Biochar as a sustainable amendment in fertilized agricultural soils; insights and trade-offs among nitrogen kinetics, carbon sequestration, and greenhouse gas emissions.

Under European and International policies, organic soil amendments are highly promoted as a cost-efficient solution to improve soil C, quality, and agrosystem sustainability. Inorganic N application is an essential agronomic practice to increase and secure crop yields, however, its long-term application has led to serious environmental problems including deterioration of soil organic C, enhanced greenhouse gas emissions, and an overall decline in environmental quality. Consequently, the co-application of organic and inorganic fertilizers is advocated as a more effective and environmentally friendly fertilization regime. This study aims, to decipher the short-term N kinetics in agricultural soils amended with organic, inorganic, and a combined application of N fertilizer, with and without biochar, and to assess the trade-off balance of soil C and greenhouse gas emissions. Therefore, we investigated the short-term (90 d) soil N dynamics of sandy soil mesocosms (2 Kg) receiving municipal sewage sludge (MSS) amendments (50 t/ha), urea-N fertilization (U; 200 kg/ha), a combined application (MSS+U), without and with biochar (1.5% w/w). An unamended soil mesocosm was included as a control. The addition of urea-N (U), municipal sewage sludge (MSS), and their combined application (MSS+U) increased the availability of soil NH₄⁺ by 3x, 5x and 12x times, relative to the control, respectively. Interestingly, we observed a tremendous release of soil NO₂^- only in the urea treatment (U; 128 mg kg^-1), and not in the other remaining treatments. Throughout the incubation approx. 12.7x, 13.4x, and 19.7x more soil NO₃^- was observed for the U, MSS, and MSS+U treatment, relative to the control, respectively. Where biochar was applied, an approx. 40% reduction in soil available NO₂^- andNO₃^- was observed. Considering the gaseous emissions of CO₂ and N₂O, that are generally products of soil respiration, nitrification, and denitrification, the addition of MSS and its co-application (MSS+U), enhanced soil CO₂ by 2.4x and 2.4x, and by 13.6x and 16.9x for soil N₂O emissions, respectively. Though biochar addition reduced cumulative CO₂ emissions by 24%, for all treatments except the control. Although biochar addition decreased cumulative N₂O emission by 65% in the U, it had no effect on cumulative N₂O emission for MSS and the combined treatment (MSS+U). Fertilization by U did not affect much soil CO₂ (526 mg CO₂-C kg^-1) and N₂O (1258 μg N₂O-N kg^-1) emissions when compared to the unamended soil treatment (C). The MSS+U reduced the N₂O emission factor, by 5x when compared to MSS treatment, however, it was well above the IPPC emission factor of 1%. Municipal sewage sludge is a source of C, though we observed that MSS (74%) and the combined treatment (MSS+U, 96%) enhanced the CO₂-equivalent emissions, indicating a complete loss of the added organic C through greenhouse gas emissions. Considering our key question, whether co-application of inorganic, and organic fertilizer with biochar is a double-edged sword, we conclude that co-application should be carefully evaluated case per case, as it affects several key soil parameters differently, and therefore we should seek new ways to minimize gaseous losses thus to improve sustainability in agrosystems.