Dynamics of leaf carbon, nitrogen and phosphorus of two dominant species in a Poyang Lake wetland

Carbon(C), nitrogen (N) and phosphorous (P) stoichiometry are critical indicators of biogeochemical coupling in terrestrial ecosystems. Stoichiometric homoeostasis plays important role in modulating structure, functioning and stability of ecosystems. However, our current understanding of C :N :P stoichiometry is mainly derived from observations across space, and little is known about its dynamics through the time. Besides, stoichiometric researches have been carried out in various terrestrial ecosystems in China, but few data are available for wetland ecosystem. Wetland plants live in more fluctuant environments than terrestrial plants, and responses of wetland ecosystem to climate change are more sensitive than terrestrial ecosystem. The C :N :P stoichiometry for wetland plants may differ from terrestrial plants. Hence, more researches on dynamics of leaf carbon, nitrogen and phosphorus concentrations, as well as their stoichiometry for wetland plants are necessary. Poyang Lake is the largest fresh lake in China, with huge areas of wetland occurred in non-flood periods in a year. In this study, a wetland dominated by plant species of Carex cinerascens and Triarrhena lutarioriparia was selected in the national nature reserve, Nanji wetlands of Poyang Lake. Subsequently, carbon, nitrogen and phosphorus concentrations, as well as aboveground plant biomass were measured every month from February to June in 2011. The objectives of this study were as follows:(1) to clarify dynamics and control factors of the dominant plants' carbon, nitrogen and phosphorus concentrations, as well as their stoichiometric ratios, (2) to discuss the current status of wetland nutrient utilization and nutrient limitation. Results showed that leaf carbon ranged from 365.3 to 386.6 mg/ g for Carex cinerascens, and 352.6 to 393.2 mg/ g for Triarrhena lutarioriparia, respectively. Leaf nitrogen differed from 6.96 to 17.59 mg/ g for Carex cinerascens, and 5.50 to 20.68 mg/ g for Triarrhena lutarioriparia, respectively. Besides, the ranges of leaf phosphorus were 0.65 to 2.14 mg/ g and 0.57 to 2.25 mg/ g for the two species, respectively. The arithmetic means were (375.5±17.4) mg/ g and (371.7±12.5) mg/ g for carbon, (11.35±1.40) mg/ g and (11.54±0.84) mg/ g for nitrogen, (1.56±0.69) mg/ g and (1.55±0.68) mg/ g for phosphorus, respectively. The arithmetic means of C :N, C :P and N :P ratios were 44.00, 1068.46 and 21.30 for Carex cinerascens and 47.90, 1059.92 and 21.19 for Triarrhena lutarioriparia. There was no significant difference of carbon, nitrogen and phosphorus concentrations, as well as stoichiometric ratios between the two species (p > 0.05). Moreover, both leaf nitrogen and phosphorous showed clear dynamic patterns. Leaf nitrogen and phosphorus concentrations peaked at the early stage of growth, and then decreased substantially during the fast growth period, but increased a little by the end of the growing season. Temperature and aboveground biomass were the two main factors controlling the dynamic patterns of nitrogen, phosphorus concentrations and their stoichiometric ratios. C :N and C :P ratios were positively correlated with aboveground biomass, which suggested nutrient use efficiency of nitrogen and phosphorus were greatly affected by the growth rate. Additionally, according to nitrogen and phosphorus concentrations both in soil and the two dominant plants' leaves, as well as the stoichiometric ratios of nitrogen to phosphorus, the plants in the wetland were more limited by nitrogen, rather than by phosphorous.