The use of biogeochemical models to evaluate mitigation of greenhouse gas emissions from managed grasslands

Authors: SANDOR, RENATA - Institut National De La Recherche Agronomique (INRA); EHRHARDT, FIONA - Institut National De La Recherche Agronomique (INRA); BRILLI, LORENZO - University Of Florence; CAROZZI, MARCO - Agroscope; RECOUS, SYLVIE - Institut National De La Recherche Agronomique (INRA); SMITH, PETE - University Of Aberdeen; SNOW, VAL - Agresearch; SOUSSANA, JEAN-FRANCOIS - Institut National De La Recherche Agronomique (INRA); DORICH, CHRISTOPHER - Colorado State University; FUCHS, KATHRIN - Institute Of Agricultural Sciences; FITTON, NUALA - University Of Aberdeen; GONGADZE, KATE - Rothamsted Research; KLUMPP, KATJA - Institut National De La Recherche Agronomique (INRA); Liebig, Mark; MARTIN, RAPHAEL - Institut National De La Recherche Agronomique (INRA); MERBOLD, LUTZ - Institute Of Agricultural Sciences; NEWTON, PAUL - Agresearch; REES, ROBERT - Sruc-Scotland'S Rural College; ROLINSKI, SUSANNE - Potsdam Institute; BELLOCCHI, GIANNI - Institut National De La Recherche Agronomique (INRA)

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/2/2018
Publication Date: 6/12/2018
Citation: Sandor, R., Ehrhardt, F., Brilli, L., Carozzi, M., Recous, S., Smith, P., Snow, V., Soussana, J., Dorich, C., Fuchs, K., Fitton, N., Gongadze, K., Klumpp, K., Liebig, M.A., Martin, R., Merbold, L., Newton, P.C., Rees, R.M., Rolinski, S., Bellocchi, G. 2018. The use of biogeochemical models to evaluate mitigation of greenhouse gas emissions from managed grasslands. Science of the Total Environment. 642:292-306.

Interpretive Summary: Identifying solutions to emerging ecological and societal challenges require improved knowledge of the underlying processes affecting carbon-nitrogen (C-N) pools and fluxes in agricultural systems. Grassland ecosystems have a potentially important role to play in meeting the challenge of climate change because they can serve as a sink for atmospheric greenhouse gases (GHGs). This study used five case studies to test the sensitivity of eight grassland models to gradients of management intensity selected for their potential to mitigate GHG emissions. Simulated model outcomes indicated input reduction would lead to an increase in the C sink strength in intensive grazing systems. Simulations also indicated a strong effect of N fertilizer reduction on nitrous oxide fluxes. Simulated decline in grazing intensity had only limited impact on N balance. The simulated pattern of enteric methane emissions was dominated by high model-to-model variability. The reduction in simulated harvest (animal intake plus cut biomass) led to a doubling in net primary production per animal. The highest nitrous oxide intensities were simulated at mown and extensively grazed arid sites. Overall, modeled outcomes confirmed grasslands can provide GHG mitigation during beef and dairy production.

Technical Abstract: Simulation models quantify the impacts on carbon (C) and nitrogen (N) cycling in grassland systems caused by changes in management practices. To support agricultural policies, it is however important to contrast the responses of alternative models, which can differ greatly in their treatment of key processes and in their response to management. We applied eight biogeochemical models at five grassland sites (in France, New Zealand, Switzerland, United Kingdom and United States) to compare the sensitivity of modelled C and N fluxes to changes in the density of grazing animals (from 100% to 50% of the original livestock densities), also in combination with decreasing N fertilization levels (reduced to zero from the initial levels). Simulated multi-model median values indicated that input reduction would lead to an increase in the C sink strength (negative net ecosystem C exchange) in intensive grazing systems: -64±74 g C m-2 yr-1 (animal density reduction) and -81±74 g C m-2 yr-1 (N and animal density reduction), against the baseline of -30.5±69.5 g C m-2 yr-1 (LSU = 0.76 ha-1 yr-1). Simulations also indicated a strong effect of N fertilizer reduction on N fluxes, e.g. N2O-N emissions decreased from 0.34±0.22 (baseline) to 0.1±0.05 g N m-2 yr-1 (no N fertilization). Simulated decline in grazing intensity had only limited impact on the N balance. The simulated pattern of enteric methane emissions was dominated by high model-to-model variability. The reduction in simulated offtake (animal intake + cut biomass) led to a doubling in net primary production per animal (increased by 11.6±8.1 t C LSU-1 yr-1 across sites). The highest N2O-N intensities (N2O-N/offtake) were simulated at mown and extensively grazed arid sites. We show the possibility of using grassland models to determine sound mitigation practices while quantifying the uncertainties associated with the simulated outputs.