Effects Of Nitrogen Deposition And Litter Input On Plant And Soil Characteristics In Songnen Meadow Steppe

Atmospheric deposition of reactive nitrogen, originating mainly from fossil-fuel burning, land use and artificial fertilizer application, has increased rapidly in recent decades. Atmospheric deposition of nitrogen can dramatically increase plant productivity, and thus enhance the accumulation of aboveground plant litter in grassland ecosystems. The changes in nitrogen inputs, via fertilization or N deposition, can also affect the quality of plant litter. The amount of nutrients in plant tissues can determine the decomposition rate of plant litter, and thus indirectly affect nutrient cycling in grassland ecosystems. Examining the combined effects of nitrogen deposition and aboveground plant litter input on plant community productivity, species diversity, as well as plant and soil nutrient status, will help us to better understand how grassland ecosystems response to global changes. More importantly, these studies could provide important implications for grassland managements and ecosystem conservations.Our study was conducted in a temperate grassland in the eastern region of the Eurasian Steppe Zone. We tested the combined effects of N addition and plant litter manipulation on plant productivity, plant diversity, as well as plant and nutrient status. During the 3-year(2010 to 2012) experimental period, we obtained some important results and conclusion as follows:N fertilization significantly decreased the species richness, Shannon-Weiner index, Simpson’s index and Pielou index, whereas N fertilization significantly increased total plant biomass, total plant cover and mean plant height. N addition also significantly enhanced plant biomass, plant cover, plant density and plant height of the dominant species(Leymus chinensis), while it has limited effects on other grasses and forbs. In 2012, Shannon-Weiner index decreased with increasing plant cover of L. chinensis, which indicated that the increases in L. chinensis abundance is the main reason of plant diversity decline. The accumulation of aboveground litter significantly decreased total plant density, and there was no general trend for the impacts of plant litter on total plant biomass and the biomass of each plant functional group. Interestingly, within each nitrogen treatment, we found that Shannon-Weiner index, Simpson’s index and Pielou index generally increased with plant litter addition in 2011 and 2012. Therefore, despite the negative effects of nitrogen on species diversity have been widely confirmed, our study revealed that the accumulation of aboveground plant litter would potentially alleviate the negative influences of nitrogen on grassland plant biodiversity, and facilitate the maintaining of grassland ecosystem functions and services.Total carbon(C) concentration in the leaves of L. chinensis was not affected by N application. However, N addition significantly increased total N concentration, C:P and N:P in leaves and culms, while decreased total phosphorus(P) content and C:N in leaves and culms of L. chinensis. Aboveground plant litter has no effect on the total C content in the organs of L. chinensis, but it significantly increased total N and total P concentration in both leaves and culms. In the plots with N addition, plant litter inputs decreased the N:P in green leaves of L. chinensis. The C:N and C:P in tissues determined the allocation patterns of above- and belowground biomass in plants, as well as the quantity and quality of plant litter, and ultimately affected the energy flows and nutrient cycling in grassland ecosystems. As the most important nature fertilizer in terrestrial ecosystems, plant litter could potential influence the soil nutrient status. Thus, understanding the importances of aboveground plant litter in regulating plant nutrient status is essential for grassland management and conservation.To our knowledge, our study is the first to simultaneously examine the effects of N deposition and litter treatments on plant nutrient resorption. Our results showed that although N fetilization had no general effects on N and P resorption efficiency in plant organs, N addition siginificantly decreased leaf N resorption proficiency by 54%, culm N resorption efficiency by 65% for dominant species in Songnen meadow steppe. Moreover, N addition increased leaf P resorption proficiency by 13%, culm P resorption profiency by 20%. Under both ambient and enriched N conditions, litter addition reduced N and P resorption proficiencies in plant organs. Moreover, the response of P resorption proficiency to litter treatments was more sensitive than N resorption proficiency: except for leaf N resorption proficiency under both ambient N conditions, N resorption proficiency was not significantly affected by litter treatments. In the contrary, P resorption proficiency in leaves and culms decreased significantly with increasing litter accumulation under both ambient and enriched N conditions. Our study confirmed the different response between N resorption and P resorption in the plant organs under elevated N condition. Moreover, our study also suggested that the accumulation of above-ground plant litter could potentially alleviate the Plimitation situation under elevated N condition, which may potentially facilitate the growth of plants.Soil total C content was not affected by N fertilization. In contrast, soil total N content, C:P, N:P were increased and soil P was significantly decreased in the plots with N addition. Soil total C, total N and total P were not significantly affected by aboveground plant litter, which may due to the short period of the experimental treatments. With the rapid industrialization proceeding in China, it was predicted that atmospheric deposition of reactive nitrogen in Songnen meadow steppe would be dramatically increasing over the next decade. Our results suggested that grassland managers should apply more P fertilizer into the grasslands to maintain a stable N:P in the soils, and thus alleviated the negative impacts of N deposition on the grasslands. Moreover, retaining a certain amount of plant litter in grasslands is nesseccesry, as it will help to alleviate the P limitation situation under N deposition.Based on the above experimental results, we obtained the further insight on the importance of N deposition and aboveground plant litter in regulating plant community characteristics, as well as plant and soil nutrient status in grassland ecosystem. N deposition caused the loss of plant diversity, while the aboveground plant litter inputs could alleviate the negative impacts of N deposition on species diversity. N fertilization significantly increased the total N content and potentially decreased the total P content in the soil, which would further resulted in P-limitation in this grassland. As the most important natural fertilizer in terrestrial ecosystems, plant litter could release substantial amounts of nutrient that were locked in plant tissues via litter decomposition, which would modify the nutrient content as well as nutrient ratios in the soil. In summary, under elevated atmospheric nitrogen deposition in China, retaining a certain amount of litter was of great significance. The aboveground plant litter inputs could restrain the growth of the dominant species in Songnen meadow steppe, and alleviate the negative impacts of N deposition on species diversity. Plant litter could release substantial amounts of nutrient into the soil, which would potentially relieve the nutrient imbalance status in the soil casued by N deposition. To our knowledge, our study was the first to simultaneously examine the effects of N deposiiton and litter treatments on plant nutrient resorption, and which was of great significance for improving the theory on plant nutrient resorption in Songnen meadow steppe. Based on the insights of plant community diversity, as well as the plants and soil nutrient status in grassland, our study further confirmed the important role of above-ground litter on grassland ecosystem function and service. This study would further help us to better understand how grassland ecosystems response to global changes. More importantly, our experimental results could provide important implications for grassland managements and ecosystem conservations: the healthy grassland requires a certain amount of above-ground litter, while long-term overgrazing and mowing would result in the lack of nutrients in soil, and be not good for the grassland productivity.