| Phosphorus(P)is the second most essential macronutrient for plant growth.However,P availability in most soil is quite lowalthough the total P concentration is very high,which significantly limited yield production.Therefore,exploring and exploiting soil P pool and improving crop P uptake and utilization efficiency have become great challenges for the sustainable development of agriculture.Intercropping is an efficient way to improve P spatial and biological availability through crops changes in root architecture,mycorrhizal colonization and rhizosphere processes,resulting in increased P uptake and utilization.Meanwhile,root turnover has great effects on soil P availability and crop P uptake and utilization.However,little information is available on how root architecture,rhizosphere mobilization and mycorrhizal colonization affect each other and their comprehensive effect on P uptake and utilization in intercropping.Especially,the relationships between root turnover and P uptake and utilization in intercropping system are largely unknown.Maize(Zea mays L.)and alfalfa(Medicago sativa L.)were studied,and a field experiment was conducted in a split-plot design with main plot treated with two P levels(P0 and P1)and subplot treated with four cropping patterns: monoculture maize(MM),monoculture alfalfa(MA),four rows of maize intercropping with six rows of alfalfa(IMA4)and six rows of maize intercropping with six rows of alfalfa(IMA6).Each treatment was replicated for three times.The characteristics of root morphology and spatial distribution,rhizosphere proton,organic acids and acid phosphatase secretion,mycorrhizal colonization and hyphal density of maize and alfalfa as well as root turnover traits in different cropping patterns at two different P levels were measured from 2013 to 2015,and the relationships of these root traits with soil P availability and crops P uptake and utilization were studied.Major results are presented as follows:(1)Alfalfa was more competitive than maize for P capture,as much as 2.3-5.6 times.In IMA4 and IMA6,P uptake of maize was slightly reduced by 17% and 20%,which was significantly complemented by the pronounced increase inP uptake of alfalfa(by 186% and 177%),resulting in significantly improved P uptake of the two intercropping systems: higher than MM by 72% and 45% and improved by 83% and 54% as compared to MA.Therefore,maize was the contributor while alfalfa was the beneficiary in interspecific interaction of P uptake.For P utilization efficiency,compared to monoculture,there was no significant changes for intercropped maize,while it was significantly reduced for intercropped alfalfa.Therefore,P utilization of maize was economical,while that of alfalfa was luxurious in intercropping.When P fertilizer was applied,crop P uptake was significantly increased;while when P was deficient,P utilization efficiency was dramatically improved.(2)In IMA4 and IMA6,the number,dry weight,length,surface area and volume of maize crown roots were significantly lower than that of MM by 18% and 16%,33% and 34%,39% and 32%,38% and 29%,and 34% and 29%,respectively;while for alfalfa,compared to MA,the dry weight,surface area and volume of axial root were significantly increased by 65% and 60%,29% and 25%,and 59% and 42%,respectively,the number,dry weight,length,surface area and volume of lateral roots were significantly improved by 31% and 37%,112% and 113%,68% and 61%,110% and 99%,and 178% and 171%,respectively,and the rooting depth,branching number and diameter of collar root were significantly increased by 25% and 26%,52% and 44%,and 25% and 25%,respectively.Roots growth inhibitions of intercropped maize suppressed its P uptake while had no effect on P utilization efficiency;root growth promotions of intercropped alfalfa enhanced its P uptake but reduced its P utilization efficiency.P fertilizer application significantly improved root growth of both maize and alfalfa.(3)Compared to monoculture,root distributions of both intercropped maize and alfalfa were shallower.In IMA4 and IMA6,maize root length density at the topsoil(0-20 cm)and its proportion in the whole soil depth were significantly increased by 61% and 47%,and 31% and 24%,respectively;the corresponding parameters of alfalfa roots were significantly improved by 55% and 40%,18% and 17%,respectively.Meanwhile,both maize and alfalfa roots extended to the intercropping profile as well as each other’s growth zone.However,root distribution changes of intercropped maize failed to improve its P uptake and utilization,and the potential reason was that soil available P in maize growth region was competed and acquired by intercropped alfalfa.Intercropped alfalfa succeeded to improve P uptake through its own changes in root distribution and competition for P from intercropped maize zone,but its P utilization efficiency was decreased.(4)Intercropping,especially at P deficiency,significantly improved rhizosphere mobilization of both maize and alfalfa.Compared to monoculture,rhizosphere pH of intercropped maize and alfalfa were decreased by 0.35 and 0.24 unit;rhizosphere total organic acid concentration,acid phosphatase activity and Olsen-P concentration of intercropped maize and alfalfa were significantly improved by 27% and 30%,21% and 41%,and 24% and 40%,respectively.For intercropped maize,the mobilized P was competed and acquired by alfalfa,besides for its own growth and development,leading to decreased P uptake but improved P utilization efficiency;for intercropped alfalfa,P uptake was significantly improved through its own rhizosphere mobilization as well as competing available P from intercropped maize,but P utilization efficiency was decreased.Therefore,maize was the contributor while alfalfa was the beneficiary in interspecific rhizosphere effect.With planting years’ growing: rhizosphere acidification of both maize and alfalfa became stronger while organic acids and acid phosphatase secretions were decreased.(5)In IMA4 and IMA6,for maize,mycorrhizal colonization rate,vesicle and hyphal abundances of colonized root were improved by 79% and 66%,186% and 182%,and 342% and 375%,respectively,and hyphal densities at 0-10 cm and 10-20 cm were increased by 37% and 31%,and 19% and 19%;for alfalfa,the corresponding parameters of mycorrhizal colonization were improved by 50% and 41%,163% and 143%,and 45% and 51%,respectively,and hyphal densities at two soil depths were increased by 30% and 23%,and 29% and 24%.The improved mycorrhizal colonization of intercropped maize inhibited its P uptake but improved P utilization efficiency at the P1 level;while at the P0 level,the improvements in mycorrhizal colonization had no significant effect on P uptake or P utilization efficiency.For intercropped alfalfa,the improved mycorrhizal colonization,especially at P deficiency,enhanced its P uptake but decreased P utilization efficiency.(6)Maize/alfalfa intercropping significantly reduced root survival rate and mean lifespan,but increased root turnover rate at the topsoil.Especially at the P0 level,for crop roots at 0-10 cm and 10-20 cm in maize/alfalfa intercropping,mean survival rates were decreased by 53% and 56%,both mean lifespans were decreased by 43%,and root turnover rates were improved by 74% and 39%,as compared to MM;while compared to MA,mean survival rates were reduced by 20% and 24%,mean lifespans were decreased by 29% and 25%,and root turnover rates were increased by 17% and reduced by 5%,respectively.The increased root turnover of intercropped crops was beneficial to improve P concentration and utilization efficiency,but had no significant effect on P uptake.(7)The variations of P nutritionof intercropped maize,including P concentration,uptake and utilization efficiency,were mainly attributed to the changes in rhizosphere acid phosphatase activity and succinic and citric acid concentrations,crown roots length as well as root dry weight density.The effect of rhizosphere acid phosphatase activity was extremely important,explaining the variabilities of P nutrition by 38.5%.For intercropped alfalfa,changes in rhizosphere citric and acetic acid concentrations and total organic acid concentration,rhizosphere acid phosphatase activity,lateral root length and volume,root dry weight density as well as root turnover greatly contributed to the variations of P nutrition.Rhizosphere citric acid concentration and total organic acid concentration played vital roles,explaining 46.7% and 14.5% of its variances,respectively.(8)The interactions of P nutrition,soil water content and light transmission of intercropped maize inhibited its yield production,and 98.7% of maize yield variability was explained by these constrained variables.Specifically,intermediate light transmission,soil water content,P utilization efficiency and P concentration had significant effects on maize yield,and explained its variabilities by 95.0%,1.3%,1.6% and 0.4%,respectively.For intercropped alfalfa,the improvements of P nutrition,soil water content and light transmission facilitated its yield production,and 76.8% of alfalfa yield variances was attributed to these parameters.Specifically,alfalfa yield was significantly affected by soil water content,P uptake and P concentration,and 50.5%,20.5% and 3.0% of its variances were explained.Therefore,competition and facilitation in P uptake and utilization of maize/alfalfa intercropping had little effect on maize yield reduction,but significantly promoted alfalfa yield improvement.(9)In IMA4 and IMA6,the slight reductions of maize yield(by 7% and 13%)were significantly complemented by the pronounced increases of alfalfa yield(by 136% and 124%).Thus,total yieldsin IMA4 and IMA6 were increased by 14% and 3% compared to MM,and by 96% and 77% relative to MA;output values per unit area were improved by 113% and 37% relative to MM,and by 45% and 18% relative to MA;land utilization efficiencies were improved by 61% and 35%,respectively.In addition,comprehensive benefits of maize/alfalfa intercropping were significantly improved when P fertilizer was applied,and it varied with the changes of precipitation and planting years.Overall,maize/alfalfa intercropping had obvious advantages in P uptake and utilization as well as yield production.InterspecificP competition and facilitation were effectively balanced and coordinated through changes in root architecture,rhizosphere mobilization,mycorrhizal colonization and root turnoverof both maize and alfalfa,resulting in efficient P uptake and utilization.Moreover,competitions and complementarities in phosphorus,water and light utilization between maize and alfalfa facilitated the improvements of intercropping in yield production and output value.Therefore,the trade-off relationship between competition and complementarity for P in maize/alfalfa intercropping was clarified from a new perspective of root-soil nutrient interaction.This study provided theoretical bases for P efficient utilization as well as for delicately regulating P fertilizer utilization in maize/alfalfa intercropping system and reducing its input and risks to environment. |
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