Photosynthetic Physiological Response And Growth Adaptation Of Major C₃ And C₄ Plants In Songnen Grassland To Variation In Precipitation Pattern

Human activities affect the processes of global hydrological cycle and nitrogen?N?cycle,resulting in changes of precipitation pattern and enhancement in N deposition rate.Precipitation pattern changes and N deposition?wet deposition?occur at the same time and may significantly affect the structure and function of grassland,which is often constrained by the timing and availability of water and N.With vast area and huge soil carbon pool,grasslands play an important role in the global carbon balance and economic development.The variation in precipitation amount,precipitation frequency and seasonal distribution could cause more unpredictable extreme precipitation events,which are predicted to have significant impacts on ecosystem structure and functions.Variation in precipitation regimes are characterized by strong spatiotemporal variability,and are likely to occur simultaneously with other global change factors,such as N deposition,which may have profound impacts on the relative abundance of grasses differing in functional traits,such as C₃ and C₄ plants.C₃ and C₄ plants may respond differently to the altered precipitation and N deposition due to functional differences in anatomical structure,photosynthetic processes,water use efficiency and N use efficiency.Seasonal variation of precipitation and N deposition could also affect the interspecific competition intensity between C₃ and C₄plants,which are differed apparently in temporal niche.Plant functional type differences in global change responses could cause substantial changes in vegetation composition and ecosystem functions.To explore the differences in photosynthetic physiological processes,biomass accumulation and distribution mechanism between C₃ and C₄ plants in response to variation in key soil resources,we designed three controlled precipitation regime experiments with different amount,frequency and seasonal distribution with one C3?Leymus chinensis?and two C4?Chloris virgata and Hemarthria altissima?grass species that co-occur widely in the meadow steppe of Northeast China.Below are the major results of the present study:?1?Using a pot experiment,C3 grass L.chinensis and C4 grass H.altissima were selected as experimental species to assess the sensitivity of photosynthetic carbon fixation and biomass distribution between aboveground and belowgroud to different precipitation amount and precipitation frequency.The variation of precipitation amount and precipitation frequency affected the biomass accumulation and distribution for C3 grass L.chinensis and C4 grass H.altissima?same change pattern?.For both C₃ and C₄ grasses,biomass accumulation increased with the increasing of precipitation amount,but the effects of precipitation frequency on the biomass accumulation were regulated by precipitation amount.Under the low precipitation amount treatment,the low precipitation frequency benefited to the improvement of plant biomass accumulation.On the contrary,the high precipitation frequency yielded the greatest plant growth under the high precipitation treatment.Precipitation variation mainly changed the pulse degree and duration of soil water content,which leaded to the change of photosynthetic carbon fixation capacity and affected the accumulation and distribution of plant biomass in C3 grass L.chinensis and C4 grass H.altissima.In general,the effect of precipitation frequency on the photosynthetic physiology and biomass accumulation of experimental species depend on the variation in precipitation amount.Under the scenario of variation in precipitation amount and frequency,C₄ grasses are likely to have photosynthetic advantages in water and N use efficiency,which eventually helped the C₄ grass to better adapt to precipitation variation than the C₃ grass.?2?To assess differences in drought resistance and resilience between the C₃ and C4 grasses,we subjected a controlled drought to one C3?L.chinensis?and two C4?Chloris virgata and H.altissima?grasses,followed by a re-watering treatment under both fertilized and unfertilized conditions in a pot experiment.Compared to the C₃grass L.chinensis,the C4 grasses C.virgata and H.altissima had greater carbon assimilation rates under moderate drought conditions.Soil water deficit induced metabolic limitations in C₄ gresses were responsible for greater drought sensitivity in C₄ grasses than to C₃ species?stomatal limitation?,which resulted in lost of advantages in photosynthetic physiological for C₄ plants at the end of the drought treatment,especially for the fertilized grasses.Nitrogen addition caused a strong increase in biomass and resulted in more severe drought stress in the later period of the drought treatment,leading to a change in the dominant photosynthetic limitation and greater downregulation of photosynthetic rates.The findings of this study indicate that the effect of N addition on photosynthesis during drought was asymmetric,especially in the plants with low photosynthetic nitrogen use efficiency such as C₃grasses.?3?Using a mesocom experiment,C3 grass L.chinensis and C4 grass H.altissima were selected as experimental species to investigate the effects easonal variation of precipitation and N addition on biomass accumulation,distribution and interspecific competition between C₃ and C₄ plants for two consecutive growing seasons.Under the condition of constant total precipitation in the growing season,the increase of precipitation in spring was beneficial to the growth of C3 grass L.chinensis,and the increase of precipitation in summer was beneficial to the biomass accumulation of C₄grass H.altissima.The N addition promoted the aboveground and belowground biomass of the two experimental species,but was more beneficial for C3 grass L.chinensis than C4 grass H.altissima on the biomass accumulation.Under the unfertilized condition,C₄ grasses have greater water and N use efficiency,which helped the C4 grass H.altissima obtained the competitive advantages.For the fertilized pots,the competition ability for light was significantly enhanced in C₃ grass L.chinensis,which enhanced carbon fixation rate and distribution of more photosynthates to belowground parts to acquire more water and N resources;higher belowground resources acquiring ability ensured the aboveground growth and eventually change interspecific competition between C₃ and C₄ plants.In summary,C₄ plants have lower sensitivity to precipitation variation in amount and frequency,which suggest that C₄ plants are more suitable than C₃ plants under the scenarios of precipitation variation.However,stomatal limitation plays a more important role for drought impacts on leaf carbon assimilation in C₃ grass than in C₄grasses;therefore,the C₃ grass would recover more quickly than the C₄ grasses after rehydration.For the studied region,the predicted occurrence of extreme precipitation events may help to maintain balance in the ratio of C₃ to C₄ photosynthetic pathway plants.Seasonal variation in precipitation distribution decoupled the coordination of soil water availability and temporal niche,which subsequently changed plant growth,biomass allocation and outcomes of interspecific competition.There are interactions between global change factors,on global change factor may totally change the effects of another factor.The present study provides information on the effects of multiple global change factors on the growth and competition between two photosynthetic functional groups and unravels the underlying mechanisms.The results are useful for the prediction of regional vegetation composition,especially the ratio of C₃ to C₄plants,under the future changed precipitation regime and enhanced N deposition.