**5. Conclusions**

Water deficit had no e ffect on the phosphorus content in the above-ground parts of spring barley and Italian rye-grass. This factor reduced phosphorus accumulation in the biomass of barley (the stems, leaves and spikes) and of rye-grass (the stems and leaves) from the emergence to the end of plant growth, both when the plants were cultivated as a single species and in a mixture. Water deficit inhibited the phosphorus accumulation in the barley biomass more than the competition of rye-grass. The competition from barley was, for rye-grass, a stronger factor hindering phosphorus accumulation in the stems and leaves than water deficit. Spring barley was a stronger competitor than rye-grass. Irrespective of the water supply of the plants, the competition intensified until the stem elongation phase. The full competition was noted at the stages of most intense barley development, i.e., during the stem elongation and heading. Underwater deficit inter-species competition for P intensified, which further weakened the P uptake both by barley and rye-grass. This phenomenon should be taken into account when recommending the undersowing of barley with Italian rye-grass for sustainable climate-smart agriculture.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2077-0472/10/8/329/s1, Table S1: Phosphorus content of the above-ground biomass of barley (g kg−<sup>1</sup> dry matter), Table S2: Phosphorus content of the above-ground biomass of rye-grass (g kg−<sup>1</sup> dry matter).

**Author Contributions:** Conceptualization, M.W., M.J. and M.K.K.; methodology, M.W., M.J. and M.K.K.; formal analysis, M.K.K. and M.J.; investigation, M.K.K., M.J., M.W. and K.T.; data curation, M.W. and M.K.K.; writing—original draft preparation, M.K.K.; writing—review and editing, M.J.; visualization, M.K.K. and M.J.; funding acquisition, M.K.K., M.J., M.W. and K.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Ministry of Science and Higher Education of Poland, gran<sup>t</sup> number N N310 082836.

**Acknowledgments:** The authors kindly acknowledge the technical support of Przemysław Makowski from the University of Warmia and Mazury in Olsztyn.

**Conflicts of Interest:** The authors declare no conflict of interest.
