Graduate Thesis Or Dissertation

 

Nutrient uptake by conifer seedlings as influenced by root CEC and competing species Public Deposited

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  • The objectives of this study were to establish a suitable method for the measurement of the root cation-exchange capacity of conifer seedlings, and to examine possible influences of this root characteristic on nutrient absorption from a Bellpine soil under greenhouse conditions. The exchange sites of fresh, excised roots were saturated with H⁺ by means of ten subsequent 15-second rinses in 0.1 N HC1. Each acid rinse was followed by a water rinse of equal length. This technique, together with the use of large volumes of acid and "lowsalt" roots, permitted complete saturation of the root exchange sites by preventing readsorption of replaced and leaked cations. The acid which was subsequently released into a neutral 0.1 N KC1 solution by these roots was titrated. Neutral pH was maintained by addition of standard KOH for up to two hours. The titration value did not represent an accurate measurement of the root CEC, since short titration periods prevented complete neutralization of the adsorbed acidity. Extended titration times led to the inclusion of large amounts of H⁺, which are derived from various other sources within the root. This latter fraction of H⁺, termed non-exchangeable, was quantitatively determined by titration of the acidity, which was liberated from Ca-saturated roots. The root CEC was measured as the difference between total and non-exchangeable acidity. The Bellpine soil was shown to considerably reduce the root cation-exchange capacity of nursery-grown conifer seedlings. Most likely, changes in the fungus-root association, variations in the N-level, or high levels of Mn are the causes of the observed CEC decreases. In a greenhouse experiment, incense cedar, noble fir, and two Douglas fir varieties were grown singly, in association with each other, and together with bentgrass and tall fescue. Manganese concentrations in the seedlings reached toxicity levels in many instances and induced a Fe-deficiency. The tolerance of the conifer species to high levels of Mn was correlated to their root CEC. Species with a relatively high root CEC took up more Ca; whereas, in most cases, seedlings with relatively low root CEC accumulated more K. Increases in P uptake were paralleled by rising root CEC. These correlations, however, do not necessarily imply a causal relationship between root CEC and nutrient absorption. Competing tree and grass species did not, in all cases,affect the nutrient uptake by the host species according to expectations from their root CEC. Other factors overshadowed possible influences of root CEC in competitive nutrient uptake.
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