The carbon, nitrogen, phosphorus, and sulfur (CNPS) ratios of soils are known to be relatively stable parameters, characterizing different land uses. We hypothesized that the long-term application of sewage sludge compost (SSC) would not change these ratios but would increase the concentration of these elements and change the quality of organic matter (OM), as well as soil enzyme activities. Hence, soil chemical and microbiological properties were studied in a 20-year long-term experiment. The plots were grouped into five blocks and treated every third year with SSC at the rates of 0, 9, 18, or 27 t ha⁻¹. Three plants, in a crop rotation, were tested and sown every year as follows: rye, rye with hairy vetch, and maize. The results showed that basic soil parameters (pH, OM content, E4/E6 ratio, NO₃-NO₂-N, AL-P₂O₅, and soil moisture content) were increased, along with the SSC doses in soil for the rye. Similar trends were found in CNPS concentrations, β-glucosidase, and alkaline phosphatase activities, while the acidic phosphatase activity was reduced. The C:N, C:S, and N:S ratios were not affected by the compost application. The main factors of treatment effects were plant-available phosphorus (ammonium lactate (AL)-soluble P₂O₅), total P, and NO₃-NO₂-N, based on principal component analysis. The canonical correspondent analyses revealed that phosphatase activities were affected by C:N, C:P, and N:P ratios and β-glucosidase was correlated with P forms and the E4/E6 ratio, while the soil pH strongly affected all soil enzymes. Based on the alkaline and acidic phosphatase activities, the role of microbes became more important with increasing compost doses in phosphorus mobilization. We conclude that the addition of SSC could improve soil health through increasing the pH, OM, nutrient content, and microbial activity. Also, some elemental ratios have an important role in the regulation of soil enzyme activities.

The treatment of wastewater and the utilization of the by-products of these processes are an important part of the circular economy. The sewage sludge, a result of wastewater treatment, could be used as a material for plant nutrient supply and/or soil-improving products. The city of Nyíregyháza, Hungary, with 120,000 citizens, has a well-planned water treatment plant operated by Nyírségvíz Ltd., which, in cooperation with the Research Institute of Nyíregyháza, developed a municipal sewage sludge compost (SSC). The closed loop of sewage water treatment and the agricultural utilization of its by-product has been developed and managed. The compost product called Nyírkomposzt was planned for acidic sandy soils. Beyond the agronomic benefits, the sustainable and environmentally sound utilization of SSC reduces sewage sludge disposal. This active involvement of a water utility company demonstrates the potential of cross-sectoral cooperation in solving environmental problems. The quality of the compost fits the Hungarian legislation. To study the effects of 0, 9, 18, and 27 t ha⁻¹ doses of compost on acidic sandy soil, a long-term small plot experiment was started in 2003. The cumulative effects of the regular (every third year, last treatment before sampling in 2021) application of the SSC showed positive changes in basic soil properties, depending on the doses used. Increasing values were found in the case of pH from 4.5 to 6, plant available P₂O₅ from 240 to 690 ppm, and plant available K₂O from 180 to 200 ppm. The plant-available zinc and copper content also increased. Soil organic matter and total N content stabilized at around 0.9% and 0.08%, respectively. The grain yields of winter rye also increased in both investigated years. The yields of 18 t ha⁻¹ treatment were about two times higher compared to the control, but only in 2022 was the difference significant. Our findings underscore the potential of well-planned SSC applications to improve the fertility of ploughed, acidic sandy soil, taking into account the theory of the circular economy by utilizing wastes and decreasing landfilling.

A Debreceni Egyetem, AKIT Nyíregyházi Kutatóintézetében 2003 tavaszán beállítottunk egy szennyvíziszap komposztos kisparcellás kísérletet. A kísérletben alkalmazott korlátozásmentesen felhasználható szennyvíziszap komposzt összetétele: 40% szennyvíziszap, 25% szalma, 30% riolit és 5% bentonit. A kísérleti területen négy kezelést vizsgálunk, a kontroll mellett 9, 18 és 27 t/ha dózisban juttatjuk ki és szántjuk be a talajba a szennyvíziszap komposztot. A parcellákat újrakezeltük 2006, 2009, 2012 és 2015-ben. a tesztnövények zöldborsó (Pisum sativum L.), tritikálé (x Triticosecale x Wittmack) és kukorica (Zea mays L.), melyek kiterített vetésforgóban követik egymást. A talajmintavétel során a növények betakarítása után, parcellánként 5 leszúrásból átlagmintát képezünk a 0-30 és 30-60 cm-es talajrétegekből. Az eredmények alapján a szennyvíziszap komposzt pozitív hatással van a talaj kémiai tulajdonságaira, növelve a kémhatás értékét és a szervesanyag mennyiségét.