Summary

Dept. of Plant and Environmental Sciences

White clover (Trifolium repens L.) is a nitrogen (N) rich legume in which a large part of the N derives from the atmosphere through the symbiosis with Rhizobium bacteria. White clover is, therefore, an essential production factor in low-input agroecosystems around the world and is commonly used in leys and pastures in temperate areas. A major challenge when using white clover in northern climate is to keep N on the field over the winter and secure the transfer of N to the subsequent growing season. This is important in order to minimise the risk of loosing a production resource much needed and the risk of environmental detriment. This task might be difficult in areas of the world with a long winter period between cropping seasons.

In the present work, the use of white clover in a northern climate has been seen in both production- and environmental perspectives. The major goals were to determine 1) at what point in time leaves, stolons and roots emerge and turn over; 2) how harvesting affects both plant organ longevity and N dynamics; 3) to what degree these plant organs lose N during the winter and 4) the fate of lost N.

To accomplish this, growth, death and N dynamics of leaves, stolons and roots of harvested and unharvested white clover plants were followed in both a pot and a combined plot and root window experiments. Recovery in seepage water of compounds lost from the aboveground material was evaluated in a four-year field study.

This work showed that the leaves were the most dynamic part of the white clover plant, with an average lifespan of 65 days. About 60% of the leaves produced during the growing season senesced, died and had disappeared by late autumn. Of the remaining 40%, approximately 70-80% died away during the winter and only traces of debris could be found in spring.
Stolons were long lived, on the average 411 days, and stolon sections close to the parent plant seemed to be functional over the entire two-year observation period. Sections towards the terminal bud, however, overwintered poorly and had a much shorter lifespan. The white clover roots had a mean lifespan of 290 days and seemed to be conserved during the winter. The roots were surprisingly persistent as compared with observations made in milder climates and the data therefore suggest that root turnover was strongly affected by temperature. Root studies have often been hampered by methodological difficulties and root viability status is seldom quantified. However in this work, a new method was developed based on a combination of viability staining with 2, 3, 5 – triphenyltetrazolium chloride and use of the image analysis-programme “WinRHIZO“ to distinguish between metabolically active and inactive parts of plant roots. Altogether, the data showed that the amount of dying and dead white clover tissue was considerable and contributed with more N to the soil–plant system than the amount which could be harvested from the field.

Removal of leaves during the growing season caused an internal re-distribution of N in the white clover plant. New leaf growth was prioritised at the expense of stolon and root development. This rendered the plants more susceptible to larger relative biomass N losses during the subsequent winter while the relative N uptake rate in the following spring was almost unaffected. In absolute figures, fluctuation in biomass N (the gains and losses) during the year was largest for unharvested white clover plants, and growth in early summer depended heavily on the plant size in spring.

A high portion of white clover N residing in the leaves in late autumn was lost during the winter. Nitrogen stored in stolons and roots was much better conserved and on the average for all experiments, 12% of stolon N and 15% of root N were lost in the cold season. Winter losses varied considerably from year to year, dependent on the prevailing climatic conditions. On the average for all experiments, 35% of the N in aboveground tissue was lost during winter and might be at risk of further transport out of the soil–plant system by gaseous emission as ammonia or nitrous oxides, by surface run-off or by leaching through soil into the soil drainage system. The results showed that approximately one-third of the winter losses appeared in a distinct pulse of N in melt water in spring that could be prone to leaching or surface run-off on frozen soil, and further suggested that gaseous N losses were moderate during the winter The experiments gave some indications that N that was unrecovered could have been immobilised by a cold-adapted microbial community which was utilising white clover carbon and actually contributing to system sustainability. Similar pathways for N could very well be similarly important for other plant species, which stand green when the frost and winter comes in temperate areas of the world.

The knowledge obtained through the detailed observation of white clover organs in this work can, hopefully, contribute to an environmentally sound use of white clover in future agriculture.
Sturite, I. 2005. Dynamics of nitrogen in white clover plants in production- and environmental perspectives. (Nitrogendynamikk i kvitkløverplanter sett i produksjons- og miljøperspektiv) Norwegian University of Life Sciences. Doctor Scientiarum Theses 2005: 30. ISBN 82-575-0681-8 ISSN 0802-3220


Oppdatert: 30.10.05
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