Stepwise incorporation of white clover (Trifolium repens L.)as fertiliser increases nitrogen fixation and improves nitrogen retention when intercropped with leek (Allium porrum L.)

Background and aims We investigated whether the incorporation of intercropped white clover as a split-dose fertiliser improves N retention of the plant-soil system in leek production. Methods White clover and leek were grown in pots where clover was stepwise incorporated into soil as fertiliser. Half of the clover was incorporated before leek transplantation (LT), the other half was incorporated 26 (S1) or 41 (S2) days after LT, compared with full incorporation of all clover (F) before LT. 15N injection and leaching studies were performed to investigate interspecific competition and the effects of precipitation on soil N availability. Results Stepwise incorporations increased or had no effect on leek growth, despite competition for N between white clover and leek. Compared to F, leek grown with stepwise incorporations had similar and lower N accumulation before the second incorporation, but N was higher at the end of the experiment. N2 fixation by white clover increased the total N input into soil and matched N availability with leek N demand and growth rate. Loss of inorganic N by leaching was lowest in S2 and highest in F. Conclusions Stepwise incorporations of white clover improved N retention in the plant-soil system and could be used to increase leek yield