围栏放牧下土壤-植被碳密度空间分布格局

Spatial distribution patterns of vegetation and soil carbon density in fenced and grazing grassland

对围封13年且放牧的冷季高寒矮嵩草草甸,进行了从围栏入口到内部不同距离植被和土壤碳密度状况的调查。结果表明:1)入口到50m植被现存碳密度平均为1298.0gC·m-2,60～180m有所下降(平均为997.3gC·m-2),200～300m反而升高(平均为1285.5gC·m-2)。当年净初级生产碳密度分布趋势与其相同,0～50m、60～180m和200～300m平均分别为742.5、571.0和745.7gC·m-2。这种分布趋势与放牧过程中绵羊觅食频度和强度有关。一般在中央地带放牧强度大,绵羊觅食时间长,边缘地带受围栏效应或围栏外环境因素影响,放牧强度相对较弱,一定程度上对植被生长发育起到了保护作用,使边缘地带植被碳密度得到提高。2)从围栏入口到草场内部土壤碳密度变化趋势表现复杂,入口到100m增加,100～170m减小,然后略有升高。土壤碳密度最高值出现在95m处(15.42gC·m-2),最低值出现在170m处(14.12gC·m-2)。目前尚不清楚为何出现这种格局,但至少认为,土壤有机质的动态转化过程受多种因素影响,与植被碳密度相比具有一定的迟滞效应。具体如何影响有机质的动态转化及其迟滞效应,有待进一步研究。

Taking a 13-year enclosed and grazing alpine Kobresia humilis meadow as test object,an investigation was conducted on the vegetation and soil carbon density at different distances from the entrance to the inside of the pasture.From the entrance to the inside 50 m,at inside 60-180 m,and at 200-300 m,the carbon density of existing vegetation was averagely 1298.0 g C·m-2,997.3 g C·m-2,and 1285.5 g C·m-2,respectively,and the carbon density of net primary production had the same variation trend,i.e.,averagely 742.5 g C·m-2 at 0-50 m,571.0 g C·m-2 at 60-180 m,and 745.7 g C·m-2 at 200-300 m,which was related to the foraging frequency and grazing intensity of sheep.Generally,in the central zone of the pasture,the grazing pressure was stronger,and the foraging time of sheep was longer.In contrast,owing to the effects of fencing or of the environmental factors outside the fence,the grazing intensity near the fence was relatively weaker,which,to some extent,played a protective role on the vegetation growth in the enclosed pasture,and eventually,led the vegetation carbon density to increase.From the entrance to the inside of the pasture,soil carbon density showed a complex pattern,i.e.,increased from the entrance to the inside 100 m,decreased at 100-170 m,and increased slightly at 170 m,with the maximum value（15.42 g C·m-2） at 95 m and the minimum value（14.12 g C·m-2） at 170 m.It remained unclear why this pattern occurred,but one of the reasons could be that the dynamic transformation process of soil organic matter was affected by many factors,and compared with vegetation carbon density,this process would have a certain hysteresis,which should be further studied.