Drivers of nitrous oxide emissions under crops and natural grasslands in the Pampas region, Argentina

Background/Question/Methods

Nitrous oxide (N₂O) is one of the major greenhouse gases, responsible for near 6% of global warming, and is the largest stratospheric ozone-depleting substance. The major sources of N₂O are soils under natural vegetation and arable lands. Due to the diverse processes involved in the production of N₂O from soils, emissions are largely variable in space and time. Understanding the drivers that control these emissions represents a key task when modeling N₂O emissions or upscalling from field measurements. Soil variables such as water fill pore space, temperature, and inorganic nitrogen content are usually identified as the main drivers of N₂O emissions from terrestrial ecosystems. However, vegetation productivity, which is closely related to these soil factors, has received less attention. In this study two questions were tackled in the main agricultural region of Argentina: Do soils under commercial crops release more N₂O than natural ecosystems set aside? And; is vegetation productivity closely related to soil N₂O emissions? A manipulative field experiment was settled in two contrasting sites of the Pampas Region. In each site, N₂O samples were taken monthly during one year with the static chamber method in soybean, corn, wheat/soybean rotation crops, and natural ungrazed grassland plots. Experimental plots received common agricultural practices like land under commercial production. We also measured soil temperature, vegetation productivity (estimated by the normalized difference vegetation index - NDVI), and we collected soil samples to determine water filled pore space, and NH₄⁺ and NO₃^- content in each sampling date.

Results/Conclusions

Our results showed that the N₂O emissions from soybean crops were higher than from natural grasslands at both sites (p<0.05). Emissions from corn and wheat/soybean double cropping were non-significantly different from natural grasslands. These, results suggest that emissions coming from natural ecosystems need to be considered as a baseline for estimating anthropogenic N₂O emissions. Since soybean crops are not fertilized with nitrogen in this region, our results evidence the role of biological nitrogen fixation by soybean as a source of reactive nitrogen for N₂O production. As expected, soil water filled pore space, inorganic nitrogen content, and soil temperature were important drivers controlling N₂O emissions. In addition, net primary productivity was as a strong indirect driver, controlling the factors mentioned before, and therefore has the potential to be used to estimate N₂O emissions from large regions using satellite NDVI time series.