Effects Of Long-term Nitrogen Deposition On Plant Community Structure And Aboveground Productivity In Typical Steppe Of Inner Mongolia

Grassland ecosystem is one of the main types of terrestrial ecosystem,which can maintain biodiversity and provide productivity.Nitrogen is an important limiting factor of grassland ecosystem.In the past decades,the global nitrogen input rate has been increasing year by year,which can directly or indirectly change the plant community structure and ecosystem function of grassland ecosystem.Many studies have reported that increased nitrogen input can accelerate vegetation growth and improve productivity,but inhibit plant diversity,which is an important factor leading to the loss of global biodiversity.But few of the current studies have been carried out on degraded grasslands.Grasslands in Inner Mongolia have experienced long-term human activities,and most grassland have been degraded to varying degrees,with reduced vegetation productivity and diversity.It is not clear whether the long-term increase of nitrogen input will have similar consequences on degraded grasslands.In order to answer this question,we conducted a 10-year control experiment on nitrogen(N)addition in a degraded Inner Mongolia typical steppe,and investigated the effects of plant community structure and above-ground productivity.The results are as follows:(1)Nitrogen supplementation increased the aboveground biomass of perennial grasses to a certain extent,but had no significant effect on the aboveground biomass of perennial grasses and annual and biennial herbs in the degraded grassland of Inner Mongolia.(2)In the community structure,perennial grasses had absolute dominance under different years and nitrogen addition conditions.Nitrogen addition increased aboveground biomass by increasing the abundance of Leymus chinensis,and changed the community structure by changing the dominance degree of species in the community.(3)For degraded grassland,the Shannon-Wiener index,Simpson index and Pielou evenness index of the community were not significantly decreased with the increase of nitrogen input.The species richness of the community was basically stable,and the community structure tended to develop to single dominant species with nitrogen addition(4)The effects of nitrogen addition on plant community diversity,productivity and community composition were different.With the increase of nitrogen input time,only the proportion of perennial grasses gradually decreased,which indicated that,in 10 years’ time scale,the increase of nitrogen input mainly affected perennial grasses(mainly Leymus chinensis)which had more advantages in resource utilization,Other species or functional groups are at a competitive disadvantage.These results suggested that nitrogen supplementation increased the competitive ability of Leymus chinensis in the community,but reduced the dominance of other species,thus changing the community structure and making the community tend to develop towards a single dominant species.In addition,since species richness in degraded grassland ecosystems is already very low,nitrogen addition will not always reduce species richness,but will affect the distribution of individual plants.