The Response Of Primary Production, Species Composition And Community Succession To Water, Nitrogen And Grazing In Typical Steppe, Inner Mongolia

Long-term livestock over-grazing caused nitrogen deficiency in Inner Mongolian steppe, if low-level nitrogen fertilization maybe a useful approach for restoring degraded grasslands, has always been a controversial issue. In arid and semi-arid steppe, water is the primary limiting factor for aboveground net primary production, the effectiveness of nitrogen is dependent on water availability. The altered species composition and losses of plant diversity caused by nitrogen fertilizer addition will adversely affect grassland ecosystems functioning; moreover, water and nitrogen availability would change the competition among species, and then could affect community succession and replacement of dominant species. Global climate change models have predicted that precipitation patterns will change significantly in the future, better understanding the response of primary productivity, species composition and community succession to water and nitrogen availability is essential for evaluating the role of nitrogen fertilization on degraded grassland restoration.Based on the background above, this study included two parts:1. Research the interaction of water and nitrogen on primary productivity, species composition and community succession through comprehensive evaluation of the long-term (2005-2014) experiment data (Chapters 3 and 4). This experiment manipulating precipitation (NP:natural precipitation and SWP:simulated wet year precipitation) and nitrogen (0,25 and 50 kg N/ha) addition was conducted at two sites with different grazing history:a moderately (MG) and a heavily (HG) grazed site in Inner Mongolia. We measured aboveground net primary production (ANPP), water use efficiency (WUE), the inter-annual variability of aboveground biomass of main species, and saved the meteorological data of experiment years and the past 20 years.2. The response of main species to water and nitrogen addition at the different soil depths by using stable isotope 13C and 15N in semi-arid grassland (Chapter 5). This field experiment was carried out in a moderately (MG) and a heavily (HG) grazed site in Inner Mongolia in 2008. The same amounts of water and 15N-labelled nitrogen fertilizer were uniformly supplied at the soil depths of 0,15 and 45 cm. We estimated aboveground biomass, 13C and 15N-abundance, nitrogen productivity of main species and aboveground net primary production, resource use efficiency at community level. The main results were summarized as following:1. The primary limiting factor of aboveground net primary production (ANPP) shifted from water to nitrogen with increasing precipitation. When the crop year water input< 250 mm, the mainly limiting factor is water; when the crop year water input was 250-450 mm, the productivity is limited by water and nitrogen; when the crop year water input> 450mm, the mainly limiting factor is nitrogen.2. The seasonal distribution of precipitation and the amount of single rainfall event have dramatic impacts on community productivity. The larger rainfall event or more winter-precipitation could reduce soil surface water evaporation loss and play an important role on improving ANPP.3. The different response of ANPP to water and nitrogen addition at heavily grazed (HG) site and moderately grazed (MG) site was dependent on the different plant functional group composition. The first five years of the experiment, ANPP of HG site was significantly higher than MG site, and then there was no significantly difference between MG and HG sites in the later five years. This result was mainly due to the increasing of perennial species and decreasing of annual species at HG site.4. A significant species succession process was exhibited at HG site. The stable water and nitrogen input can promote the dominance of perennial grasses, and help species succession. However, under the fluctuation in water availability, the inter-annual variation of the replacement of dominant species was significantly, nitrogen addition further exacerbate the inter-annual variation, would impede species succession.5. The species richness and shannon-wiener index at MG site was significantly higher than that at HG site. Water input increased species richness significantly while no effect on shannon-wiener index; under irrigation, nitrogen supply significantly decreased species richness and shannon-wiener index.6. At both the heavily grazed site (HG) and the moderately grazed site (MG), the aboveground net primary production (ANPP), water-use efficiency (WUE) and 15N-fertilizer use efficiency (15NUE) significantly increased with the increasing depths of irrigation and fertilization at community level. ANPP and WUE were higher at HG than MG site, while 15NUE was significantly lower at HG site.7. Aboveground biomass and 15N-abundance of annual specie Salsola collina was highest and increased with the depths of irrigation and fertilization. Using isotope methods can indirectly distinguish plant root distribution and competition for water and nitrogen.