Responses Of Plant Community Structure And Function To Warming And Nitrogen Addition In A Desert Steppe Of Inner Mongolia

Human activities have resulted in increasing of global temperature and atmospheric nitrogen deposition, which is a hot topic in the field of global change. These changes would have profound impact on biodiversity and plant community structure, and subsequently will influence on ecosystem function and feedback to global change. Desert steppe is an important part of Inner Mongolia grasslands and plays an important effect in biodiversity conservation and maintaining of the grassland animal husbandry production. However, understanding and cognition of community composition and functional services under global change are still scarce.This research focused on desert steppe with dominant species of Stipa breviflora. The random split plot experiment (2×2factors) was conducted on natural grassland to mimic the climate warming and enrichment of nitrogen deposition using the infrared heaters and nitrogen fertilizer addition. The effects of controlled warming and nitrogen addition on plant community structure, plant phenology, seed rain and nitrogen resorption efficiency in desert steppe were measured, in order to test the response mechanism of desert steppe ecosystem to global climate change. The key conclusions are summarized as follows:Warming significantly increased daily mean soil temperatures at soil depths of Ocm,10cm,20cm and30cm by1.34℃,0.87℃,0.87℃and0.76℃, respectively (P<0.01). The interaction of warming and nitrogen addition with the same warming effect was increased by1.22℃,0.80℃,0.78℃and0.61℃, respectively (P<0.01). With the increase of soil depth, the variation in the scope of the soil temperature decreased and warming effect was higher in autumn and winter than in spring and summer.Warming had no effect on community species richness, Shannon-Wiener index, Margalef richness index and Pielou evenness index (P>0.05). Nitrogen addition significantly decreased species richness,Shannon-Wiener index and Margalef richness index (P<0.05). The interactive effect either warming x year or nitrogen x year was observed (P<0.05).Either warming or nitrogen addition influenced total community coverage (P<0.05). However, differential response trends were found among different functional groups. Warming decreased the proportion of perennial grasses (Stipa breviflora and Cleistogenes songorica), but the nitrogen addition increased the proportion of perennial grasses. Both warming and nitrogen addition decreased the proportion of semi-shrubs (Kochia prostrata). In addition, warming had a negative effect on the coverage of perennial forbs, whereas nitrogen addition had a positive effect on the coverage of perennial forbs.Warming significantly advanced the budding, flowering and fruiting time by3.10day,2.28day and1.88day per year, respectively, which were mainly due to warming significantly advanced the budding, flowering and fruiting time of Stipa breiflora, Allium tenuissimum and Kochia prostrata. No significant difference was detected in nitrogen addition, with advancing the budding, flowering and fruiting time by0.27day,0.27day and0.19day per year, respectively, which were mainly due to nitrogen addition no significantly advanced the budding, flowering and fruiting time of any species. In the whole plant community, warming advanced both flowering and fruiting times, advanced flowering time is greater than fruiting time results in delayed reproductive duration with0.84day per year (P<0.05). However nitrogen addition advanced0.25day per year of reproductive duration. Each species showed a specie-specific effect among growing seasons. Nitrogen addition or its interaction with warming did not affect plant phenology.Warming was not significantly influenced by both above-ground and below-ground net primary productivity (P>0.05). At species level, warming significantly decreased Convolvulus ammannii above-ground net primary productivity (P<0.05). No effects of nitrogen addition on each species above-ground net primary productivity (P>0.05). At functional group level, neither warming nor nitrogen addition affected the net primary productivity of ecological functional group. But warming reduced biomass of C3plant (P<0.05), no effects was observed on C4plant (P>0.05). Nitrogen addition improved the belowground net primary productivity of plant community associated with0-10cm soil depth. The net primary productivity had a significantly difference among growing seasons (P<0.05).The annual average of seed rain density is1231.5±144.7seeds per square meter in desert steppe among5growing seasons from2006to2010. At plant community level, neither warming nor nitrogen addition had significantly influenced the seed rain density (P>0.05). At species level, warming significantly enhanced seed rain density of Kochia prostrata. All of treatments have marginally interannual variation (P<0.05).No main and interactive effects of warming and nitrogen were detected on Shannon-Wiener index, Margalef richness index and Pielou evenness index (P>0.05). We could not detect main or interactive effects on the Sorensen and Jaccard similarity between aboveground vegetation and seed rain, either (P>0.05). Significantly annual variations were observed at the diversity and similarity index (P<0.01). Warming significantly increased nitrogen concentration in green and senescent plants by5.5%and11.3%, respectively, and decreased nitrogen resorption efficiency by7.0%(P<0.05). Nitrogen addition significantly increased nitrogen contents in green plants by5.2%and decreased nitrogen resorption efficiency by2.9%(P<0.05). There was significant interaction between warming and nitrogen addition to increase nitrogen concentrations of green and senescent plants and to reduce nitrogen resorption efficiency (P<0.05). The nitrogen concentration and nitrogen resorption efficiency of the five species showed higher species-specific responses in different treatments. The simple linear correlation showed that except for Convolvulus ammannii and Kochia prostrate, Stipa breviflora and Cleistogenes songorica have significant negative relationships between green or senescent plants nitrogen concentration and nitrogen resorption efficiency, but the significant negative correlation was just found between senescent plants nitrogen concentration and nitrogen resorption efficiency in Artemisia frigida.