Effects Of Water And Phosphorus Supply On Physiology,Growth And Interspecific Relationship Of Switchgrass And Bushclover

Artificial grassland establishment is an important measure for vegetation restoration and ecological construction in semiarid loess hilly region.In view of soil water and phosphorus（P）deficiency,and lack of applicable gramineae species and unreasonable structure in artificial grassland establishment in the region,to study the effects of soil water and P supply on the productivity and interspecific relationship of introduced gramineae species and native leguminous species under intercropping could provide theoretical basis for artificial grassland establishment in the region.An introduced applicable gramineae species switchgrass（Pancium virgatum L.）and a native leguminous species bushclover（Lespedeza davurica S.）were selected in present study.A pot experiment was conducted under two soil water regimes[high water（75%±5%field capacity,FC）and low water（35%±5%FC）],three P treatments(0,0.05 and 0.1 g P₂O₅·kg^-1 dry soil)and five mixture ratios[12:0（switchgrass:bushclover）,8:4,6:6,4:8,4:8 and 0:12]to clarify the effects of water and P supply on photosynthetic physiology,leaf nitrogen（N）and P stoichiometry,biomass production,water use efficiency（WUE）and interspecific relationship of switchgrass and bushclover under intercropping.The main results were as follows:（1）In the establishment year,under different water and P treatments,the average net photosynthetic rate（P_n）values of switchgrass in mixtures were significantly higher than those under monoculture,and the average P_n values of bushclover were opposite（p<0.05）.In the second year,under high water regimes,the average P_n values of switchgrass at tillering and jointing stage were significantly increased in mixtures,while average P_n values of bushclover at branching stage were significantly decreased;under low water regimes,the average P_n values,maximum photochemical efficiency（Fv/Fm）values and photochemical quenching coefficient values of switchgrass at jointing stage and those of bushclover at flowering and mature stage were significantly increased in mixtures compared with monocultures.Under low water regimes,the average P_n and Fv/Fm values of bushclover at flowering and mature stage,as well as the photosynthetic water use efficiency values of switchgrass at tillering and jointing stage and bushclover at each growth stage were significantly increased under 0.05 g kg^-1 P application treatments compared with no extra P application treatments,but no significant difference were detected between 0.05 and 0.1 g kg^-1 dry soil.（2）Under different water and P treatments,the photosynthetic N-use efficiency（PNUE）values and photosynthetic P-use efficiency（PPUE）values of switchgrass were significantly higher than those of bushclover;the leaf nitrogen（N）concentration,N:P values and PPUE values of switchgrass were significantly increased in mixtures compared with monocultures,and the leaf N and P concentration of bushcolver were significantly decreased while PNUE and PPUE values were significantly increased.Under both water regimes,the leaf N:P and PPUE values of both species were significantly decreased,and the leaf P of switchgrass and leaf N and P of bushclover were significantly increased under P application treatments compared with no extra P application treatments.（3）The plant height values of switchgrass and buchclover reached to maximum in their mature stage.At tillering stage of switchgrass,the average plant height values of switchgrass and bushclover reached up to 64.2%and 22.2%of their maximum height values respectively,indicating relatively rapid growth in plant height of switchgrass.Regardless of P application,plant height values of both species were significantly lower under low water regimes than those under high water regimes,indicating growth in plant height of both species were restrict under low water regimes.In the establishment year,biomass production per plant of switchgrass showed an increased trend as its mixture ratio decreased in their mixtures,and those of bushclover were opposite;in the second year,under low water regimes,the biomass production per plant of both species reached to maximum at 4:8（switchgrass:bushclover）mixture ratios,and greater in P application treatments than no extra P treatments,but no significant difference were detected between P application of 0.05 and 0.1 g kg^-1 dry soil.（4）Under different water and P treatments,the relative yield total（RYT）values of switchgrass and bushclover mixtures were between 0.91～1.34,implied resource complementarity and high land use efficiency in most mixtures.The results of the two-year experiment showed that under low water regimes,switchgrass to bushclover at 4:8 mixture ratios obtained the highest total biomass production and RYT values among all the mixture ratios and had higher WUE compared with other mixture ratios;total biomass production and WUE in their mixtures were significantly increased under P application treatments compared with no extra P treatments（P<0.05）,while no significant difference were detected between P application of 0.05 and 0.1 g kg^-1 dry soil.（5）The competitive ratio（CR）values of switchgrass were greater than 1.0 among all treatments,and CR values of switchgrass and relative competition intensity（RCI）values of bushclover were significantly lower under low water regimes than those under high water regimes,and significantly lower in the second year than those in the establishment year,implied that switchgrass was the dominant species in their mixtures,and its relative competitive ability decreased under low water regimes and in the second year,while bushclover showed opposite.In the second year,under low water regimes,CR values of switchgrass and RCI values of bushclover were significantly lower under P application treatments than those under no extra P treatments,implyed P application improved the competitive ability of bushclover under low water regimes,while decreased the competitive advantage of switchgrass.In the establishment year,RCI values of switchgrass were lower than 0 and those of bushclover were the opposite,while in the second year,RCI values of both species at 4:8 mixture ratios were greater than 0 under low water regimes,implying benefit from mixtures at 4:8 mixture ratios.In conclusion,switchgrass was the dominant species in their mixtures,and its relative competitive ability decreased under low water regimes and in the second year,while bushclover showed opposite.P application could improve the competitive ability of bushclover under low water regimes,while decreased the competitive advantage of switchgrass.Despite competition between switchgrass and bushclover,there existed resource complementarity in their mixtures,and showed mixture advantage.Our results implied that switchgrass and bushclover could be intercropped in the region,and 4:8 mixture ratio of switchgrass to bushclover with 0.05 g P₂O₅ kg^-1 dry soil application could help to improve total biomass production and WUE,and help them gradually reach the pattern of mutual benefit in their mixtures.