Co-application of organic amendments and urea-N in a loamy soil reduced the N2O emission factor but substantial amounts of organic C were lost as CO2.
Authors: Giannopoulos Georgios , Pasvadoglou Elpida , Kourtidis Georgios , Elsgaard Lars , Zanakis George , Anastopoulos Ioannis 
Published: March 2024

Under the framework of Circular Economy, EU Green Deal, and UN Sustainable Development Goals the addition of organic amendments to agricultural soils is highly promoted as a cost-efficient solution to improve soil quality and agrosystem sustainability. Nonetheless, their agronomic use comes with an uncertainty of their potential to release ample plant-available N, and to emit soil greenhouse gases.This mesocosm study investigated short-term (90 d) soil N dynamics of a loamy soil receiving four organic amendments (50 t ha-1) (i) cow manure compost (CMC), (ii) food waste compost (FWC), (iii) used digestate substrate (UDS) and (iv) municipal sewage sludge (MSS), without and with N fertilization (160 kg N ha-1; urea). An unamended soil mesocosm was included as a control (C). During the incubation soil NO2-, NO3-, NH4+, N2O and CO2 were regularly monitored.During the incubation, org. amendments did not affect NH4+ availability (AUC) compared to unamended soil, except MSS treatment which had 5.7x more NH4+ than C. The co-application of urea increased available NH4+ by 2.9x, 4.1x, 4.4x, 4.6x, and 5.9x for MSS, UDS, CMC, FWC, and C, respectively. There was no difference in available NO2- among org. amendment treatments and the C, except MSS (2.4x). There was a substantial and temporal accumulation of NO2- (2.4x to 3.6x) when urea was co-applied with org. amendments. Co-application of urea with org. amendments increased AUC NO3- in all treatments ranging to 2.7x from 13.6x, except MSS. Considering cum. CO2 we did not observe any differences between org. amended treatments without and with urea. However, org. amendments increased cum. N2O emission by 1.4x, 1.6x, and 3x, for UDS, FWC, and MSS, and reduced by 0.6x for CMC relative to C, respectively. The co-application of urea increased cum. N2O emissions for MSS, UDS, and CMC by 6%, 65%, and 90%, respectively, and reduced by 58% for FWC, compared to the corresponding org. treatment without urea.Interestingly, co-application of urea with org. amendments reduced N2O emission factor (EF) by 4x, 6x, 6x, and 9x, relative to org. amendments without urea, for CMC, MSS, UDS and FWC, respectively. However, the EF N2O exceeded 1% in most cases. Treatments with urea lost substantial amounts of org. C as CO2-equivalent emissions, for instance, UDS+U and MSS+U lost 22% and 68%, respectively. &#160;In conclusion, our preliminary results indicate that the co-application of org. amendments with urea-N could potentially fuel soil N2O emissions, thus offsetting any favorable aspects of the aforementioned policies. Org. amendment, urea-N, and their interaction were significant factors (p&#8804;0.05) driving CO2 and N2O emissions. The quality and composition of the amendments may stimulate soil microbial N transformations, and further investigation will elucidate the intrinsic role of soil microbes and their dynamics in regulating CO2 and N2O emissions from soils.The research project was supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the &#8220;2nd Call for H.F.R.I. Research Projects to support Post-Doctoral Researchers&#8221;; Project #01053 awarded to P.I. Dr Georgios Giannopoulos. This project was co-implemented with industrial partner Corteva Agriscience Hellas SA. &#160;&#160;&#160;&#160;