A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Isotopic Signature of Nitrous Oxide in dry season forest soils -implications for seasonal production of N2OTibisay PerezSusan Trumbore, Plinio Barbosa de Camargo, Enir Salazar da Costa, StanlyTyler, Michael Keller, Patrick Crill and Eric DavidsonUniversity of CaliforniaMeasurements of stable isotopes in N 2 O provide useful constraints for the globalN 2 O. Tropical rain forest soils are the largest natural source of N 2 O to the atmosphere.Variations in the flux and isotopic signature of N 2 O from tropical soils reflectmicrobiological processes that produce and consume N 2 O, and physical controls of therate at which N 2 O escapes from the soil pore space to the overlying atmosphere. Ourprevious work in the Amazon basin and in a Costa Rican forest suggested that soil textureaffects the isotopic composition of the N 2 O emitted from the soil surface. We presenthere measurements of N 2 O isotopic composition across a soil texture gradient during thedry season of 2001 in the N 2 O Tapajós National Forest (TNF), Para State, Brazil. Weselected three soil types within the TNF (km 83 site) for study: sandy, transitional (sand +clay) and clay soils. Soil pits (0 to 2 m) were dug at each site and sampled for soilcharacteristics, and tubes for sampling soil gases were installed in each pit. We alsocollected samples at a dry down experiment (TNF, km 67) at 4 pits (0-11 m depth) tocompare variability in the N 2 O isotopic composition affected by drought and at greaterdepths. During the dry season at all sites the N 2 O mixing ratio in soil air increased withdepth from ~340 ppb near the surface to ~600 ppb in the sandy soil (2 m depth) and to~2000 ppb in the transition and clay soils (2 m depth) of the km 83 site. At the km 67site the mixing ratio increased to 1000 ppb at 11 m. The isotopic signature of δ 15 N andδ 18 O of N 2 O and became more depleted in heavy isotopes with depth at all sites. Theδ 15 N 2 O values ranged from 3 to 4 ‰ in the surface to ~2 at 2 m depth in the sandy soiland to ~-4 ‰ in the transition and clay soils at 2m depth. We suggest that little is N 2 O isproduced in these soils during the dry season and that most of the N 2 O emitted during thistime was produced during the wet season and is being slowly released from the large soilcolumn to the atmosphere. Modeling of the diffusion of gases from the soil columnsupports this hypothesis, as do the N 2 O isotopic differences observed between soils withdifferent texture. Soils with higher overall effective diffusivity (sandy soil) release theN 2 O faster than the more compacted soils (e.g. clay and transitional) and therefore havelower N 2 O concentrations at depth and N 2 O with an isotopic value closer to that of theatmospheric N 2 O compared to the soils that have less effective diffusivity.