Article,

The effects of elevated CO2 on the C:N and C:P mass ratios of plant tissues

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Plant and Soil, 224 (1): 1--14 (2000)
DOI: 10.1023/a:1004790612630

Abstract

The influence of elevated CO2 concentration (CO2) during plant growth on the carbon:nutrient ratios of tissues depends in part on the time and space scales considered. Most evidence relates to individual plants examined over weeks to just a few years. The C:N ratio of live tissues is found to increase, decrease or remain the same under elevated CO2. On average it increases by about 15\% under a doubled CO2. A testable hypothesis is proposed to explain why it increases in some situations and decreases in others. It includes the notion that only in the intermediate range of N-availability will C:N of live tissues increase under elevated CO2. Five hypotheses to explain the mechanism of such increase in C:N are discussed; none of these options explains all the published results. Where elevated CO2 did increase the C:N of green leaves, that response was not necessarily expressed as a higher C:N of senesced leaves. An hypothesis is explored to explain the observed range in the degree of propogation of a CO2 effect on live tissues through to the litter derived from them. Data on C:P ratios under elevated CO2 are sparse and also variable. They do not yet suggest a generalising-hypothesis of responses. Although, unlike for C:N, there is no theoretical expectation that C:P of plants would increase under elevated CO2, the average trend in the data is of such an increase. The processes determining the C:P response to elevated CO2 seem to be largely independent of those for C:N. Research to advance the topic should be structured to examine the components of the hypotheses to explain effects on C:N. This involves experiments in which plants are grown over the full range of N and of P availability from extreme limitation to beyond saturation. Measurements need to: distinguish structural from non-structural dry matter; organic from inorganic forms of the nutrient in the tissues; involve all parts of the plant to evaluate nutrient and C allocation changes with treatments; determine resorption factors during tissue senescence; and be made with cognisance of the temporal and spatial aspects of the phenomena involved.

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