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Grass-legume mixtures are suitable for crop rotations in organic farming. However, seasonal development of below-ground organs of Trifolium pratense in mixtures and on different soils was neglected. We asked (1) how the diameter of the root neck, the maximum order of branching as well as (2) the nodule traits are affected by locality and time, and (3) how above-ground plant traits of red clover vary in space and time. Red clover was investigated in grass-legume mixtures in the first year of vegetation. Five sites in S Germany were sampled at the day of cut at the end of May, the beginning of July, and at the end of August, respectively. Under similar climatic conditions root traits (diameter of the root neck, order of root branching, size of nodules, and proportion of senescent nodules) differed with soil conditions and time within the season. Root diameter increased during the season. Higher sand content fostered root branching and branched roots developed more nodules. Thinner roots had more active nodules (pink). Root diameter and non-active senescent (green) or moribund (brown) nodules increased at the end of season. Nodule activity differed more according to season than to soil conditions. The number of nodules per plant (12.5-19.5) decreased from May to August. Cylindrical nodules were found on 85-100% of the plants and branched nodules only on 0-25%. The height of plants was lowest in May and increased in July. The mean number of stems per plant (3.3-6.3) was highest in August. Understanding red clover traits and N2 fixation is interesting scientifically as well as agronomical. Organic and conventional farmers can both benefit of our findings. [ABSTRACT FROM AUTHOR]