The Effect of Waste Organic Matter on the Soil Chemical Composition After Three Years of Miscanthus × giganteus Cultivation in East-Central Poland

The circular economy practice of using waste to fertilize plants should be more widespread. It is a means to manage natural resources sustainably in agriculture. This approach is in line with organic and sustainable farming strategies, reducing the cultivation costs. Organic waste dumped into a landfill decomposes and emits greenhouse gases. This can be reduced through its application to energy crops, which not only has a positive impact on the environment but also improves the soil quality and increases yields. However, organic waste with increased content of heavy metals, when applied to the soil, can also pose a threat. Using Miscanthus × giganteus M 19 as a test plant, an experiment with a randomized block design was established in four replications in Central–Eastern Poland in 2018. Various combinations of organic waste (municipal sewage sludge and spent mushroom substrate) were applied, with each dose containing 170 kg N ha −1 . After three years (in 2020), the soil content of total nitrogen (N t ) and carbon (C t ) was determined by elemental analysis, with the total content of P, K, Ca, Mg, S, Na, Fe, Mn, Mo, Zn, Ni, Pb, Cr, Cd, and Cu determined by optical emission spectrometry, after wet mineralization with aqua regia. For the available forms of P and K, the Egner–Riehm method was used, and the Schachtschabel method was used for the available forms of Mg. The total content of bacteria, actinomycetes, and fungi was also measured. The application of municipal sewage sludge (SS) alone and together with spent mushroom substrate (SMS) improved the microbiological composition of the soil and increased the content of N t and C t and the available forms of P 2 O 5 and Mg more than the application of SMS alone. SMS did not contaminate the soil with heavy metals. In the third year, their content was higher after SS than after SMS application, namely for Cd by 12.2%, Pb by 18.7%, Cr by 25.3%, Zn by 16.9%, and Ni by 14.7%.