Evaluation of mitigation practices to reduce N2O and increase soil C; multi-model assessment in five croplands worldwide

Understanding the impacts of the agronomic practices on greenhouse gas (GHG) emissions and soil carbon (C) storage is a key aspect to counteract climate change, mitigate emissions and develop adaptation strategies. Modelling an uncertain future provides stakeholders with a range of potential outcomes to better lead decision making, facilitating the construction and analysis of future scenarios. Notwithstanding mitigation strategies to attenuate GHG emission as nitrous oxide (N2O) and increase soil C stock are well known, their inclusion in process-based models is still limited (Brilli et al., 2017), as well as an exhaustive evaluation based on a variety of model outputs (e.g. crop productivity).
The objective of this work is to evaluate the response and uncertainties of predictions from an ensemble of crop
models, which were run with a set of management options recognized for GHG mitigation and soil C sequestration, i.e., nitrogen (N) fertilisation regimes, irrigation amount and the handling of crop residues. This work was carried out in the framework of the FACCE-JPI project CN-MIP (C and N Models Inter-comparison and Improvement to assess management options for GHG mitigation in agrosystems worldwide; Ehrhardt et al., 2018, Sándor et al.,2018). Results highlight that a reduction of fertilizer N input is accompanied by a more than proportional reduction of N2O emissions compared to a decline in biomass production: A fertilization at 20% of the maximum N dose for each crop and in each site is expected to reduce N2O emissions of about 25% (0.3 kg N2O-N ha-1 y-1), with a reduction of 6% of the aboveground biomass (AGB; 0.6 t DM ha-1 y-1). Furthermore, the effect of N dose reduction decreases by 25% and 30% nitrate leaching and ammonia emission, respectively. High irrigation volumes (up to +50% from the baseline) combined with high N rates increase N2O emissions by about 3% with respect to the baseline irrigation, accompanied by a 3% increase in AGB. Reducing both irrigation to -50% from the baseline and N to the lowest doses causes a reduction down to 60% in N2O emissions compared to the high N input and +50% of irrigation, with 18% reduction in AGB. A significant effect on N2O emissions and soil C stock was observed with crop residues management. Multi-model uncertainty varied with the output and generally increased with low N doses and high irrigation volumes.This work demonstrates the capability of multi-modelling assessment to quantify the impact of mitigation options on N2O emissions, crop production and soil C stocks, providing an estimate of the uncertainties associated with the ensemble modelling.