| Carbon(C),nitrogen(N)and phosphorus(P)are the essential mineral elements for plant growth and development,and the cycling of these elements is the key part of ecosystem functioning.Plant C,N and P stoichiometric characteristics are significantly related to the nutrients cycling of terrestrial ecosystems,and can indicate multiple plant responses to the environment conditions.Human activities have induced significant global changes,including increases in N and P inputs,climate warming and precipitation alteration.These global changes may profoundly affect plant growth across terrestrial ecosystems.Grassland is an important component of terrestrial ecosystems,and plays a key role in sustaining food supply for an increasing human population and maintaining the stability of the Earth's climate.Global climate change and nutrient inputs may significantly alter the relative availability of soil nutrients for plants,and modify plant nutrient acquisition and the relative composition of nutrients in plant tissues,that is,the nutrient stoichiometry.However,the effects of global change factors on plant C,N and P stoichiometric characteristics are still poorly understood in grasslands.Alpine meadows on the Tibetan Plateau and the semi-arid grassland on the Loess Plateau are two important grassland types in China.These ecosystems are very fragile,and sensitive to global change.In addition,they are located in the areas where global climate change will be ones of the most significant.Therefore,to study the effects of nutrient inputs and climate change factors on plant C,N and P stoichiometric characteristics in these two grassland ecosystems,is not only theoretically important,but also provides a scientific basis for the management of grassland ecosystems.We conducted three field experiments,including a long-term N and P addition experiment,and two global change factor manipulation experiments,one in an alpine meadow on the Tibetan Plateau and the other in a semi-arid grassland on the Loess Plateau,to study the effects of the global change factors on plant C,N and P concentrations and stoichiometry.Main results are summarized as follows:(1)The results from long-term N and P additions experiment showed that in general N additions increased plant N concentrations at species,functional group and community levels,but did not significantly affect plant P concentrations at any scale.Given the unchanged plant P concentrations and significant increase of N concentrations in response to N addition,higher plant N:P ratios were observed,suggesting that P limitation intensified under the N additions.Meanwhile,P additions significantly increased plant P concentrations and reduced N:P ratios,but did not significantly affect plant N concentrations,at both functional group and community levels,suggesting that nutrient limitation of plant growth was transferred from P to N.In addition,the scaling of N to P under N additions was similar to the previously reported pattern with a uniform 2/3 slope of the regression between Log N and Log P at both community and functional group levels,which was significantly different from that under P additions with a smaller slope(0.455).Also,graminoids had different responses from forbs.In addition,the responses of N and P stoichiometric characteristic to N and P additions were species-specific.(2)The results from the global change factor experiment in the Tibetan alpine meadow showed that N addition significantly increased leaf N concentrations,C:P and N:P ratios,but reduced leaf P concentrations and C:N ratios;warming reduced leaf P concentrations but increased leaf C:P and N:P ratios,while precipitation alteration did not significantly alter leaf C,N and P stoichiometry characteristics.Moreover,the combination of N addition and warming increased leaf C:P and N:P ratios,while the combination of warming and precipitation alteration affected leaf P concentrations,C:N and C:P ratios.Specifically,under the control condition,leaf P concentrations increased and leaf C:N and C:P ratios reduced with increased precipitation level,and the responses of leaf P concentrations and C:P ratios to precipitation alteration were affected by warming.Furthermore,the precipitation alteration affected the responses of leaf P concentrations,C:N and C:P ratios to warming.The three-factor interaction have no significant effect on leaf C,N and P stoichiometric characteristics.The leaf dry matter content(LDMC)as one of the main plant leaf functional traits significantly affected the leaf C,N and P stoichiometric characteristics and there was an interaction effect of N addition and plant leaf functional traits on leaf P concentrations,leaf C:P ratios and N:P ratios.Additionally,we found that LDMC significantly correlated with leaf nutrient concentrations and C:N:P ratios.These results indicated that the global change factors can significantly alter plant leaf nutrient stoichiometry in the Tibetan alpine meadow,suggesting that plant leaf morphological traits may critically mediate the responses of plant leaf nutrient stoichiometry to global change factors.The results also suggest that the combinations of two global change factors are mostly additive and the magnitude of ecosystem response to global change factors may decline with higher-order interactions.(3)Results from the global change factor experiment in the semi-arid grassland on the Loess Plateau showed that none of N addition,warming and precipitation had significant effect on plant leaf C concentrations;N addition increased leaf N concentrations and N:P ratios but decreased leaf C:N ratios;leaf P concentrations were increased in the warming treatment and reduced in the precipitation reduction treatment,but leaf N:P ratios was increased in the precipitation reduction treatment.The combination of N addition and warming decreased leaf C concentration,C:N and C:P ratios,but increased the leaf N concentrations and N:P ratios,and their effects were synergistic.Combination of N addition and precipitation alteration significantly influenced plant leaf stoichiometry,and the effects were additive,specifically,except the leaf C:P ratios,the responses of leaf N,P concentrations,C:N and C:P ratios to precipitation alteration can be affected by N addition.Warming and precipitation alteration had no effect on plant leaf C,N and P concentrations and their stoichiometry,indicating that the interaction of warming and precipitation alteration was antagonistic.The combination of N addition,warming and precipitation alteration only significantly affected leaf N:P ratios,indicating that the magnitude of leaf C,N and P stoichiometry response to global change drivers declined with higher-order interactions.In addition,the plant growth was limited by P in this semi-arid grassland(N:P=19.12),and the limitation was exacerbated by N addition alone and interactively with other factors.We also found that LDMC not only had direct effects,but also interacted with global change factors to affect leaf C,N and P concentrations and stoichiometry.LDMC significantly and positively correlated with leaf C concentrations,C:N ratios,C:P ratios and N:P ratios,but negatively correlated with leaf N and P concentrations,which was different from the results of the experiment in the alpine meadow.These results indicated that the coupling effects between leaf morphological traits and leaf C,N and P concentrations depend on the ecosystem type,and that leaf morphological traits can influence the response of leaf C,N and P stoichiometry to global changes.In summary,results from this study showed that nutrient(N and P)additions and climate change factors can significantly affect plant C,N and P stoichiometry.However,the response of plant C,N and P stoichiometry to N and P addition,warming and precipitation alteration varied among plant species,ecosystem types.In addition,the plant stoichiometry in the alpine meadow and the semi-arid grassland were insensitive to precipitation alteration and warming,respectively.The plant growth was limited by temperature and nutrient supply in the alpine meadow,but it was limited by precipitation and nutrient supply in the semi-arid grassland.these results indicate that these two grassland types have different mechanisms of their responses to global change factors.Therefore,to further improve our understanding of how grasslands may respond to future global change,we need to pay more attention to long-term ecosystem-scale studies testing multi-factor effects on plant stoichiometry across different regions,which can provide a scientific basis for policy-making to address environment and global change in the future. |
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