Nitric and nitrous oxide fluxes from intensifying crop agriculture in the seasonally dry tropical Amazon–Cerrado border region

Abstract

Rapid expansion and intensification of crop agriculture in the tropics may accelerate ecosystem losses of reactive nitrogen (N). We quantified emissions of nitric oxide (NO) and nitrous oxide (N 2 O) in forest, single-cropped soybean [Glycine max (L.) Merr.], and N-fertilized double-cropped soybean-maize (Zea mays L.) at three N fertilizer levels within the largest area of recent cropland expansion on earth, in the Amazon and Cerrado biomes in Brazil. The NO emissions were 2.1 kg N ha -1 yr -1 in forest, 0.6 kg N ha -1 yr -1 in soybean, and 1.3 kg N ha -1 yr -1 in soybean-maize.The N 2 O fluxes were <1.1 kg N ha -1 yr -1 across all land uses. As fertilization levels increased from 80 to 160 kg N ha −1 yr -1 in soybean-maize double-cropped sites, NO emissions increased from 0.6 to 6.7 kg N ha −1 mo -1 in the month immediately after fertilization, but N 2 O emissions only increased from 0.6 to 1.8 kg N ha −1 mo -1 .These results indicate that NO emissions do not increase when forests are converted to croplands under current fertilization levels, and that NO will respond more strongly than N 2 O fluxes to increases in fertilizer applications. Our findings suggest that if N fertilization rates in the region were increased, NO fluxes could increase rapidly. INTRODUCTIONExpanding croplands and increasing fertilizer use in tropical regions can be a growing source of reactive nitrogen (N) emissions to the atmosphere. A global frontier for tropical agricultural intensification is the Amazon and Cerrado biomes in Brazil, where large-scale commodity cropping has propelled Brazil to become the world's leading exporter and the second largest producer of soybeans [Glycine max (L.) Merr.], and the world's second largest exporter and third largest producer of maize (Zea mays L.) by 2018 (FAO, 1997). Approximately 12% of tropical forest in the Amazon biome has been cleared (MapBiomas, 2019), and about half of the Cerrado (savanna) biome has been converted to agriculture (Klink &This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Land Use, Temperature, and Nitrogen Affect Nitrous Oxide Emissions in Amazonian Soils

“…In the Brazilian Amazon, especially in the deforestation arc, recent changes in land use have advanced with the suppression of humid tropical forest areas [4,24,25]. In part of these areas, the insertion of agricultural and livestock production systems can directly influence the emission of greenhouse gases into the atmosphere, such as N 2 O [26,27]. Therefore, the results obtained in the present study contribute to the understanding of the importance of these changes.…”

“…These high emissions are likely due to the N fertilization used in this area. In agricultural areas, such as pepper cultivation, the fertilization and periodic maintenance of the crop increases N in the soil and, consequently, the emission of N 2 O from the system [26,28]. Another essential aspect is the presence of soil discovered between crop lines due to the spacing between plants contributing to the emission of N 2 O, which in this study was 2 × 2 m in the cultivation of pepper.…”

“…The concentration of N in the soil influence N 2 O emissions, as they can directly affect soil microbiological dynamics [12,26,29,32], especially nitrification and denitrification [39]. In addition to these factors, soil moisture and temperature also cause changes in the proliferation of these microorganisms and, consequently, changes in flux and N 2 O emissions.…”

Modeling Nitrous Oxide Emissions From Large-Scale Intensive Cropping Systems in the Southern Amazon

“…Estimating N 2 O emissions from soils is challenging because measurement is time consuming, labor intensive and costly, and because emissions vary in space and time. To date there have been few field-based estimates of N 2 O emissions that correspond to varying levels of fertilization, especially from tropical croplands (Hickman et al, 2015(Hickman et al, , 2017Meurer et al, 2016;Jankowski et al, 2018;Huddell et al, 2021). Process-based models that simulate biogeochemical activity involved in ecosystem C and N cycling while accounting for site characteristics (e.g., climate and soils) are useful to estimate N 2 O emissions.…”

“…Often tropical soils show different emergent properties compared with temperate soils of similar clay content, including low bulk density (0.7-1.2 Mg m −3 ), high permeability (Ksat ∼10-1,000 mm h −1 ) and low available water capacity (70 mm m −1 ), all of which are explained by the micro-aggregated structure of tropical soils Hodnett, 1996, 2005). These physical attributes of tropical soils contribute to differences in their relative low rates of nitrification and denitrification, as well as to the relative low quantity of N 2 O and nitric oxide (NO) produced when compared with temperate fertilized croplands (Kiese et al, 2005;Hickman et al, 2015;Meurer et al, 2016;Huddell et al, 2021). Accurate application of current process-based models to estimate N 2 O emissions from fertilized, cultivated tropical soils requires that these distinct soil physical characteristics be considered.…”