Similar Nitrous Oxide (N2O) Emissions in Soybean Crops and Set-Aside Grasslands of South America.

Nitrous oxide (N₂O) is a major greenhouse gases (GHG) emitted by agricultural activities and responsible of stratospheric ozone depletion. Estimates of N₂O emissions from croplands are challenging because in the absence of crops, soils will still release N₂O to the atmosphere, either under spontaneous vegetation or bare soil. Currently, IPCC protocols do not discount baseline emissions from bare soils or natural ecosystems. Here we report a comparison of N₂O emissions in croplands and nearby adjacent set-aside grasslands, achieved by a well-replicated field experiment. Our objectives were: i) to analyze the seasonal variations of N₂O emissions from single crop and double crop rotations compared to a reference ecosystem (set-aside grasslands); ii) to evaluate N₂O emissions from croplands row and inter-row spaces; and iii) to evaluate the importance of environmental factors in controlling N₂O emissions. Total N₂O emissions did not differ significantly among the three land cover types, although significant differences were found during summer (January), when set-aside grasslands had higher emissions than soybean crops. Nitrous oxide emissions were low during cold months in all treatments, and high in warmer months under grasslands and during crop senescence in soybean plots. Our results suggest that counting all N₂O emissions from croplands without subtracting emissions from nearby background, unmanaged or set-aside lands can lead to overestimations of the true contribution of crops to increases in atmospheric N₂O concentrations. Inclusion of baseline situations is encouraged for future IPCC guidelines and therefore extensively measuring N₂O emissions in baseline, natural, unmanaged or set-aside lands, is urgent.