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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]

Abstract: The acidicole behavior of many species is given by their inability to acquire enough P in alkaline soil, but reasons for acidicole (syn. calcifuge) behavior of Trifolium arvense have never been studied experimentally. We asked how emergence of seedlings, the survival of plants, the growth of aboveground organs, the nodulation and the growth of roots of this species is affected by different N, P and K supply in alkaline soil. In the years 2010 and 2011, we performed a pot experiment (ten N, P and K fertilizer treatments) with seeding of T. arvense into alkaline soil. The acidicole behavior of T. arvense proved to be connected with the inability of seedlings to acquire enough P during their emergence. In all treatments, T. arvense was not able to flower in the seeding year and in the second year it flowered only in P treatments. This indicates the biennial character of the species in the case of late seeding and points to a strong P limitation of flowering and seeds production in plants grown on alkaline soil. Without P addition, T. arvense was not able to develop beyond the seedling stage. Nodulation was positively affected by P application and negatively by N application. Although T. arvense is a typical species for P poor soils, it does not suffer from P toxicity under high P supply. We concluded that the acidicole behavior of T. arvense is based on its P limitation when growing on alkaline soil. [Copyright &y& Elsevier]