Study On The Effect Of Grazing And Enclosure On Soil Health In Rough Fescue Grassland

Grassland ecosystem is an important component of terrestrial ecosystem.Due to their widely distribution and great carbon sequestration potential,grassland plays an important role in promoting local economic development as well as in regulating global climate change.Soil health is the foundation of grassland ecosystem health,it is also one of the most critical factor that affecting the stability of ecosystem functions.However,due to the diversity of soil health assessment indicators and the synergy and trade-off among different indicators,until now,no systematic soil health assessment system were formed for grassland ecosystem.This study was based on a large-scale cattle grazing experiment that was established in 1949 at a Rough fescue grassland in southwest Alberta,Canada.This study including six treatments:CK(no grazing,0 Animal unit month hm-2:AUM hm-2),Light grazing(LG,1.2 AUM hm-2),Heavy grazing(HG,2.4 AUM hm-2),Very Heavy grazing(VHG,4.8 AUM hm-2),grazing exclusion after 50 years H grazing,grazing exclusion after 50 years VH grazing.We systematically analyzed the effects of grassland utilization and landscape position on totally 27 soil physical,chemical,biological and community indicators.A comprehensive soil health assessment was made according to the Cornell Soil Health Assessment Manual.The key findings are summarized as following:1.Soil clay and silt contents were lower in VHG than LG;total porosity,air capacity,and saturated water content followed the same pattern.However,the opposite was true for sand content,penetration resistance,and bulk density,with differences between the two grazing treatments greater at the top than the bottom slope position.Grazing changed the shape of soil moisture retention curve in both slope positions with the curve steeper in LG than HG and VHG.The S-index and saturated hydraulic conductivity were reduced by grazing but their responses to slope position and its interaction with grazing intensity were not significant.2.VHG significantly increased soil pH,with greater increase in top position.Grazing increase the content of available phosphorus(AP)only in the bottom position while no difference were found among three grazing treatments in top position.Soil organic carbon(SOC),total nitrogen(TN)and NH4+-N were higher in LG than VHG only in bottom position,but there was no difference among three grazing treatments in top position and all of them were lower than CK.SOC and TN were same between May and September,but NH4+-N was higher in May than in September.3.In bottom position,HG and VHG had lower active carbon(AC),protein and water extractable nitrogen(WEN)than LG,three grazing treatments were all lower than CK in top position.VH increased soil microbial respiration carbon(MRC)only in top position,and no difference were found among three grazing treatments in bottom position.Similar to NH4+-N,AC,WEOC and WEN were higher in May than in September.4.Relative to corresponding grazing area,the exclosures has lower soil surface and subsurface hardness and bulk density but higher total porosity,clay and silt contents.However,no significant difference were found between inside and outside of the exclosures as respect to most soil hydraulic properties.The reason may attributed to the widespread of invasion species.Soil biology properties was also improved by grazing remove.Specially,SOC,TN,AC(only in HVG exclosures),PMN,WEOC and WEN(only in HG exclosures)were higher in exclosures than the corresponding grazing area.At the same time,our results show that slope position also play an important role during soil recovery.5.Grazing decreased litter and total biomass.At the same time,it induced decrease of native dominant species but increase the cover of invasive specie especially in bottom position.Relative to grazing area,the exclosure increased the aboveground net primary productivity and litter biomass as well as total biomass and reached a stable level 5 and 10 years after grazing ceased for HG and VHG enclosures,respectively.Community species composition also changed along with time.The relative cover of invasive species were increase along with time with inside higher than outside of the enclosures.However,the relative cover of native dominant species only increased slightly.Species richness and Shannon-Weiner diversity also decreased with time in both grazing and exclosure area,although the fluctuate was different among three positions.6.Soil health index under different treatments were evaluated,with the rank of CK>HG exclosures>VHG exclosures>LG>HG>VHG.For different slope position,the results show that bottom position has higher soil health index than top position in LG and HG exclosures;whereas,middle position was higher than top and bottom positions in VHG;only slight difference were found among three positions for HG and VHG exclosures.