Effects of daylength and soil compaction on white clover growth and N2-fixation

In order to detect the efficiency of the nitrogen (N-2) fixation in clover-grass leys in northern climate conditions, we studied how soil compaction affects growth and N-2-fixation of white clover (Trifolium repens L.) under contrasting growth conditions. A pot experiment was carried out under controlled climatic conditions in the phytotron at Holt (Tromso) Sandy soil was compacted to two levels, 60% and 85% of the standard degree of compactness (SDC). Four seedlings of white clover plants or timothy (Phleum pratense L.) were carefully planted in each pot. Timothy was used as reference plant. The plants were placed at 15 degrees C for twelve weeks and subjected to 18 or 24 h daylight. The N-15 isotope dilution method was used to assess N-2-fixation. Results suggest that 24 h daylight increased white clover biomass production as compared to 18 h daylight and favoured leaf and stolon production significantly more at 85% of SDC than at 60% of SDC. However, for white clover plants grown at 18 h day-length, higher compactness reduced the root development. On average, white clover derived 44-58% of its total N from N-2-fixation grown at 60% of SDC and 46-47% at 85% of SDC, regardless of light conditions. The N-2-fixation was somewhat higher at 24 h day length only under the low soil compaction level.