Contribution from stolons and roots to estimates of the total amount of N₂ fixed by white clover (Trifolium repens L.)

Growth and N-accumulation rates in leaves, stolons and roots of individual white clover plants were studied in three experiments using two methods. In a growth chamber experiment, the relative differences between tissues were found to be almost constant for a wide range of clover plant sizes. The stolon dry matter (DM) production was 56% and the root DM production 40% of the DM production in leaves.

The N yield of stolons was 30% while N yield in roots was 34% of N yield in leaves. The effect of N application on these relations was investigated in a glasshouse experiment. Application of N reduced the root:shoot N ratio from 0.50 to 0.28, whereas the stolon+root:leaf N ratio (i.e. for abovevs.below cutting-height tissues) was only reduced from 0.97 to 0.80.

In a field trial with two contrasting N regimes, growth and N accumulation were measured on individual clover plants. Dinitrogen fixation was estimated by15N isotope dilution based on analysis of leaves-only or by including stolons. Using leaves-only did not affect the calculation of percentage of clover N derived from N2fixation (% Ndfa) since the15N enrichment was found to be uniform in all parts of the clover.

A correction factor of 1.7 to account for N in below cutting-height tissue is suggested when N2fixation in white clover is estimated by harvesting the leaves only.