Summary
The efficient use of phosphorus (P) in agriculture is essential caused by the limited P resources. Therefore, the present thesis investigated the role of weeds for an efficient use of the soil P pool in maize (Zea mays L.) and potato (Solanum tuberosum L.).

The effects of the weed species Chenopodium album, Tripleurospermum perforatum, Polygonum convolvulus, Echinochloa crus-galli, Solanum nigrum and Viola arvensis separately and the site-specific mixture of weeds on P mobilization and P uptake of both field crops were tested in three pot experiments and two field experiments.

The weed species Chenopodium album, Echinochloa crus-galli and Tripleurospermum perforatum revealed the potential to improve the P supply to maize, but not to potato from the soil P pool. This effect was limited to specific weather conditions only. Weeds were able to promote mycorrhiza formation of maize, irrespectively of their own mycorrhizal dependency. The species-specific P-use efficiencies of weeds differed significantly. In particular, Echinochloa crus-galli with a low P use efficiency in combination with high enzymatic P mobilization and mycorrhiza promotion increased the P use efficiency in maize. In addition to the effects on the P use efficiency, increased tolerance of abiotic stress (drought, heavy precipitation) by crop plants was indicated in some of the experiments. In general, the potential of increased P use efficiency by weeds was indicated. However, also a strong dependency of this effect on the seasonal weather conditions was revealed. The future challenge will be, to quantify consequences of weeds for the site-specific P fertilisation.