Contribution from Stolons and Roots to Estimates of the Total Amount of N2Fixed by White Clover (Trifolium repensL.)

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 above vs. 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 by 15 N 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 N 2 fixation (% Ndfa) since the 15 N 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 N 2 fixation in white clover is estimated by harvesting the leaves only.