Influences Of Phosphorus Supply On Interactions Between Roots And Soil Microbes And Post-silking Partitioning Of Shoot Carbon/phosphorus In Field-grown Maize

Determination of the optimized application of phosphorus(P)chemical fertilizer and the critical soil available P concentration in crop production is important to avoid over-use of P fertilizer and increase its use efficiency,explore the biological potential of crop and soil microorganisms and ensure the sustainable development of agro-ecosystems.In this work,muti-years' field experiments at the Shangzhuang Experimental Station of China Agricultural University were conducted to:a)determine the critical application rate of P fertilizer and the corresponding soil available P concentration for achiving the highest maize grain yield;b)study the effects of P fertilizer supply on post-silking P remobilization/redistribution between the vegetative and reproductive organs and the carbohydrate concentration in leaves and ear;c)investigate the arbuscular mycorrhizal(AM)fungal community structure and potential contribution to host plant P nutrition in maize roots under varying P-fertilizer inputs(0-300 kg ha-1)and soil depths(0-20 cm,20-40 cm)by using molecular biological methods;d)compare the differences of transcriptomes and microbial community structures in maize axial and lateral roots and the effects of soil microorganism on maize growth under low and adequate P-fertilizer inputs,using technologies of RNA-Seq,16S,ITS sequencing and soil irradiative sterilization.The main results were as follows:1)The critical P chemical fertilizer(P2O5)input for the highest maize grain yield at the Shangzhuang Experimental Station was 75-100 kg ha-1,the corresponding Olsen-P concentration in the 0-20 cm soil layer was 6-10 mg kg-1.Over-use of P fertilizer did not significantly increase grain yield further,however,led to decrease of P fertilizer use efficiency.The soil Olsen P concentration in the 20-40 cm soil layer remained 3-6 mg kg-1,even under adequate or excessive P-fertilizer inputs.The shoot P concentration of maize under optimized P supply was 3 mg g-1 at V8 stage(total shoot biomass was about 2 t ha-1)and decreased to 1.6 mg g-1 after silking(total biomass was about 8 t ha-1),and remained relatively stable until maturity.2)Phosphorus deficiency significantly enhanced P remobilization in the lower leaves during the early post-silking period,but did not significantly influence the P remobilization in upper leaves,stem and the P harvest index at maturity.Phosphorus deficiency did not significantly influence the post-silking photosynthesis rate of ear leaf,while promoted the synthesis of starch in ear leaf and ear during the early post-silking period.The shoot P remobilization efficiency and carbohydrate concentration of leaf/ear were not affected by P supply at the dough stage.3)AM colonization rate,arbuscule abundance,intensities of fungal alkaline phosphatase(ALP)/acid phosphatase(ACP)activities and expression of ZmPhtl;6(AM fungi colonization inducible Pi transporter gene)and ZmCCD8a(Strigolactones synthesis gene)in maize roots decreased with increasing P-fertilizer input and reached their minima at P application rates of 75-100 kg ha-1,despite continued further application of P-fertilizer.The AM fungal potential contribution to host plant P nutrition in roots from the 20-40 cm soil layer,in terms of the above parameters,was greater than those from the 0-20 cm.Neither P input nor soil depth influenced the Shannon-Wiener Index or number of AM fungal OTUs present in roots,but P-fertilizer input,in particular,influenced community composition and relative AM fungal abundance.There was no difference of AM fungal community composition in roots between soil layers.4)Maize axial and lateral roots showed obvious differences in transcriptomes and community structures of fungi inhabiting in them,and the differences were more divergent than those caused by P supply.At root transcriptional level,"cell wall","secondary metabolism" and "biotic stress"related metabolism were more active in lateral roots than in axial roots under the adequate P-fertilizer input.Phosphorus deficiency caused up-regulation of many genes both in the axial and lateral roots.The number of differentially expressed genes caused by P deficiency was more in axial roots than those in lateral roots,and mainly enriched in "biotic stress" related metabolism.AM fungi colonized mainly in lateral roots,rather than in axial roots.Phosphorus deficiency affected fngal community structure preferentially in lateral roots.5)The results of the pot experiment combining with soil irradiative irradiation showed that the irradiation on P-deficient soil suppressed maize growth and led to extreme P deficiency in plant compared with that grown in the same soil without irradiation.By contrast,irradiation on P adequate soil promoted maize growth.The analyzing results of the root transcriptomes showed that soil irradiation caused down-regulation of many genes expression.The down-regulated expression genes in roots of plants grown in the sterilized P-sufficient soil enriched in "response to biotic stimulus","proteolysis","chitin catabolic process","oxidation reduction" related metabolism and so on,while those in the P-deficient soil enriched in "DNA replication initiation","fatty acid biosynthetic process","cell wall organization or biogenesis" related metabolism and so on.Phosphorus supply did not significantly affect the bacterial and fungal community structures in maize roots.Overall,optimized application of P chemical fertilizer could not only achieve high grain yield in maize,but also substantially improve P fertilizer use efficiency.Neither insufficient nor excessive P input could increase P harvest index at maturity.Under adequate or excessive P-fertilizer inputs,the potential contribution of AM fungi to host plant P nutrition in maize roots from the 20-40 cm soil layer was greater than that from the 0-20 cm soil layer.The beneficial effects of soil microbe for plant growth were greater than their harmful effects when plants grew under insufficient P input,while it was opposite when grew under adequate P supply.Fungal colonization and "biotic stress" related metabolism activity in lateral roots were greater than those in axial roots under adequate P-fertilizer supply.Phosphorus deficiency preferentially affected fungal community structure in lateral roots,and induced "biotic stress" related metabolism in axial roots,suggesting that P supply could affect not only the functional separation of the axial and lateral roots,but also the biological potential of soil microbe for improving soil P utilization.Fungal colonization intensity and community structure in lateral roots,which are the main organ for acquisition of soil nutrients and water,were more susceptible for P supply.Meanwhile,the significant up-regulation of gene expression related to "biotic stress" in axial roots could reduce the risk of harmful microorganisms for plant growth.