Can the Urea Fatty Fraction Support Sustainable Agriculture in the Improvement of Soil Properties?

One of the assumptions of the circular economy is the introduction of nitrogen (N) fertilizers into soil in the form of by-products, such as urea fatty fraction (UFF). Another recommended sustainable agriculture treatment is to plough post-harvest straw into soil to improve the organic matter (OM) balance. We aimed to verify the efficacy of UFF as a N fertilizer applied with wheat or rape straw by examining its effect on the total carbon and N contents, pH, enzyme activity, OM mineralization and stabilization of soil. For this, we conducted a 120-day-long incubation experiment in which we compared the effect of UFF fertilizer applied with urea (both with and without a Ure inhibitor) on soil properties. Our main findings were that UFF acidified the soil (pH was lowered to 5.93) more than the urea (pH was above 6). Both fertilizers administered with straw slightly increased the soil carbon (to above 14 g kg−1) and N contents (to around 1.4 g kg−1) compared to the control treatment and caused an increase in enzyme activity at the beginning of the experiment, followed by a gradual decrease. The UFF application accelerated the OM decomposition, although urea had a more stabilizing effect on the OM expressed by larger (above 16%) areas occupied by stable, aggregated OM than UFF (below 10%). We concluded that UFF can replace urea as an environmentally friendly N fertilizer, and that it has a similar effect to urea on soil properties.