Soil Seed Bank Germination Response To Nitrogen And Water Supply In Natural Grassland At Loess Hilly-Gully Region

In order to reveal the dynamic potential and vegetation construction in the semi-arid regions of Loess Plateau, the response of soil seed germination to different water and nitrogen supply conditions have been clarified. Using the natural grassland soil of loess hilly-gully region as the research object, effects of water and nitrogen supply conditions on the number of seed germination, species, dynamic condition and species distribution have been investigated using two nitrogen fertilizers (nitrate nitrogen-NO3--N and ammonium nitrogen-NH4+-N) under six nitrogen levels (0,0.05,0.1,0.15,0.2,0.3 g N Kg-1 dry soil) and two water treatments (sufficient water supply-water every day to control soil moisture content at 80% FC; drought stress treatment-water every two days to control soil moisture content at 80% FC). The experiments were carried out utilizing sand culture from April to June and July to September in 2014. The main results are summarized as follows:1. Applying nitrogen fertilizer could promote the germination of seed bank and the main species under sufficient water supply condition. During April to June,0.1 g/kg nitrate addition is beneficial for seed germination, while the number of seed germination decreased with excess nitrogen, and 0.3 g/kg ammonium nitrogen is beneficial for seed germination. During July to September, the promoting effect on seed germination is enhanced with the increase of ammonium nitrogen application rate, and the promoting effect at 0.15 g/kg ammonium nitrogen is the best, while 0.3 g/kg nitrate nitrogen could promot the germination of seed bank significantly. When sufficient water is supplied,0.15 g/kg nitrate nitrogen is advantageous to the germination of bluegrass, while the number of seed germination decreased with excess nitrogen.0.2-0.3 g/kg nitrate nitrogen is advantageous to sweet wormwood germination. Nitrate nitrogen inhibits green bristlegrass herb germination; During April to June,0.15 g/kg ammonium nitrogen is advantageous to green bristlegrass herb germination, while excess nitrogen inhibit it.0.15-0.3 g/kg ammonium nitrogen is advantageous to bluegrass germination.0.2-0.3 g/kg ammonium nitrogen is advantageous to sweet wormwood germination during April to June, while 0.05-0.1 g/kg ammonium nitrogen is more advantageous during July to September.2. During April to June, within the scope of 0.05 to 0.2 g/kg, the promoting effect of nitrate nitrogen on seed germination is better than that of ammonium nitrogen under well water supply condition. Under drought stress, the promoting effect of ammonium nitrogen is better. During July to September, the promoting effect of ammonium nitrogen under different water treatment is better.3. Under drought stress,0.2-0.3 g/kg ammonium nitrogen is advantageous to the germination of seed bank and bluegrass during July to September. In addition, nitrogen fertilizer has no effect on seed germination under drought stress.0.15-0.3 g/kg nitrogen could delay the initial seed germination time under drought stress.In summary, drought stress inhibits the germination of soil seed bank, and there is no effect with nitrogen fertilizer treatments, while sufficient water supply is beneficial for seed germination, and nitrogen application significantly affect seed germination. Therefore, nitrogen deposition will not affect the seed germination at dry season, but have obvious influence on the germination at rainy season. All these suggest moderate amount of nitrogen deposition is advantaged to increase the number of seed germination and species in natural grassland of Loess Plateau, community richness and stability will be also improved. However, with the increase of nitrogen deposition, the number of germination and germinated species will be decreased, and community richness would decrease while the risk of grassland degeneration would increase.