Effects Of Water And Nitrogen Regimes On The Growth And Interspecific Relations Between Switchgrass And Old World Bluestem

Water and nitrogen are the main factors affecting plant growth,distribution and competitive ability in semiarid Loess Plateau.This study was intended to compare competitive abilities between native and introduced grass species under different water and nitrogen supply conditions,and thus to evaluate the potential ecological risk for growing introduced species in large scale,and provide theoretical basis for re-vegetation purposes and ecological restoration in the regions under climate change conditions such as changes in rainfall patterns and increasing atmospheric N deposition.Swichgrass(Pancium virgatum L.)is a vigorous perennial introduced C4 grass from North America.Old world bluestem(Bothriochloa ischaemum L.)is a native C4 perennial grass species in the region.One pot experiment was conducted using a replacement series design method,in which old world bluestem and switchgrass were grown with twelve plants per pot at seven planting ratios(12:0,10:2,8:4,6:6,4:82:10,0:12)under three nitrogen treatments(0,0.05 and 0.1 gN/kg dry soil)and three water regimes: [80% field capacity(FC),60% FC and 40% FC].The photosynthetic characteristics,biomass accumulation and allocation and relative competitive ability of the two species were determined in response to different water and nitrogen regimes.The main results were as follows:1.The response of switchgrass to a short-term drought and re-watering treatment during tillering stage showed that: net photosynthetic rate(Pn),stomatal conductance(gs),transpiration rate(Tr)andthe averaged maximum photochemical efficiency(Fv/Fm)all decreased as soil water decreased,and the reduced extent of Pn and gswere larger under nitrogen application.After re-watering,all these parameters recovered and approached to the level of control treatmentat the end of the experiment,while the aboveground biomass under 40% FC regimes with nitrogen application was significantly lower,which indicated that nitrogen increased the degree of drought stress under 40% FC regimes.2.The growth performance and competitive ability of switchgrass and old world bluestem under different water regimes indicated that the plant height,total biomass per plant,of old world bluestem were lower in mixtures than in monoculture,and were lower than those of switchgrass.The aggressivity(A)values of old word bluestem and relative competition intensity(RCIba)of switchgrass increased with soil water content decreased and were smallerthan 0,Besides,these values in 2015(the second year after planting)were lower than in 2014,while the relative competition intensity(RCIab)of old world bluestem showed an opposite trend,thus we conclude that the relative competitive ability of switchgrass will be enhanced and displaced old world bluestem as soil water increased or as growedyear prolonged after sowing.3.The photosynthetic characteristics of switchgrass and old world bluestem were investigated under different water and nitrogen regimes.Results showed that the Pn,WUEi and non-photochemical quenching values(NPQ)of switchgrass was higher than those of old world bluestem,whereas,Fv/Fm and photochemical quenching(qP)values of switchgrass were lower than old world bluestem.During each growth period,the averaged Pn values of switchgrass in mixtures were significantly higher than in monoculture,vise versa for old world bluestem.All these indicated that mixture plantation improved the photosynthetic performances of switchgrass while decreased those of old world bluestem.4.The biomass production and allocation of switchgrass and old world bluestem were measured under different water and nitrogen regimes.Results showed that nitrogen addition can significantly increase the plant height and total biomass per plant of the two grasses under 80%FC and 60%FC conditions.Besides,water or nitrogen addition resulted in a greater increase in biomass productionof switchgrass than that of old world bluestem.Nitrogen supply had little effect on switchgrass under 40%FC regime.The biomass production switchgrass for each plant in mixtures were higher than in monoculture.Without nitrogen fertilization condition,there were no significant differences among mixture ratios in total biomass production of the two grasses,whereas,with nitrogen fertilization condition,the highest total biomass production occurred in monoculture of switchgrass.5.The competitive ability between switchgrass and old world bluestem under different water and nitrogen regimes were compared.The results indicated that in most treatments,the aggressivity(A)of old world bluestem and relative competition intensity(RCIba)of switchgrass were less than zero,and both the two values decreased with nitrogen regimes increased under 80%FC and 60%FC regimes.The two values increased with nitrogen regimes under 40%FC regime,indicating thatthe competition ability of switchgrass was higher than old world bluestem.The competitive ability of old world bluestem increased with nitrogen regimes under 40%FC condition,while decreased with nitrogen regimes under 80%FC and 60%FC water supply condition.In summary,water and nitrogen supply changed the competitive ability between the old word bluestem and switchgrass.Low water and nitrogen availability had little effect on competition between the two grasses,while high soil water and nitrogen addition is in favor of the switchgrass,which may result in a wide dispersal of switchgrass.