Tree species of the Central Amazon and soil moisture alter stable isotope composition of nitrogen and oxygen in nitrous oxide evolved from soil

The use of stable isotopes of N and O in N₂O has been proposed as a way to better constrain the global budget of atmospheric N₂O and to better understand the relative contributions of the main microbial processes (nitrification and denitrification) responsible for N₂O formation in soil. This study compared the isotopic composition of N₂O emitted from soils under different tree species in the Brazilian Amazon. We also compared the effect of tree species with that of soil moisture, as we expected the latter to be the main factor regulating the proportion of nitrifier- and denitrifier, derived N₂O and, consequently, isotopic signatures of N₂O. Tree species significantly affected δ¹⁵N in nitrous oxide. However, there was no evidence that the observed variation in δ¹⁵N in N₂O was determined by varying proportions of nitrifier- vs. denitrifier-derived N₂O. We submit that the large variation in δ¹⁵N-N₂O is the result of competition between denitrifying and immobilizing microorganisms for NO^-₃. In addition to altering δ¹⁵N-N₂O, tree species affected net rates of N₂O emission from soil in laboratory incubations. These results suggest that tree species contribute to the large isotopic variation in N₂O observed in a range tropical forest soils. We found that soil water affects both ¹⁵N and ¹⁸O in N₂O, with wetter soils leading to more depleted N₂O in both ¹⁵N and ¹⁸O. This is likely caused by a shift in biological processes for ¹⁵N and possible direct exchange of ¹⁸O between H₂O and N₂O.