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Content archived on 2024-04-16

Internal cycling of nitrogen in deciduous and evergreen forest trees


The project has 2 objectives. Firstly, to study the processes of internal nitrogen cycling in forest trees under controlled conditions and secondly, to study internal nitrogen cycling in field grown trees.

4 experiments will grow seedling trees of contrasting species in sand culture, under laboratory conditions. Contrasting treatments of nitrogen and water will be applied in each experiment and a detailed study made of the processes of internal nitrogen cycling. A number of field sites will be chosen and the seasonal pattern of nitrogen uptake and partitioning in relation to nitrogen and water supply quantified in contrasting tree species.

The project will establish the principles of internal nitrogen cycling by testing 2 basic hypotheses: that within a species the internal cycling of nitrogen to support spring growth is independent of the current availability of nitrogen from the soil and that withdrawal of nitrogen from senescing leaves depends upon the interaction between water and nitrogen supply with the leaf demography typical of each species. By identifying the quantitative and qualitative importance of these processes in young trees, it will be possible to study them in field grown trees. The coupling of laboratory and field studies will then allow the effect of nitrogen and water supply on the processes of internal nitrogen cycling in forest trees to be determined.

The project will use 2 approaches to achieve these aims. First, laboratory based sand culture experiments will use techniques that have already been developed to quantify internal cycling using isotopically labelled nitrogen (nitrogen-15). Trees will be supplied with either a generous or poor supply of nitrogen and with contrasting water supplies and their growth monitored by destructive harvesting at frequent intervals. The recovery of nitrogen-15 in leaves, woody tissues and roots will be used to determine seasonal patterns of nitrogen uptake. In this way the contribution of nitrogen remobilized from storage tissues to the seasonal growth will be quantified. The contribution of nitrogen withdrawn from senescing leaves to augmenting overwinter stores of nitrogen will also be quantified in relation to the influence of nitrogen and water supply on leaf demography and nitrogen use efficiency. These experiments on young trees, grown under controlled conditions, will identify the processes important in the inte nal cycling of nitrogen in trees.

The second approach used by the project will be to establish a series of field sites to study the effect of contrasting supplies of nitrogen and water on the processes of internal nitrogen cycling identified by the laboratory based experiments. These field studies will measure tree canopy demography, nitrogen content and leaf gas exchange characteristics. In addition, nitrogen-15 will be applied to trees before the onset of spring growth and the subsequent recovery of nitrogen-15 in leaves used to determine the pattern of nitrogen uptake by the roots to support spring leaf growth. Separate field studies will determine the patterns of nitrogen storage over winter by harvesting branches of different age classes during autumn, winter and spring, and analysis done to identify the major over winter storage, compartments and compounds in different climate types of Europe.

The data from the field studies will be used to validate the results from the sand culture experiments. The project will, therefore, identify the varying nitrogen and water supplies on these processes in the field.


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