Emissions of nitrous oxide and methane after field application of recycled fertilisers

Soils naturally emit nitrous oxide (N2O), a potent greenhouse gas (GHG). But excessive N fertilisation and manure management in agriculture significantly increased N2O emissions over the last decades, fueling climate change. On the other hand, soils behave as carbon (C) sinks, with a net methane (CH4) uptake, which could be an important asset in mitigating climate change. The use of biogas digestates as fertilisers, i.e. recycled fertilisers, and the degree with which they are impacting the environment are still debated and widely unresearched. In this project, N2O and CH4 emissions occurring after the application of two types of recycled fertilisers (liquid digestate and digested slurry) were compared with emissions occurring after the application of cattle slurry. The comparison was extended with the use of a mineral fertiliser (ammonium sulfate) and a negative control (no fertilisation). Additionally, the potential impact of biochar addition with digested slurry was assessed. The soil was a luvisol with a silt loam texture and planted with winter wheat. Fertilisers were added in two applications of 70 kg N ha-1 on 26 March and 16 April 2019. N2O and CH4 emissions were measured using closed static chambers for 91 days (19 February to 21 May). NH4+, NO3- concentrations and soil water content (SWC) were assessed weekly. The pH and microbial biomass were measured only twice, before and after the two fertiliser applications. The results showed first that the addition of 2 t ha-1 biochar to the digested slurry had no significant influence on N2O and CH4 emissions, nor on soil properties (NH4+, NO3-, microbial biomass, pH). CH4 uptake was independent of the types of fertilisers but was dependent logically on SWC. The mean CH4 flux was -11 μg CH4-C m-2 h-1. On the contrary, N2O emissions were dependent on soil properties (such as NH4+), environmental conditions (SWC, temperature) and on fertilisers characteristics (Ntot, C, C/N ratio), as well as the total volume of fertiliser applied. The first fertilisation event had no effect on N2O emissions. Contrarywise, the second fertilisation induced high emissions of up to 552 μg N2O-N m-2 h-1. Both recycled fertilisers were not significantly different from cattle slurry in terms of N2O emissions, with cumulated emissions of 328.7 ± 28.6, 403.6 ± 83.1 and 364.6 ± 100.1 g N2O-N ha-1 for the liquid digestate, digested slurry and cattle slurry, respectively. In comparison, the cumulated N2O emissions (91 days) induced by the mineral fertiliser or from the unfertilised plots were 221.4 ± 41 g N2O-N ha-1 and 70.7 ± 22.7 g N2O-N ha-1, respectively, and showed no peak production after the second fertilisation event. Detailed analysis of the results provided clear indications showing that denitrification was the dominant process linked with N2O production. The addition of C by organic fertilisers strongly promoted this process. However, long-term research would be necessary to better identify and compare the respective impacts of the added fertilisers and the influence of environmental parameters on GHG emissions. Finally, the detailed analysis carried out so far allowed for the presentation of some recommendations for a rational usage of these fertilisers in the future