Effects Of Different Mowing Intensity On Belowground Net Primary Production And Root Dynamics In The Grassland Of Farming-Pastoral Zone In Northern China

Grassland is one of the important components of the global terrestrial ecosystem,covering nearly 42%of the total land area.Youyu grassland is located in the northwest of Shanxi Province in China and is the core area of the farming-pastoral zone.Due to the increasing population in recent years,the farmers have been mowing the grassland frequently in pursuit of higher economic benefits,resulting in serious grassland degradation.As an important part of plants,root system is an important organ for plants to absorb water and nutrients,and its growth dynamics is extremely important to evaluate the function of grassland ecosystem.In order to find the suitable mowing intensity for grassland restoration and sustainable use,it is important to investigate the effects of different mowing intensity on belowground net primary production（BNPP）and root dynamics in the northern farming-pastoral zone.However,the research on the effects of mowing on plant root system from the whole plant perspective is not comprehensive enough,especially the study on the effects of different mowing intensity on plant root dynamics remains particularly unclear.This study was conducted in Youyu natural grassland with a completely randomized block design,and we set four different mowing intensity:no mowing（M0）,light mowing（10 cm stubble,M1）,moderate mowing（5 cm stubble,M2）and heavy mowing（2 cm stubble,M3）,with four replications for each treatment.BNPP was determined using the Ingrowth cores method,and root dynamics were monitored in situ using Improved root windows method.Based on data from three years of field experiments,the following main findings were obtained:1.Regardless of the mowing intensity,the dominant species in all plots was still Leymus secalinus after three years of mowing.In particular,light mowing increased the relative biomass of Leymus secalinus by 16.9%in the grassland community,indicating that light mowing could not only ensure the relative biomass of the grass,but also maintain the sustainable utilization of grassland resources.2.Values of BNPP obtained by the Ingrowth cores method varied between101.7 and 521.7 g m^-2,BNPP was 521.7 g m^-2 in 2018,was significantly higher than2017 and 2019 by 63%and 416%;the root production measured by the Improved root windows method varied between 121.6 and 148.5 mm cm^-2.The root production in2018 was 148.5 mm cm^-2,which was significantly higher than that in 2019 by 22.1%.In addition,different mowing intensity had a significant effect on root production only in 2017 and 2019,with light mowing having the highest root production with values of 447.3 g m^-2 and 125.5 mm cm^-2,respectively.This is because light mowing promotes plant compensatory growth.The response of f _BNPP to mowing was similar to that of BNPP,which was strongly affected by interannual precipitation variation,and its numerical variation ranged from 28%to 66%,which was significantly higher in2018 than in 2019 by 132%.3.Root dynamics showed an obvious seasonal changes,but different mowing intensity had little effect on root seasonal dynamics.Specifically,both root production and root standing crop reached their peaks in August because summer is the peak of the growing season,when hydrothermal conditions are favorable,and plants need as many roots as possible to absorb water and nutrients.Root mortality peaked in September,as annual species began to die at that time.4.Different mowing intensity had a significant effect on root turnover only in the drier year 2019,which showed that root turnover first increased and then decreased with the increase of mowing intensity.Our results also found that light mowing can ensure the absorption of water and nutrients to cope with drought stress through strategies of rapidly growing new roots,dying old roots,and increasing root turnover.In conclusion,light mowing can maintain the competitive advantage and relative biomass of dominant species,increase the proportion of photosynthetic products transported to the root system,and indirectly improve root turnover by increasing root production.Therefore,light mowing is the optimal mowing intensity.The results of this study can provide data support for how the grassland belowground roots in the farming-pastoral zone in northern China respond to mowing intensity,and provide theoretical support for the rational use of grassland resources in the production practice of animal husbandry.