,under LNS treatment compared to FFP.4.The field experiment showed that under LNS conditions,application of CRU was able to increase the ammonium nitrogen（NH₄⁺-N）content of the soil in the root zone and prolong the storage time of fertilizer N in the soil with better root morphological indicators compared with U application,which contributed to dry matter accumulation and nitrogen uptake in rice,which in turn increased the yield and N fertilizer uptake utilization.Mainly,ammonium N content,total root surface area,total root volume,mean diameter,dry matter accumulation and N uptake were significantly increased by 52.28%,42.59%,52.68%,22.63%,17.90%and 14.75%,respectively,under CRU treatment compared with U treatment.5.The field experiment showed that total root surface area,volume,mean diameter and root biomass were significantly increased by 19.30%,23.76%and 9.68%,respectively,under M treatment compared to R under LNS conditions,which favored nitrogen uptake.For R,concentrated and high ammonium nitrogen（NH₄⁺-N）concentration promoted rice growth and yield,with 15.59%,3.81%,11.76%and 3.03%increase in NH₄⁺-N concentration,tiller number,total N uptake,dry matter accumulation and yield under R treatment compared to M,respectively.In addition,the apparent nitrogen loss was significantly reduced by 50.04%under CRUR treatment compared to CRUM.Therefore,urea can be selected for low and medium N application in the root zone,while controlled-release urea is preferred for high N application levels.In the rice cropping system of Jianghan Plain,LNS can increase or maintain rice yield and improve nitrogen use efficiency,which is worth to be promoted.When considering the risk of soil nitrogen leaching and environmental protection,R should be selected as the recommended fertilization method,and the combination of CRU and R is the most effective for sustainable rice production.