| Grazing is the main way of grassland management and utilization,and rotational grazing is one of the most important techniques of modern grassland management.Biological soil crusts(BSCs)are important component of typical grasslands.Biological soil crusts are complex community of organisms comprise of cyanobacteria,lichens,mosses,and other microorganisms that live within or immediately on the top of the soil surface as a thin horizontal layer.BSCs play an important role in the integrity of biological soil crusts in the grassland ecosystems.In this study,we investigated the effects of grazing on vegetation,BSCs and physicochemical properties of underneath soil of a rotationally grazed grassland by Tan Sheep in Loess Plateau.The main results are as follows:1.Effects of grazing intensity on distribution of plant and biological soil crustThe coverage of lichen was ranged from 27.51%-40.74% and 37.82%-42.19% in 2015 and 2016 respectively.In both years,the lichen coverage was significantly higher than vegetation,moss,bare land and litter(P<0.05).The coverage of BSCs was significantly higher than the vegetation(P<0.05).In 2015,the coverage of moss crust was 14.64% in 5.33 sheep/ha which was significantly lower than the other stocking rates.In 2016,the coverage of moss was decreased with increasing stocking rate;16.11% at 8.67 sheep/ha treatment was significantly lower than that in 0,2.67 and 5.33 sheep/ha(P<0.05).The vegetation compensation growth after grazing in the 2.67 sheep/ha stocking rate treatment resulted vegetation coverage of 33.14% and 32.81% in 2015 and 2016 respectively which were significantly higher than that at 0 and 8.67 sheep/ha(P<0.05)treatments.The vegetation coverage at 5.33 sheep/ha was similar to 2.67 sheep/ha stocking rate treatment.The vegetation coverage between 0 and 5.33 sheep/ha was not significantly different(P>0.05),but significantly higher than of 8.67 sheep/ha(P<0.05).2.Effects of grazing intensity on chlorophyll content of biological soil crustThe chlorophyll content of the biological crusts varied with different grazing intensitiesand the chlorophyll content of different types of crust were different.The chlorophyll content of lichen crust decreased in 0-5.33 sheep/ha stocking rates then increased at 8.67 sheep/ha.The chlorophyll content of lichen crust was 7.72 mg/g at the 0 sheep/ha treatment,which was significantly higher than that of 2.67,5.33 and 8.67 sheep/ha(P<0.05).The chlorophyll content of lichen crust was 5.24 mg/g at 2.67 sheep/ha,which was significantly higher than that of 5.33 sheep/ha,but significantly lower than that of 8.67 sheep/ha(P<0.05).The variation of chlorophyll content in the moss was opposite to lichen crust,with the increase of the stocking rate,the chlorophyll was increased first then decreased.The order of chlorophyll content in the moss in different stocking rates was;5.33> 8.67> 2.67> 0,and the difference of stocking rates were significant(P<0.05).3.Effects of grazing intensity on soil moisture content underneath the BSCsIn 2015 and 2016 the order of soil moisture underneath the BSCs was moss > lichen > bare land;,indicating that moss was conducive to drought in the Loess Plateau area to protect water,to prevent water and soil flow.Soil moisture content in different stocking rates increased up to 2.67 sheep/ha and then decreased up to 8.67 sheep/ha indicating moderate grazing was beneficial to maintain the soil water avilability.4.Effects of grazing intensity on soil physical and chemical properties underneath BSCsMild(moderate)grazing was beneficial to increase the storage of soil carbon while heavy grazing would reduce the soil organic carbon in grassland.The moss crust was beneficial to improve the soil organic carbon content.;The soil total nitrogen content,NO3--N and NH4+-N content were all decreased with the increase of stocking rate.Under the same stocking rate,the contents of total nitrogen,NO3--N and NH4+-N followed the same trend;: moss crust> lichen crust> bare land.Under different stocking rate,the soil availablephosphorus contents were in the order of moss> lichen> bare land.,T he soil available phosphorus content in the lower layer of each crust type decreased with the increasing stocking rate.The results showed that at optimum grazing intensity,could promote the growth and development of BSCs,and improve the soil physical and chemical properties of underneath of it. |
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