Effect Of Root-soil Interaction On Phosphorus Use Efficiency In The Zea Mays/Medicago Sativa Intercropping System

Phosphorus is the second important nutrient element for crop growth,however for most agriculture soil,total phosphorus content is very high,while available phosphorus concentration is low and not sufficient enough for plant growth.Therefore,the effective use of soil phosphorus pools and the improvement of phosphorus absorption efficiency are now important strategies to deal with these serious challenges in agricultural production.In intercropping systems,changes of root architecture and interspecific rhizosphere effect by alteration of protons,organic acid and acid phosphatase secretion can improve phosphorus uptake of plants,which can help increase utilization efficiency of light,heat,water and fertilizer.Maize（Zea mays L.）is an important food and forage crop,and alfalfa（Medicago sativa L.）is a leguminous forb in farming-pastoral zone of northeast China.Alfalfa/maize intercropping has important significance on sustainable development of agriculture and animal husbandry,as well as eco-environmental security.However,previous studies on alfalfa/maize intercropping advantages mainly focus on aboveground part,but there are few studies to explore the relationships between root architecture,rhizosphere microecology and efficient phosphorus uptake and utilization in the field.A field experiment was conducted in a split-plot design with four replications.The mainplot included no phosphorus fertilizer addition（P0）and phosphorus fertilizer addition(P1,120.0 kg P₂O₅ hm^-2 for maize and 135.0 kg P₂O₅ hm^-2 for alfalfa).The subplot was composed of five cropping patterns:alfalfa monoculture（MA）;maize monoculture（MM）;2rows maize and 6 rows alfalfa intercropping,spacing 30 cm（IMA23）;4 rows maize and 6rows alfalfa intercropping,spacing 20 cm（IMA42）;4 rows maize and 6 rows alfalfa intercropping,spacing 30 cm（IMA43）.This study was intended to evaluate the characteristics of root architecture,pH,organic acids concentration and acid phosphatase activities in the rhizosphere soil under different intercropping patterns.Major results were presented as follows:（1）There was significant interaction between phosphorus addition and intercropping patterns on maize yield.Maize yield was mainly affected by root morphology（explained41.6%）,in which total root volume（explained 20.5%）and crown root number（explained17.7%）played the vital role.Phosphorus addition significantly increased maize yield by18.7%,and regardless of phosphorus addition level,maize yield in monoculture was higher than that in intercropping patterns.In P0 treatment,maize yield and phosphorus uptake in intercropping patterns were lower than those in monoculture,because crown root number,total root volume,total root length,total root surface area and root length density in 0-10cm were reduced in intercropping patterns,which led to a reduction in the capacity of absorbing nutrition.But in P1 treatment,maize yield in IMA43 was increased,while maize yield in IMA23 and IMA42 was significantly decreased compared to monoculture,which were attributed to higher crown root number and total root volume in IMA43 than other patterns.（2）No significant interactive effect was found between phosphorus addition and intercropping patterns on maize phosphorus use efficiency（PUE）.Regardless of phosphorus addition level,maize PUE in IMA43 pattern was lower than that in other patterns.Maize PUE was mainly affected by rhizosphere soil microenvironment（explained30.1%）,in which citric acid concentration was the most important parameter（explained12.3%）.In IMA43 pattern,maize PUE was lower than that in other patterns,because maize rhizosphere soil acid phosphatase activities,acetic,lactate,citric and total organic acids concentration were increased,which led to the improvement of maize phosphorus uptake.Therefore,the phosphorus uptake,yield and stalks phosphorus concentration of maize in IMA43 pattern were higher than those in other patterns.（3）There was no significant interactive effect between phosphorus addition and intercropping patterns on alfalfa yield.Regardless of phosphorus addition level,alfalfa yield in intercropping patterns was higher than that in monoculture,especially alfalfa yield in IMA43 pattern was significantly higher than that in IMA42 and IMA23 pattern.Alfalfa yield was mainly affected by rhizosphere soil microenvironment（explained 46.1%）.In P0treatment,there was no yield penalty,because the citrate concentration in the rhizosphere soil was significantly increased and pH value was also significantly decreased,which can increase phosphorus availability.In intercropping patterns,alfalfa lateral root diameter was greater than those in monoculture,and lateral root surface area was increased,which led to the enhancement in the capacity of absorbing nutrition and alfalfa growth,especially for the IMA43 pattern with most obvious advantages.（4）No significant interactive effect was observed between phosphorus addition and intercropping patterns on alfalfa PUE.In P1 treatment,alfalfa PUE was 31.6%lower than that in P0 treatment.Alfalfa PUE was mainly influenced by rhizosphere soil microenvironment（explained 40.4%）,in which citric acid concentration in the rhizosphere soil played the dominant role（explained 24.6%）.In P0 treatment,alfalfa PUE was higher than that in P1 treatment,because the citric concentration,total organic acids concentration and soil acid phosphatase activities in the rhizosphere soil were significantly increased and pH value was significantly decreased,which led to an increase alfalfa phosphorus availability.In P1 treatment,alfalfa phosphorus condition was much improved,and alfalfa phosphorus concentration and phosphorus uptake were significantly increased.Therefore,alfalfa phosphorus absorption efficiency was remarkably increased and PUE was significantly decreased.（5）Land equivalent ratio（LER）was significantly affected by the interaction between phosphorus addition and intercropping patterns.In P0 treatment,IMA42 and IMA43 had yield advantage（LER>1）,while only IMA43 had yield advantage in P1 treatment.There was no significant interactive effect between phosphorus addition and intercropping patterns on output value per unit area（OVPUA）.OVPUA in P1 treatment was 1.11 times more than that in P0 treatment averagely.Irrespective of phosphorus addition level,the OVPUA of IMA43 treatments was higher than that of MA and MM by 4.3%and 23.5%,respectively;the OVPUA of IMA42 and IMA23 were higher than that of MM by 2.2%and 19.6%,respectively and lower than that of MA by 18.3%and 0.7%.The IMA43 pattern had a significant economic advantage.In conclusions,intercropping distance in 30 cm and maize alfalfa lines for 4:6（IMA43）was much better than other patterns from yield advantage and phosphorus recovery efficiency aspects.Our results suggest intercropping pattern IMA43 has obvious advantages in the plot level.