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.

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.  

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≤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 “2nd Call for H.F.R.I. Research Projects to support Post-Doctoral Researchers”; Project #01053 awarded to P.I. Dr Georgios Giannopoulos. This project was co-implemented with industrial partner Corteva Agriscience Hellas SA.