## 3 Results and Discussion
### P Content and Accumulation in Plant Biomass
#### 3.2.2 Weeds  
In arable fields, weeds compete for soil P resources with crop plants [73; 74]. It follows from earlier reports that the richness of soil in P may be even more important to weeds than nitrogen (N) or potassium (K), and that the response to the availability of P depends on the species [75]. To date, however, weeds have not been the subject of research in terms of their use of P from recycled fertilisers. The range of P content determined in the weed biomass in the current research (Table 7) was within the biological plant variability limits [50]. Many times, the P content in weed biomass exceeded the P content in grain. Available literature confirms that weeds very often have a higher capacity for P accumulation than arable crops. According to [PERSON] and [PERSON] [76], P content in the weed biomass in the rye field was as much as twice as high as in the rye biomass. In our research, the variability of weed biomass P content in individual experiments may be related to the species composition of weed communities [73], but negative correlations between P content in weeds and P content in soil (Table 8) and between P content in weeds and P content in wheat grain and post-harvest residues (Table 9) were found. These relationships indicate stronger weed competition for P with lower soil P abundance. In experiment II, wheat under +PP (poor weed competition) showed a higher grain P content than wheat under -PP (strong weed competition). According to [PERSON] et al. [54], _Avena fatua_ L. was capable of reducing the P content of barley (_Hordeum vulgare_ L.), and soil P content affected the level of competition between these species. However, studies by [PERSON] and [PERSON] [75] demonstrated that both the weed P content and wheat shoot P content markedly increased with added P.  
None of our studies have demonstrated any significant effect of the applied P-fertilicr treatments on the P content in the weed biomass (Table 7). No plant protection effect nor interaction between plant protection and fertilisation in this matter was found (experiment II). The P content in the post-herbicide multi-species weed biomass usually results from a combination of factors, including species diversity and species-specific P content (together with the variability related to the development stages), competitive ability and vulnerability to herbicide (i.e., to a particular active substance). In our study (experiment II), regardless of weed biomass reduction, changes in the species and biochemical structure of the community caused by herbicide (+PP) produced such a resultant weed biomass P content that did not differ from the one under -PP. Similar effects were also reported by other authors [76; 77; 78]. The P accumulation in weed biomass was of quantitative importance in the wheat under -PP (Table 7). In experiment II, in wheat field under -PP, the P accumulation in the weed biomass exceeded even P accumulation in straw and, at the same time, would be enough to produce about 1 t of wheat grain. The P accumulation in weed biomass showed a strong positive correlation with the biomass volume, and a weaker but negative correlation with the P content (Table 11). In the wheat fields under -PP, weed species of lower P content, such as _Raphanus raphanistrum_ L. [79] found in experiment II, had a greater share in the total weed biomass. In experiment I, weeds accumulated less P in their biomass when A + H\({}_{2}\)O or A\({}_{8}\)_Bm_ were applied than under PR treatment (Table 7). PR seemed to favour weed biomass development (probably with an increased Ca content [80]), which also resulted in a higher P accumulation in the biomass. In experiment II, SP applied at a P dose of 35.2 kg/ha caused an increase in P accumulation in weed biomass as compared to SP at 26.4 kg P/ha, and +PP treatment limited P accumulation in weed biomass against -PP. These effects were a consequence of the lower weed biomass. There was no interaction between plant protection and P-fertilisation.