Response Of Plant Diversity And Productivity To Grazing Exclusion And Grassland Management On The Semiarid Loess Plateau

Grassland,one of the largest terrestrial ecosystems in the world,is important for the protection of biodiversity,the conservation of soil and water,carbon sequestration and livestock production.As climate change and human activities,grassland suffers great degradation,especially on the semiarid Loess Plateau.Grazing exclusion is an effective management for restoring semiarid degraded grassland,which could increase plant productivity and diversity.However,long-term fencing decreases plant productivity and diversity,which limits the sustainable development of grassland.Therefore,we examined the effects of grazing exclusion chronosequence(10,27 and 35,and grazing grassland as control)and slope aspect(north and south slope)on plant diversity,aboveground biomass and soil water and nitrogen(N)dynamics in a grazed grassland on the Loess Plateau,and tried to reveal the underlying mechanism for grassland degradation resulted from long-tern fencing.Meanwhile,focused on the grassland degradation(as characterized by the decrease of plant productivity and diversity)in long-term fencing,we examined the effects of grassland managements(clipping,fire and additions of water,N and phosphorus(P))on plant diversity,aboveground biomass and soil water and N dynamics,which could help to the sustainable management of the long-term grazing excluded grassland.Main results are as follows:(1)Plant aboveground biomass firstly increased with the increasing of grazing exclusion chronosequence,and stabilized during the late stage of fencing(27 a-35 a).Plant diversity was not affected by grazing exclusion.Soil moisture and mineral N content increased with the increasing of fenced time,while soil net N mineralization rate decreased with that.The effects of grazing exclusion on soil net N mineralization and mineral N were larger at 0-10 cm than those at 10-20 cm,with more decrease of soil net N mineralization rate and more increase of mineral N content.Litter accumulation was important for restoring fenced grassland,which promoted the restoration during early and mid-stage of fencing but hindered the restoration during late stage.Therefore,grazing exclusion is not effective in long-term run.The control of litter accumulation is important for the management of grasslands that suffered degradation in long-term grazing exclusion.(2)Slope aspect significantly affected the vegetation and the dynamics of soil water and nitrogen,and regulated the effects of grazing exclusion.North slope had the higher plant canopy cover,grass biomass,soil moisture,mineral N contents but the lower forb biomass and soil net N mineralization rates than south slope.Slope aspect regulated the effects of grazing exclusion through its controlling on soil moisture and nutrients availability,with more increase of plant canopy cover,aboveground biomass,soil moisture and mineral N contents but more decrease of net N mineralization rates on the north slope than those on the south slope.Species richness was increased after 10 years of fencing and decreased after 27 years of fencing on the north slope,but was not affected by fencing on the south slope.Therefore,the management of water and nutrients is important for long-term fenced grassland.(3)N addition enhanced plant aboveground biomass,soil moisture,mineral N and nitrate contents,but did not change plant diversity and soil net N mineralization rates.P addition increased plant aboveground biomass but decreased plant diversity.Water addition amplified the effects of P addition on plant diversity and aboveground biomass.Additions of water,N and P exerted limited effects on plant aboveground biomass,and decreased plant diversity,which was not a appreciate management for long-term fenced grassland.(4)Clipping significantly increased species richness and Shannon-Wiener index,decreased plant aboveground biomass,soil moisture and mineral N contents,but did not change soil net N mineralization rates.Additions of water,N and P could facilitate the compensatory growth of grass and the increase of mineral N after clipping,but did not alter the effects of clipping on plant diversity and aboveground biomass of forb.Therefore,clipping+ additions of water,N and P could be helpful to restore the grassland of the late stage of exclusion.(5)Fire did not affect plant diversity,but decreased plant aboveground biomass,soil moisture and mineral N contents.Additions of water and/or N could facilitate the compensatory growth of aboveground plant after fire,but did not change the effects of fire on plant diversity,soil moisture,mineral N and net N mineralization.Therefore,fire is not a appreciate management for long-term fenced grassland.Through examining the effects of grazing exclusion and slope aspect on grassland,we found litter accumulation was the key factor during the restoration of fenced grassland;slope aspect could regulate the restoration of excluded grassland through its controlling on soil moisture and nutrients.Thus,litter accumulation,water and fertilization were important for the restoration of grasslands in long-term fencing.Through analyzing the effects of grassland managements on grassland,our study found clipping+water+fertilization could increase plant diversity while least decreased plant productivity.Therefore,clipping+water+fertilization is an important tool for the management of long-term fenced grasslands.