| Soil seed bank is one of the material foundations of sustainable grazing management in grassland.Grazing is one of the important factors restricting grassland soil seed bank on a global scale.However,the influence of grazing intensity and grazing season on grassland soil seed bank,and its responses to climate change and atmospheric nitrogen deposition is unclear.Therefore,we study the influence mechanism and mechanism of factors such as grazing management,rainfall and nitrogen deposition on the soil seed bank in long-term Tan-sheep rotation grazing system in Huanxian Grassland Agricultural Trial Station of Lanzhou University.The main findings are as follows:1.Characteristics of soil seed bank density and richness in grazing areas.In dry and normal rainfall years,the soil seed bank density of warm season is 1.7-2.6 times and 1.5-1.6 times that of cold season,respectively.There is no significant difference between the two seasons in wet year.In normal rainfall year,the grazing season has no significant effect on soil seed bank richness.The soil seed bank richness of warm season is 1.3-2.3 times that of cold seasons in dry year.In wet year,cold season is 1.5-2.3 times that of warm seasons.In warm season,grazing intensity reduces the variation rate of soil seed bank density to rainfall,but increases the variation rate of soil seed bank richness to rainfall;in cold season with 5.33 sheep/ha,the variation of soil seed bank density and richness to rainfall is relatively small.Therefore,in warm and cold seasons,5.33 sheep/ha can increase the density and richness of the soil seed bank in response to rainfall and maintain it at a higher level.2.Effects of grazing on the relationship between grassland vegetation and soil seed bank.The soil seed bank density of Artemisia capillaris Thunb.,one of the dominant species,contributes the most to the total seed bank density.The vegetation characteristics of A.capillaris significantly affected the density and richness of the soil seed bank.In dry year,warm-season grazing increased the soil seed bank density by increasing the plant height of A.capillaris;grazing intensity increased the density of soil seed banks by increasing the total number of reproductive branches of vegetation.In normal rainfall year,warm-season grazing reduced the soil seed bank density by increasing the plant density;grazing intensity reduced the soil seed bank density by reducing the total number of reproductive branches of vegetation.In wet year,warmseason grazing increased the total density of soil seed banks by increasing total number of reproductive branches of vegetation;grazing intensity reduced the soil seed bank density by reducing the A.capillaris biomass.In dry year,warm-season grazing increased the soil seed bank richness by increasing the height of A.capillaris.In normal rainfall year,warm-season grazing reduced the soil seed bank richness by increasing the A.capillaris biomass;grazing intensity reduced the soil seed bank richness by reducing the coverage of A.capillaris.In wet year,warm-season grazing increased the soil seed bank richness by increasing the number of above-ground reproductive branches;grazing intensity reduced the soil seed bank richness by reducing the total aboveground biomass.3.Effects of grazing on the relationship between the grassland soil physical and chemical properties and soil seed bank.In dry year,the soil moisture in warm and cold seasons decreased with the increased stocking rate,and both had a significant impact on soil seed bank density,the standardized coefficient ?(the effect size)was-0.76 and-0.87,respectively;in normal rainfall year,the fractal dimension of soil particle size in warm and cold seasons increased with the increased stocking rate,and both have a significant impact on the soil seed bank density,with ? being-0.71 and-0.98,respectively;The bank density was not significantly affected by the soil physical and chemical properties.The soil p H of cold-season grazing land increased with the increased stocking rate,which has a significant impact on the soil seed bank density(? = 0.52).In dry year,soil clay volume fraction and soil moisture showed a "U"-shaped and linear downward trend with increasing stocking rate in warm-season grazing land,and had a significant impact on seed bank richness,with ? being-0.54 and-0.51,respectively,while there is no significant relationship between the seed bank richness and soil physical and chemical properties in cold season;in normal rainfall year,the seed bank richness has no significant relationship with soil physical and chemical properties in warm-season grazing land.Soil ammonium nitrogen and available phosphorus showed a linear decline and a "U"-shaped change trend with increased stocking rate in cold-season grazing land,and both had significant effects on the soil seed bank richness with ? being-0.55 and 0.61,respectively.In wet year,the soil temperature increased linearly with the increased stocking rate in warm-season grazing land,and it was significantly related to the soil seed bank richness(? =-0.82),while in cold-season grazing land,the soil seed bank richness had no significant relationship with the soil physical and chemical properties.The physical and chemical properties of soil can be used as an important index to predict the density and richness of soil seed bank,by it was dependent on the rainfall.4.Effects of grazing and rainfall on vegetation seed production and seedlingsThe seed production and number of seedlings of the three dominant species in the study area are affected by rainfall and grazing management.Increased rainfall promotes seed production;the seed yield of A.capillaris and Stipa bungeana Trin.both showed a downward trend,while the seed yield of Lespedeza bicolor Turcz.showed a downward and upward trend with the increased stocking rate in warm-season and coldseason grazing land,respectively;grazing in warm season increased the seed yield of A.capillaris and S.bungeana,and the influence of the grazing season decreases with the increase of stocking rate,at 8.67 sheep/ha,there is no difference between the two seasons.Grazing in the warm season reduced the seed yield of L.bicolor,and the impact of the grazing season increased with the increased stocking rate.The number of A.capillaris,L.bicolor and the total seedlings increased with the increased rainfall(P <0.05).Drought significantly reduced the number of S.bungeana seedlings(P < 0.05);the number of A.capillaris and the total seedlings significantly increased with increased stocking rate(P < 0.05).Compared with enclosure,grazing significantly increased the number of seedlings of S.bungeana(P < 0.05);the grazing season had no significant effect on the number of A.capillaris,S.bungeana and the total seedlings.The number of L.bicolor seedlings in warm-season grazing land was significantly lower than that of cold-season grazing land by 30.6%.Although grazing reduced the yield of seeds,it can increase the number of grassland seedlings,which is conducive to grassland regeneration,and rainfall plays a role in promoting this process.5.Effects of grazing and nitrogen deposition on the density and richness of soil seed banks.Nitrogen deposition increased the seed bank total density(18.2%)by increasing the density of seed bank of Gramineae and Compositae,and reduced the soil seed bank total richness(12.4%)by reducing the richness of the legume;while grazing reduced the total soil seed bank density(38.5%)by reducing the density of the Asteraceae and other families,and increased the total soil seed bank richness(84.7%)by increasing the soil seed bank richness of Gramineae,Leguminosae and other families.Grazing weakened the impact of atmospheric nitrogen deposition on the total density and richness of soil seed banks.Therefore,the impact of atmospheric nitrogen deposition on the soil seed bank of grazing grassland can be ignored.Grazing management responds to the effects of rainfall and atmospheric nitrogen deposition on grassland soil seed banks by affecting above-ground vegetation and soil physical and chemical properties.The soil seed bank density and richness have higher stability at 5.33 sheep/ha in both warm-season and cold-season grazing land,it can cope with changes in rainfall.The stocking rate of 8.67 sheep/ha can eliminate the influence of atmospheric nitrogen deposition on the density and richness of the soil seed bank.This study attempts to reveal the effect of grazing on grassland soil seed bank and its response to climate change and nitrogen deposition,which will help us improve grazing methods,promote grassland regeneration. |
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