Yield and Nitrogen Management of Festulolium braunii (K. Richt.) A. Camus Treated with Spent Mushroom Substrate and Mineral Fertilizers

More efficient use of nutrients by crops and their reduced dispersion in the environment are essential elements of sustainable agriculture. The purpose of the present experiment was to determine the effects of mineral (Nmin) and spent mushroom substrate (SMS) nitrogen on Festulolium braunii yield, but also on the uptake of that chemical element, use efficiency, and its accumulation in the soil. Results indicated that organic waste applied together with mineral fertilizers increased plant utilization of nutrients, their soil content and, consequently, the yield. SMS was applied once at the beginning of the experiment at three levels: SMS1—10; SMS2—15; SMS3—20 Mg·ha−1, supplying plants with 75, 112, and 150 kg N·ha−1. Supplementary mineral nitrogen was applied at three levels as well: N1—30; N2—68; N3—105. Additionally, 180 kg N·ha−1 was applied without SMS (N4). Treatment significantly affected grass yield, daily growth, and productivity of 1 kg of nitrogen. Their values were the highest on the N2 + SMS2 plot (68 + 112 kg N·ha−1). Nitrogen content was the highest in grass treated with mineral nitrogen without SMS (N4). When the share of SMS nitrogen was higher, its content in the biomass was lower. The absorption of nitrogen (Nup) and its use efficiency (NUE) by plants on the plots with SMS and mineral fertilizers (105 + 75 kg N·ha−1, as well as 68 + 112 kg N·ha−1) were similar to the values recorded on the plot with mineral nitrogen only (N4). After two years, there was no increase in total nitrogen soil accumulation as a result of applied treatment. Mushroom substrate nitrogen allowed for a reduction of nitrogen fertilizer doses by 40 to even 60%. Such fertilizer treatment fits into the closed economy model based on minimizing the consumption of raw materials and on increasing environmentally friendly waste disposal.