N2O emissions in southeastern Amazonia: The effect of agricultural intensification
The Amazon is not only an exceptionally biodiverse and carbon-rich tract of tropical forest, it is also a case study in land use change. Over the last 30 years, Amazonia has been home to extraordinary growth in agricultural production, in part from agricultural expansion, but also due to more intense management on Amazonia’s existing croplands. In Southeastern Amazonia, soybean cropland intensification often occurs when maize is planted directly after soybean harvest (double cropping) and fertilized twice with inorganic N. We use field measurements to estimate how cropland intensification in Mato Grosso, Brazil affects the emission of nitrous oxide (N2O) and soil N dynamics.
We find that dry season N2O emissions in single-cropped (soybean only) fields, double-cropped (soybean/maize) fields and reference tropical forest are uniformly near zero, or ~0-0.5 ngN/cm^2/hr. Surprisingly, wet season emissions rates remain low as well, between 1-4 ngN/cm^2/hr, for both cropland types and reference forest. By contrast, isolated post-fertilization spikes in N2O emissions are large, with a maximum increase of ~800% and a mean increase of ~400%, though these flux increases resolve rapidly as rates return to their low baseline. Finally, we explore the role that soil moisture, soil N availability, and soil C availability play in regulating N2O fluxes. Open questions surround how the Amazon’s land resources can be leveraged to increase agricultural production at the least harm to the environment. Here, we quantify the consequences of land use change on N2O, a powerful greenhouse gas, in a critical ecosystem undergoing novel agricultural intensification.