The Root-Soil Interaction Mechanism Of Improved Yield And Phosphorus Uptake And Utilization Under Phosphorus Application And Maize/Alfalfa Intercropping In Low P And Saline-Alkali Soil

Phosphorus（P）is one of the most limiting nutrients for crop growth.Traditional agricultural production often apply a large quantity of P fertilizer for high yields,which leads to low P use efficiency accompanied with severe environmental pollution.Thus,it is necessary to enhanced P uptake and utilization efficiency of plants for the sustainability of intensive agriculture.Intercropping is an effective agronomic approach to improve soil P uptake and utilization.Intercropping can improve crop yield and economic benefits by adjusting root system and rhizosphere process under the premise of reducing P fertilizer application.In addition to low P in soil,there are large areas of land affected by salinity-alkalinity as well in the farming-pasture zone in Northeast China,which severely limits P uptake and crop growth.Therefore,for the agricultural sustainability in Northeast China,it would be important to explore the potential intercropping system for enhanced P-use efficiency under different P application level in combination with different intercropping patterns.The study aims to clarify the role of root architecture and mycorrhizal colonization on the enhanced yield,P uptake and utilization efficiency of intercropped crops.Maize（Zea Mays L.）,an annual crop widely cultivated in Northeast China,and alfalfa（Medicago sativa L.）,a perennial forage with high quality,were grown in field and greenhouse.The field experiment was undertaken between 2015-2018 as a split-plot design with two P levels as the main plots and different cropping patterns as the subplots,with four replicates.In field,maize and alfalfa were grown as either sole（monoculture maize,MM or monoculture alfalfa,MA）or three intercropping patterns as IMA23（two rows of maize intercropped with six consecutive rows of alfalfa）,IMA42（four rows of maize with six rows of alfalfa）and IMA43（six rows of alfalfa with four rows of maize）,with（P1）or without P application（P0）.The greenhouse experiment consisted of two P levels,with P fertilizer application（P1）or without（P0）,two root separation modes,with plastic separation（PS）or without separation（NS）,and two inoculation treatments,inoculation with 200 g of sterilized inoculum（F0）or 200 g of inoculum（F1）,with five replicates.The microbial inoculum was Glomus mosseae,which is cultured in a mixture of sand,soil,horticultural vermiculite and perlite.The 4-year field and 1-year greenhouse experiments investigated the synergistic effects of root architecture and root mycorrhizal colonization on improving P uptake and utilization in the maize/alfalfa intercropping system,and explored whether it could sustainably enhance the yield of maize/alfalfa intercropping system.The study aimed to advance the theory of root-soil interaction for efficient P-use efficiency in the intercropped crops.The main results were presented as the following.（1）Maize/alfalfa intercropping patterns improved total yield and economic benefits compared with monocultured maize（MM）and monocultured alfalfa（MA）in slightly saline-alkali and low P soil,but the magnitude of increase differed under different P levels.Under P0,the mean total yield and output value per unit area under P0 over four years were 3.2 t ha^-1,4.6 t ha^-1,5.3 t ha^-1,5.5 t ha^-1,6.1 t ha^-1 in MM,MA,IMA23,IMA42 and IMA43,respectively.All three intercropping patterns under P0increased the total yield by 66%-91%compared with MM and 15%-33%compared with MA,taking an average of four-year results.Under P1,the mean total yield over four years were 4.9 t ha^-1,6.4 t ha^-1,6.6 t ha^-1,6.8 t ha^-1 and 7.7 t ha^-1 in MM,MA,IMA23,IMA42 and IMA43,respectively.All three intercropping patterns under P1increased the total yield and output value per unit area by 35%-57%compared with MM and 3%-20%compared with MA,taking an average of four-year results.The application of P fertilizer significantly increased the total yield by 24%-53%compared to no P application under all cropping patterns.Moreover,the coefficient of variation was lower and the sustainable yield index was higher in all maize/alfalfa intercropping system,indicating that the intercropping system was more stable and sustainable with a combination of P fertilizer application.Regardless of the P level,the yield increase of the intercropping system was mainly due to the significant increase in alfalfa yield but a slight decrease in maize yield.The land equivalent ratio（LER）of the intercropping system was 1.11-1.56,indicating that the maize/alfalfa intercropping system had significant advantages in slightly saline-alkali soil.（2）The P uptake was 7.0 kg ha^-1 in MM and 9.4 kg ha^-1 in MA under P0 level.Three intercropping patterns increased total P uptake by 63%-101%compared with MM and 21%-50%compared with MA,taking an average of four-year results under P0.The four-year average P uptake was 12.2 kg ha^-1 in MM and 14.8 kg ha^-1 in MA under P1 level.Three intercropping patterns increased total P uptake by 43%-75%compared with MM and 18%-45%compared with MA,taking an average of four-year results under P1.In the maize/alfalfa intercropping system,the CR_P of alfalfa was 1.2-4.0 times higher than that of maize,therefore P uptake of alfalfa in IMA23,IMA42 and IMA43 increased by 55%,110%and 132%compared to MA;while that of maize decreased by 38%,9%and 2%compared to MM,averaged across the two P levels.The results of the greenhouse experiment showed that total P uptake of NS treatment was significantly higher than that of PS treatment.The application of P fertilizer significantly increased total P uptake of maize and alfalfa by 34%-55%compared to no P fertilizer application.（3）Maize/alfalfa intercropping inhibited root growth of intercropped maize,while promoted root growth of intercropped alfalfa in slightly saline-alkali soil.The root morphological characteristics of maize and alfalfa were positively correlated with soil water content in the maize/alfalfa intercropping system.Compared with MM,intercropping decreased soil water content by 8%-20%for maize,resulting in a decrease in crown root length,crown root surface area,crown root volume,crown root number and crown root dry weight of intercropped maize by 15%-33%,averaged across two P levels.Meanwhile,compared with MA,intercropping increased soil water content by 10%-23%for alfalfa,resulting in an increase in lateral root length,lateral root and taproot surface area,lateral root and taproot volume,lateral root number,lateral root and taproot dry weight,and taproot diameter of intercropped alfalfa by 20%-75%averaged across two P levels.In addition,intercropping inhibited the root distribution of maize in the surface soil layer with its root length density decreased by25%compared to MM.In contrast,root length density of intercropped alfalfa was significantly increased along the entire soil profile.Thus,the intercropped alfalfa had more competitive advantages in water and P uptake than the intercropped maize.（4）Maize/alfalfa intercropping could promote the mycorrhizal colonization of both maize and alfalfa.Compared with MM,mycorrhizal colonization intensity and hyphal abundance of maize root in IMA42 increased by 6%and 12%under P0,6%and9%under P1,respectively.Compared with MA,mycorrhizal colonization intensity,vesicle abundance and hyphal abundance of alfalfa root in all intercropping patterns increased by 26%,20%and 8%under P0,17%,67%and 17%under P1,respectively.In the greenhouse experiment,root mycorrhizal colonization intensity and vesicle abundance in maize and alfalfa of NS were greater compared to those in PS when inoculated with AM fungi.The application of P fertilizer significantly inhibited mycorrhizal colonization of maize and alfalfa in both field and greenhouse experiments.（5）The study on the relative contribution of root architecture and mycorrhizal colonization to P uptake and utilization in the intercropping system showed that crown root surface area of intercropped maize was the main limiting factor for P uptake in intercropped maize under both P0 and P1.For alfalfa,the primary root morphological traits affecting P uptake and utilization of intercropped alfalfa were different under P0and P1.The enhanced P uptake and utilization of intercropped alfalfa under P0 was primarily due to an increase in lateral root surface area,while it was mainly related to the lateral root number under P1.Therefore,compared with mycorrhizal characteristics,root architecture plays a more critical role in P uptake and utilization in maize/alfalfa intercropping systems.Overall,the maize/alfalfa intercropping system can improve yield and P uptake through the synergism of root architecture and mycorrhizal colonization in soil with low P and salinity-alkalinity in the farming-pasture zones in Northeast China.The maize/alfalfa intercropping system had significant advantages and was more stable and sustainable with P fertilizer application.Among the different planting patterns,IMA43had the highest LER,yield and P uptake,therefore it was the optimal maize/alfalfa intercropping pattern for the region.This intercropping advantage was mainly due to a significant increase in lateral root surface area and lateral root number of alfalfa,contributing to its significantly increased yield and P uptake.From the perspective of soil-root interaction,our study revealed the trade-off and coordination relationship between P competition and compensation in the maize/alfalfa intercropping system.Our data provided a theoretical basis for implementing and extending maize/alfalfa intercropping system in saline-alkali degraded soil of farming-pasture zone in Northeast China.