Effects Of Sheep Grazing On Soil Seed Bank And Seedling Recruitment On The Typical Steppe Of Eastern Gansu

The rangeland is an important part of terrestrial ecosystem,overgrazing is one of the most important factors leading to grassland degradation.The density and species richness of soil seed bank and seedlings play a key role in the self-repair of degraded grassland.In this experiment,the typical grassland on the Loess Plateau was taken as the object to study the aboveground vegetation,soil physical and chemical properties,soil seed bank,seedling regeneration and their interrelations under different grazing gradients.The main results are as follows:1.The coverage and biomass of aboveground vegetation decreased with the increase of grazing intensity,while no significant difference was observed among treatments.Among them,the coverage and biomass of vegetation on enclosed plot are the highest,85.6%and 120.1g/m~2 respectively,while those on heavily grazing plot are the lowest,78.4%and 94.7g/m~2 respectively.The density,height,coverage and important values of aboveground vegetation varied with grazing intensity.Soil bulk density and soil water content showed a decreasing trend with the increase of grazing intensity.Total nitrogen,total phosphorus,total potassium,organic carbon,available phosphorus,ammonium nitrogen and nitrate nitrogen varied with grazing intensity.2.The total seed yield of aboveground vegetation firstly increased and then decreased with the increase of grazing intensity.The total seed yield of vegetation in the moderate grazing plot is significantly higher than that in the heavily grazing plot.The total seed yield of vegetation in the moderate grazing plot was the highest,reaching 4,582 seeds/m~2,while that in heavily grazing plot was the lowest,reaching3,258 seeds/m~2.The seed yield of three dominant plants,Artemisia capillaris,Stipa bungeana and Lespedeza bicolor,firstly increased and then decreased with the increase of grazing intensity,and the seed yield of three plants in the moderate grazing plot was the highest.3.The density of soil seed bank first increased and then decreased with the increase of grazing intensity.The density of soil seed bank in moderate grazing plot was the highest,1416.67±174.01 seeds/m~2;The density of soil seed bank in light grazing land is the lowest,1150.00±132.30 seeds/m~2.With the increase of soil depth,the density and richness of soil seed bank decreased significantly.4.Grazing intensity affects the number of seedlings to different extent depending on the counting season.The total number of seedlings peaks in moderate grazing plot(714/m~2),while decreased to 460/m~2 in the enclosed plot.The total number of seedlings ranked as June(1285/m~2)>July(496/m~2)>August(379/m~2)>September(143/m~2)>October(107/m~2).5.Generally,the similarity between seedlings and aboveground vegetation>the similarity between soil seed bank and seedlings>the similarity between soil seed bank and aboveground vegetation.The similarity between seedlings and aboveground vegetation increased first and then decreased with the increase of grazing intensity.The similarity between vegetation and seedlings in the moderate grazing plot is 2.89.However,the similarity between soil seed bank and seedlings and between soil seed bank and aboveground vegetation increases first and then decreased with the increase of grazing intensity except for enclosed plot.