Nitrogen, phosphorus and potassium nutritional status of semiarid steppe grassland in Inner Mongolia
In grazed semiarid steppe ecosystems, much attention has been paid to aspects of growth limitation by water. So far, potential limitation of primary production by plant nutrients was rarely considered. This knowledge is essential for identification of sustainable land-use practices in these large and important ecosystems on the background of over-exploitation and climate change. In the present study plant nutrient concentrations and ratios were investigated with factorial additions of water and N fertilizer at two sites with contrasting soil nutrient availability. Combined analysis of nutrient concentrations, contents, biomass production, and plant N:P ratios consistently confirmed primary growth limitation by water and a strong N limitation when sufficient amounts of water were supplied. P limitation only occurred at the site with low P availability when in addition to the natural supply, water and N fertilizer were given. According to reported thresholds of N:K and K:P ratios, K was not limiting in any plot. The observed nutritional patterns in the plant community were related to the dynamics of species composition and their specific nutrient status. Stipa grandis had the highest N:P ratio whereas Artemisia frigida showed lowest N:P. These nutrient characteristics were related to growth strategies of dominant species. Accordingly, the relative biomass contribution of S. grandis and A. frigida strongly affected the nutrient status of the plant community. Plant N:P ratios indicate the relative limitation by N or P in the semiarid grasslands under sufficient water supply, but other methods of nutritional diagnosis should be used when plant N:P ratios remain below critical values.
KeywordsStoichiometry Nutrient concentration Element ratio Co-limitation Water supplementation N fertilizer supplementation Rangeland management
We would like to thank Dr. Holger Brueck for providing valuable comments on an earlier draft of this manuscript, and Inner Mongolia Grassland Ecosystem Research Station of Botany Institute, Chinese Academy of Science for providing working facilities and meteorological data. We are very thankful to two anonymous reviewers who provided very helpful comments to this manuscript, and we also thank German Academic Exchange Service (DAAD) for providing a doctoral fellowship to XYG. This study was supported by the National Nature Science Foundation of China (NSFC 41071207 / 30821003) and the Deutsche Forschungsgemeinschaft (DFG, DI 546/3-2 and TA 215/3-3).
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