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