Response Of Ecosystem Structure And Function To Nitrogen Addition In Inner Mongolia Grassland

With the nitrogen fertilization is widely used,global nitrogen(N)enrichment is critical for projection of changes in structure,functioning,and service of terrestrial ecosystems.To assess the effect of nitrogen deposition on the community in Inner Mongolia grassland,we analyzed plant presence-absence and biomass data based on network theory and machine learning algorithm.(1)Based on presence-absence data,the species cooccurrence network is constructed,and it is found that all the species cooccurrence networks have small-world,non-random but not scale-free,and the rate and frequency of nitrogen addition did not affect the three statistical properties.Based on presence-absence data and lift degree,we found that Stipa grandis was a neutral species,Leymus chinensis was a repellent species,and Achnatherum sibiricum was a lift species.Based on aboveground biomass data and Spearman correlation coefficient,and it was found that significant positive correlations were dominant among species.(2)Based on the Apriori algorithm,it was found that there are at most four dominant species combinations,among which Cleistogenes squarrosa,Stipa grandis,Carex korshinskyi,Achnatherum sibiricum are the most dominant combination in natural conditions.The dominant species combination existing under natural conditions remains stable after the experiment,which indicates the dominant species combination is stable.Chi-square test showed that four dominant four-species combinations weakened with increasing nitrogen addition rate,and the combinations were weakest under nitrogen addition rate 50 g N m^-2 yr^-1.(3)The community turnover was measured by species-exchange ratio,and our studies have shown that decreases in species richness following nitrogen addition are associated with decreases in the gain of new species.The community Beta diversity was calculated based on Jaccard distance and Bray-Curtis distance,and it was found that nitrogen addition had a positive effect on Beta diversity.The spatial asynchrony fluctuated around 0.70,and the species asynchrony fluctuated around 0.65,and the nitrogen addition had no significant effect on them.