Professor ETH Zurich, Institute of Agricultural Sciences Zurich, Switzerland
Agriculture considerably contributes to the increase of global greenhouse gas (GHG) emissions. Hence, information about the magnitude and the drivers of temporal variations in CO2, N2O and CH4 fluxes in grasslands and croplands are urgently needed to develop sustainable, climate-smart agricultural management practices. Despite the widespread notion to develop nitrogen (N) reduction pathways in agriculture, long-term measurements of N2O fluxes, particularly from croplands, are very limited. Thus, climate-smart management options to reduce N2O losses to the atmosphere as well as nitrate leaching are still to be developed and tested.
With the Swiss FluxNet, a network of six long-term ecosystem sites, we measure GHG fluxes covering all major land-use types in Switzerland (grassland, cropland, forest). Currently, we have >117 site-years of eddy-covariance flux data, all openly available. Here, we report about high resolution N2O flux measurements from grasslands and croplands carried out with an eddy covariance system equipped with a laser spectrometer. Continuous meteorological measurements as well as resin sticks to assess nitrate leaching below the main rooting horizon complemented the flux measurements.
Replacing organic fertilisation by increasing the legume fraction in grasslands resulted in a 40 to 50% reduction in N2O emissions. Yields were reduced by about 10% compared to the business-as-usual management with organic manure fertilisation, but feed quality was higher due to the higher protein contents. Reductions of N2O losses were robust over the longer term. Also nitrate leaching was strongly reduced. However, the N2O mitigation experiment also triggered the grassland turning into a small carbon source since organic carbon imports were stopped. Whether this trade-off between C sink and N2O loss is consistent across different grassland systems remains to be tested.
Measuring N2O fluxes on croplands revealed a strong competition for N between plant roots and microorganisms. Independent of soil water conditions, N2O losses were small when plants were large and active. N2O losses were high when plants were small, as during crop establishment, or on bare soils, i.e., after harvest and prior to sowing the next crop. Thus, plant-demand adapted N fertilization schemes, precision farming or the use of nitrification inhibitors during such special time periods could be climate-smart options to reduce N2O emissions.
