| Phosphorus（P）as an irreplaceable macro trophic element for plant growth,its movement in the soil is mainly diffusion.P applied to the soil is easily fixed,and the moving distance is only 3-5 cm,resulting in its enrichment in the topsoil layer,but relatively insufficient in the deep soil layer,which seriously restricts the improvement of crop yield and phosphorus fertilizer utilization efficiency.Root system is a vital organ for the water and nutrient uptake by crops.It shows a high degree of plasticity in response to spatial changes in the availability of nutrients.Improved the absorption and utilization of nitrogen and phosphorus by optimizing the spatial matching between soil nutrients and root system was a potential measure to improve the grain yield of summer maize.In order to study the regulation effect of different phosphorus application depth on root configuration and function of summer maize,elucidate the physiological and ecological mechanism of improving nutrient efficient utilization and yield of summer maize from the aspects of the coupling of the spatial and temporal distribution of nitrogen and phosphorus nutrients between root and soil and the interaction between root and soil,and explore the regulation of root growth and distribution by different application depths of nitrogen and phosphorus fertilizer,as well as the contribution to yield improvement and nutrient efficient absorption and utilization.Three different experiments were set up in this study.The fist experiment was local phosphorus supply,with low phosphate treatment（LP）and local phosphate treatment（HP）.The second experiment took place with four P placement depths:5,10,15,and 20 cm,denoted D5,D10,D15,and D20,respectively.The third experiment took place with two P application levels(105 and 84 kg hm-2 P2O5,denoted P1 and P2,respectively),two N placement depths（5 and 15 cm,denoted D5 and D15,respectively）,and two P placement depths（5 and 15 cm,denoted D5 and D15,respectively）.The main results were as following:（1）Local HP treatment significantly increased root dry weight,axial root length,total root length and root surface area of the treated（HP-H）and non-treated areas（HP-L）.In particular,it promoted significantly increase the lateral root density（RLD）.Compared with the LP,the 1st RLD of HP-H and HP-L increased by 245.9%and 86.7%,the 2nd RLD increased by 650.8%and 157.9%.Local HP treatment significantly increased a higher number of root cortical aerenchyma（RCA）,combined with a larger cortical cell size（CCS）,and decreased the total respiration rate of roots.The RCA of HP-H and Local HP-L increased by 121.6%and 149.0%,the CCS increased by 13.0%and 32.0%,the total respiration rate of axial root decreased by35.6%and 11.2%,the total respiration rate of lateral root decreased by 36.5%and 13.5%,respectively.Local HP treatment significantly increased the contents of auxin,salicylic acid,jasmonic acid and trans-zetin,which were conducive to promoting the lateral roots growth and establishing an excellent root system structure.Furthermore,Local HP treatment significantly increased the relative expressions of high affinity phosphorus transporters PHT1.1,PHT1.3,PHT1.6,PHT1.8,PHT1.11,PHT1.12 and PHT3.5 in the roots of HP-H,and increased the H+efflux rate,which improved the absorption of P in roots.（2）Placing P fertilizer at an appropriate depth significantly promoted the growth and development of roots,especially placing P fertilizer at 15 cm.Compared with the D5 treatment,the root dry weight,total root length and root surface area of D15 treatment significantly increased by 14.9%,20.1%and 21.3%.Placing P fertilizer at an appropriate depth stimulated the growth of deep roots,and optimized the spatial distribution of roots.For plants of the D15treatment,the RLD in the 20-60 cm soil layer was significantly increased.The D95 was 13.4%higher than that for the D5 treatment.In addition,the RCA and CCS of D15 treatment were increased by 120.7%and 22.0%,the root N cost and root P cost were decreased by 3.3%and4.1%,which drove more nutrients to the growth of lateral root or biomass accumulation and yield formation.（3）Placing P fertilizer at an appropriate depth significantly increased the photosynthetic performance of leaves.Compared with the D5 treatment,the D15 treatment significantly improved the antioxidant capacity of leaves.The leaf superoxide dismutase（SOD）,peroxidase（POD）,catalase（CAT）activities and soluble protein content increased by 42.5%,25.9%,38.8%and 34.1%,respectively,but the malondialdehyde（MDA）content decreased by 24.2%.Which was benefited for delaying the senescence of leaves,maintaining a higher leaf area index,photosynthetic potential,chlorophyll content and net photosynthetic rate of post-R1,and improving the light use efficiency of plants.The leaf senescence index of D15 treatment from the R2 to the R6 stages was 31.4%lower than that of D5 treatment,the leaf area index and chlorophyll content at the R6 stage was 10.2%and 18.6%higher,the photosynthetic rate at the R3 stage was 25.9%higher,the light use efficiency and grain light production efficiency was11.0%and 20.4%higher.（4）With an increasing P application depth,the biomass and grain yield of summer maize increased first and then decreased.D15 treatment had the maximum values.Compared with the D5 treatment,D15 treatment significantly increased the biomass accumulation by 5.8%,especially the proportion of post-R1 biomass accumulation significantly increased by 6.4%.D15 treatment optimized the distribution of dry matter,promoted the transport of photosynthates to grains,increased the harvest index by 2.6%,and then achieved a 18.0%higher grain yield.（5）Moderately deep placement of P fertilizer decreased the P availability in the topsoil,and the roots need to explore a larger soil space to obtain the required nutrients.When the contact area between roots and soil P fertilizer was increased,the roots were induced to deep soil,which increased the N uptake and utilization by the roots in deep soil.From the vertical direction,the root length density in the soil layer below 30 cm of D15 and D20 treatment was significantly increased,soil total N and Olsen P content contents were significantly reduced.It indicated there was a highly coordination between the spatial distribution of roots,soil N and P,which enhanced the root system to absorb N and P.The N and P assimilation of plant of D15treatment was 16.5%and 20.7%higher than D5 treatment,respectively;the P recovery efficiency,the P agronomic efficiency,the partial factor productivity of P and the partial factor productivity was 71.8%,149.6%,20.8%and 20.9%higher.（6）Root growth significantly decreased when the rate of P fertilizer was reduced.Combined deep placement of N and P fertilizer optimized root system structure and root distribution,and delayed the senescence of roots.Compared with N5D5 treatment,the root dry weight in R3 stage of N15D15 treatment was 12.0%higher at P1 application rate and 23.2%higher at P2 application rate.It also enhanced the efficient absorption of soil nutrients by plants,more nutrients were provided to above-ground growth and development,the N accumulation and P accumulation were 7.1%and 18.5%higher at P1 application rate and 5.9%and 17.8%higher at P2 application rate.Therefore,the biomass accumulation and yield formation were significantly increased.Under the same N and P placement depth,the biomass was decreased by 6.9%-13.7%,the grain yield was decreased by 8.8%-14.8%when the P application rate decreased by 20%.In addition,there was no significant difference in grain yield between P2N15D15 and P1N5D5 treatments.In conclusion,local HP treatment promoted the proliferation and growth of lateral roots by changing the content of root endogenous hormones.Moreover,local HP treatment increased the RCA and CCS,which reduced root respiratory consumption and metabolism,and promoted the transfer and utilization of root nutrient elements to lateral roots.Based on the above results,this study set different P fertilizer placement depth in field production to study its regulatory effect on root system architecture and function.The results showed that placing P fertilizer at15 cm was beneficial to coordinate the spatial distribution of roots and soil nutrients,improved the uptake and utilization of nutrients by roots.Which promoted the growth of the shoot,improved the production and transport of photosynthate,and realized improvement of yield and nutrient use efficiency.Placing N and P fertilizer at 15 cm can reduce the application P fertilizer rate on the premise of ensuring the yield. |
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