Study On Ecological Stoichiometry Of C, N And P At Temporal Scales In Lucerne Grasslands On The Rainfed Loess Plateau

Ecological stoichiometry is an excellent measure to study multiple elements at a time within the plant, showing the responses of a plant to diverse changes and the adaption to different environments. Thus it is of significant importance to elucidate how a plant adapts to the adverse environments through further investigation on the stoichiometry of key elements and how to regulate it. Previous researches indicated that plant ecological stoichiometry was generally influenced by plant phenotype,habitat feature, and human disturbance etc., but whether it varied temporally with plant growth is still far from known. Hence in this study, the ecological stoichiometry of C, N and P were studied at different temporal scales, including growth stages, cuts,ages, and cultivation time, in lucerne(Medicago sativa L.) grasslands. And the correlations of stoichiometry between lucerne and soil were analyzed. The key results were shown below:1) Different characteristics at eco-physiological and stoichiometric levels were found with growth stages within each cut. There were higher net photosynthetic rate and sucrose concentration at the early growth stage. With lucerne growth, lucerne biomass, organic C, C:N, and C:P increased, while total N and total P tended to decrease, leading to inconsistent change in N:P. According to the threshold of lucerne N:P, lucerne growth might be restricted by N or P in the field, while there was mainly N-limited in the greenhouse. In addition, the concentrations and stoichiometric ratios of lucerne C, N, P would be affected by soil N, P concentrations and N:P.2) There was an obvious cut effect on lucerne C, N and P stoichiometry. With the increase in cuts, organic C concentration increased in the root, while it hardly changed in the stem and leaf. Lucerne total N concentrations decreased firstly and then increased, while total P concentrations rarely changed. Lucerne C:N decreased, but no significant differences were observed if over 4 times of cutting would be performed in the same year. Lucerne C:P hardly changed with cuts within the same year in the field,while it decreased in the greenhouse. Lucerne N:P firstly kept stable and then increased in the field, while in the greenhouse, there were no significant differences in N:P in the root and leaf, and in the stem, N:P increased firstly and then decreased. The N-limited growth of lucerne existed in all cuts of 2012 and 2013 in the field and all cuts in the greenhouse, while there was P-limitation on lucerne growth in 2014 in the field. In this study, lucerne C, N, P stoichiometric characteristics were affected significantly by soil C, N, P stoichiometric characteristics.3) Lucerne C, N and P stoichiometry were age-specific. As the stand aged, there were no significant changes in organic C and total N concentrations of the stem and leaf, but total P concentration decreased. There was no significant change in leaf C:N,while in the stem, the highest C:N was shown in 5 year stand of lucerne. Stem and leaf C:P increased, and N:P at age 6 was higher than those at age 4 or 5. According to the threshold of N:P, the N-limitation appeared in the 4 and 5 year stands, while the P-limitation was in the 6 year stand. There were some significant correlations between lucerne and soil stoichiometric characteristics of C, N and P, but inconsistent changes were observed in the stem and leaf.4) The concentrations and ratios of lucerne N and P were significantly affected by the cultivation time. In this study, the concentrations of organic C, total N and total P were generally higher in the younger lucerne stands(4 or 5 year) than the older ones(8 or 11 year), while the opposite trends were found for the ratios. The increase in soil total P concentration led to the increase in the concentrations of total N and total P in the root and stem, and to the decline in C:N, C:P and N:P. There would be positive correlations between lucerne and soil C:N, C:P, N:P if there were significant effects except that there were significantly negative correlations between leaf C:N and soil C:N or C:P.Conclusively, the stoichiometric characteristics of C, N and P in lucerne were very different among growth stages, cuts, ages and cultivated times, showing obvious temporal variation. This might be one of the most important mechanisms how lucerne adapts to the environment in the rainfed loess plateau of China. In addition, there were some close links of lucerne stoichiometry with the soil.