Responses Of Roots To Precipitation Amount In Inner Mongolia Typical Steppe

Human activities cause global warming and alter general circulation of atmosphere,consequently,leading to substantial changes in precipitation regime in the future.Arid and semi-arid grassland is the most widespread terrestrial ecosystems in the world,which is sensitive to precipitation changes.As the hidden half of grassland plants,roots play an important role in absorbing water and nutrient,and therefore maybe more sensitive to the changing precipitation amount.Precipitation has been demonstrated to influence the growth,mortality,and turnover of roots,as a result greatly affects ecosystem carbon cycling.However,due to the fact that roots are the hidden half underground and the limitation in methods of root examination,how plant roots in grassland ecosystem respond to changing precipitation regimes remains poorly understood.To address this research gap,we conducted a rain-out shelter experiment with 9levels of precipitation amount from 100 mm to 500 mm with a step of 50 mm in Inner Mongolian Grassland Ecosystems Research Station,Chinese Academy of Science.Three methods for root examination,including minirhizitron,taking up soil cores by drills and root ingrowth method were simultaneously used in this study.The key findings are as follows:(1)Effects of precipitation amount on soil moisture and soil temperature: with the increase of precipitation amount,soil moisture(0-10cm)increased linearly,while soil temperature decreased linearly.There was a linear negative correlation between soil moisture and soil temperature.(2)Effects of precipitation amount on roots: with the increase in precipitation amount,root biomass,root production,root vertical distribution did not change significantly.By contrast,root longevity significantly decreased under the highest precipitation level(500mm).With the increase of precipitation amount,the increased root growth was offset by the increased root death,as indicated by higher root turnover,resulting in relatively stable root production and root biomass.In the soil of0-10 cm from the ground,root production displayed a linear positive correlation with soil water content and a linear negative correlation with soil temperature.The variations in root production can be better explained by soil temperature rather than soil moisture.(3)Effects of precipitation amount on biomass allocation between above-and belowground organs: the allocation pattern of biomass between above-and belowground parts supported the optimal partitioning hypothesis.With the increase in precipitation amount,R/S ratio exhibited a significant decrease,suggesting that more biomass synthesized by grassland plants were allocated to aboveground organs relative to belowground ones.Such a pattern was mainly caused by the inconsistence between responses of aboveground net primary productivity(ANPP)and belowground net primary productivity(BNPP)to the precipitation amount.ANPP increased markedly with the precipitation following a nonlinear trajectory.By contrast,BNPP showed no significant change.In summary,the productivities of roots in Inner Mongolia typical steppe did not significantly change across the precipitation amount gradient.Due to the different responses of ANPP and BNPP,biomass allocation pattern between above-and belowground parts following the optimal allocation trajectory rather than isometric allocation.