Effect Of Different Grazing Intensity On Belowground Net Primary Production And Root Dynamics In Grassland Of North Agro-pastoral Ecotone Abstract

Grassland ecosystems,as one of the most important vegetation types in terrestrial ecosystems,account for nearly 30%of the global land area and nearly 42%of China's land area.Youyu grassland is located in the northwestern part of China,it is a typical north agro-pastoral ecotone.Grassland degradation in the region has become more and more serious due to the increase of grazing intensity in recent years.Therefore,exploring the optimal grazing intensity could help the sustainable use and development of grassland resources and also ensure the economic benefits of herders.So far,previous studies mainly focused on the effect of grazing on grassland vegetation,biomass and soil physical and chemical properties.This evaluation system is one-sided as it ignored the importance of the root.The dynamics of root is the important factors that regulating ecosystem carbon balance and nutrient cycling.However,there were few studies on the impact of grazing intensity on BNPP,fBNPP and root dynamics from the whole plant perspective.In this study,we carried out a field experiment for three successive years in the grassland of the north agro-pastoral ecotone abstract.The field experiment was conducted with a completely random block design including four different grazing intensities:No grazing?G0?,light grazing?G1?,moderate grazing?G2?,and heavy grazing?G3?,and each grazing intensity had four replicates.In this research,the ingrowth core method was used to determine BNPP and we used root windows to measure the root dynamics.The objective of our research was to investigate the response of BNPP and root dynamics to different grazing intensities.The results would enrich the data of plant belowground under different grazing intensities in grassland of north agro-pastoral ecotone abstract.Based on the three-year field experiment,the main results were reached:1.Results showed that the dominant species was the Leymus secalinus?Georgi?Tzvel.in Youyu grassland.However,the relative biomass of Leymus secalinus?Georgi?Tzvel.was different with the increase of grazing intensity.Generally,there was a trend of no grazing?moderate grazing>light grazing>heavy grazing.And with the increase of grazing years,the relative biomass of Leymus secalinus?Georgi?Tzvel.was decreased.Heavy grazing had the highest species diversity,while moderate grazing could not only ensure a certain species diversity,but also maintained the relative biomass of the Leymus secalinus?Georgi?Tzvel.Considering the sustainable use of grassland and the yield of high-quality pasture,moderate grazing intensity should be given great consideration in grassland management.2.The response of NPP and BNPP to different grazing intensity would vary with rainfall.the NPP ranged from 137.7?888.6 g m^-2and the BNPP ranged from 5.1?352.5 g m^-2in the three years.In 2018,the effects of different grazing intensity on NPP and BNPP were not significant with sufficient precipitation.However,NPP showed the shape of“?”with the increase of grazing intensity,and moderate grazing was the highest in 2017 and 2019.Compared to light and heavy grazing,moderate grazing increased BNPP by 49%and 104%in2017 of the 0-10 cm soil layer.Moderate grazing increased BNPP with the increase of soil depth,which suggesting plants would choose to transfer more photosynthesis to the belowground in dry years.These results was also consistent with f_BNPP,showing f_BNPPNPP was increased first and then decreased with the increase of grazing intensity,and moderate grazing was also the highest f_BNPP.3.Root productivity,mortality and standing crop had a clear seasonal dynamic,and the patterns were mainly affected by precipitation and temperature,but not related to the grazing intensities.Root productivity and standing crop in the 0-10 cm and 10-20 cm soil layers showed a peak on August.This was mainly due to plants produced more roots in order to better absorb soil water and nutrients in the growing season.After the growth season,a large proportion of root began to die,resulting in a peak of mortality in September.In addition,the plant would increase root standing crop to ensure the growth at the beginning of the growing season,and root standing crop was gradually decreased at the end of growing season.4.In 2018,the effects of different grazing intensity on root productivity and mortality were not significant with sufficient rainfall,while the standing crop was decreased first and then increased with the increase of the grazing intensity.In 2017 and 2019,root productivity and standing crop were the highest under light grazing treatment with less rainfall.In 0-10 cm soil layer,root mortality was decreased with the increase of grazing intensity,while in 10-20cm soil layer,it did not change significantly.This was mainly attributed to the comprehensive effect of grazing on plant species and soil physical and chemical properties.5.The root turnover was 0.7 yr^-11 in 2019,which was significantly higher than 0.52 yr^-11 in2018.Compared with other grazing intensities,moderate grazing had the highest root turnover rate.Grazing intensity had no significant effect on the root survival rate in the depth of 0-10 cm soil layer,while it had a significant effect in the 10-20 cm.About 60%of root died within 15 days of birth,and the mortality gradually decreased with the growth of plant.In both soil layers,root longevity was longest under no grazing,and moderate grazing had the shortest root longevity.Moderate grazing minimized input ratio and maximized efficiency of water and nutrient uptake by rapidly propagating new roots,discarding old roots and shortening the roots survival time.In conclusion,moderate grazing could improve the species diversity,but also maintain the relative biomass of dominant species of Leymus secalinus?Georgi?Tzvel.More importantly,moderate grazing improved BNPP and the biomass allocation to the belowground,and increased root turnover rate by reducing root lifespan.This study would provide some data for predicting root production and root dynamics with different grazing intensity,and also provide strong scientific theoretical support for herders'rational use of grassland resources in production practice.