Effects Of Soil Nitrogen And Phosphorus Ratio On Plant Growth And Interspecific Interaction In The Songnen Grassland

Since the industrial revolution,the rapid increase of nitrogen in the ecological environment has broken the chemometric relationship of C-N-P in the terrestrial ecosystem.Especially,the imbalance of N:P ratio in soil significantly affects plant growth and community structure composition,and seriously threaten the production and ecological function of terrestrial ecosystem.Currently,nitrogen deposition and excessive application of agricultural nitrogen and phosphorus fertilizers have caused the imbalance of nitrogen and phosphorus input in the Songnen Grassland located in the northeast agricultural and pastoral ecotone.In this context,it is of great value to study the response of Leymus chinensis,a dominant species in the Songnen Grassland,and its main companion plants and their interspecific interactions to the changes of soil N:P ratio.It will contribute to understanding the impact mechanism of global climate change on grassland community structure and diversity.L.chinensis,the dominant plant in Jilin Songnen Grassland,and the main companion species of Chloris virgata and Phragmites australis were selected as experimental materials in this experiment.During the growing seasons of 2021 and 2022,pot experiments with randomized block design were conducted to explore the effects of soil nitrogen phosphorus condition on plant growth and interspecific relationship in two paired-competition groups of L.chinensis and C.virgata,L.chinensis and P.australis respectively.The soil nitrogen phosphorus conditions included three soil nitrogen phosphorus ratios with available nitrogen and phosphorus mass ratios of 5,15 and 45 for low,middle and high soil N:P ratios respectively,and two nitrogen phosphorus supply levels(low and high levels).Three cultivation modes were designed in the experiment,L.chinensis monoculture,C.virgata(or P.australis)monoculture,mixed cultivation of L.chinensis and C.virgata(or mixed cultivation of L.chinensis and C.virgata)respectively.The main conclusions were obtained as follows:1.In the competition group of L.chinensis and C.virgata,the biomass of plant is mainly influenced by the interaction between soil N:P ratio and interspecific interaction.Under both monoculture and mixture conditions,high N:P ratio treatment significantly reduced the biomass of L.chinensis about 45.0%-63.9%,and the effect of high N:P was significant lower than both low and middle N:P ratio.The high N:P ratio also significantly reduced 9.2%-30.9% biomass of C.virgata in monoculture,but had no significant impact on C.virgata in mixture,showing that interspecific interaction reduced the limiting effect of high N:P ratio on C.virgata.In addition,the high N:P ratio treatment limited the growth of L.chinensis and C.virgata,including the fitness,relative growth rate and specific leaf area.2.In the competition group of L.chinensis and P.australis,plants were mainly influenced by the interaction between N:P ratio and supply level.L.chinensis and P.australis both accumulated higher biomass under low and middle N:P ratio conditions,while the high N:P ratio treatment significantly reduced the biomass of L.chinensis and P.australis about 11.8%-60.3% and 40.4%-49.0% respectively.Moreover,two species could accumulate more biomass at low supply level than at the high supply level.In addition,the high N:P ratio limited the growth of L.chinensis and P.australis,including the numbers of tiller and bud,fitness,relative growth rate and specific leaf area.3.The N:P ratio significantly affected root architecture of L.chinensis,C.virgata and P.australis.The high N:P ratio limited the root growth of L.chinensis and P.australis,and reduced their root length,root surface area,root volume,fiber root length,fiber root surface area and fiber root volume.The root of C.virgata was influenced by the interaction between N:P ratio and interspecific interaction.The root length,root surface area and root volume of mixed C.virgata were reduced by the middle and high N:P ratios.4.The competitive effect indicated that interspecific interaction significantly affected plant growth and was influenced by soil nutrient(nitrogen and phosphorus)conditions.In the competition group between L.chinensis and C.virgata,interspecific interaction significantly reduced the biomass of L.chinensis but significantly increased the biomass of C.virgata.The competitive advantage of C.virgata was highest in the high N:P ratio treatment with strong light competitive advantage.In the competition group between L.chinensis and P.australis,interspecific interaction promoted the growth of P.australis,except for the treatment at high supply level with low N:P ratio.The competitive effect of L.chinensis is negative at low N:P ratio and positive at high N:P ratio,indicating that interspecific competition and competition effect are regulated by the soil nutrient condition.5.The interspecific relationship of L.chinensis and C.virgata was influenced by the soil N:P ratio and supply level.The competition ability of C.virgata was stronger than L.chinensis and became the dominant competitor in the mixture.Under most experimental conditions,the effect of interspecific competition on the growth of L.chinensis was stronger than that of intraspecific competition,but there was no significant difference between the effects of intraspecific and interspecific competition on C.virgata.There is strong resource competition between L.chinensis and C.virgata as indicated by the total relative yield(RYT)at low N:P ratio treatment.Conversely,the two species could coexist under both middle and high N:P ratios conditions.6.The interspecific relationship of L.chinensis and P.australis was influenced by the soil N:P ratio and supply level.In this experiment,P.australis occupied a competitive advantage at low supply level with low N:P ratio and middle N:P ratio,while L.chinensis was the dominant competitor at high supply level with high N:P ratio.Generally,there was no significant difference between inter-and intraspecific competition for L.chinensis,while interspecific competition had less influence than that of intraspecific competition for P.australis.In addition,there was intense competition for resources between L.chinensis and P.australis at high supply level with low N:P ratio.Under other conditions,the two species could coexist.Based on above conclusions,this study not only clarified the regulatory effect of soil N:P ratio on the growth and interspecific relationships of dominant plants in the Songnen Grassland,but also provided experimental data for further predicting species changes and community dynamics of the Songnen Grassland under the background of global climate change characterized with nitrogen deposition.Finally,we propose that the addition of proper phosphorus to the Songnen Grassland can not only avoid the inhibition of plant growth by high soil N:P ratio,but also increase the abundance and dominance of perennial grasses as well as increasing the productivity and stability of the Songnen Grassland.