TG-09 Group Augmented Abstract

Stable Isotopes of N2O Emitted from Tropical Soils: Causes of Spatial Variability and Implications for Constraining the Global N2O Budget

Susan E. Trumbore-- Department of Earth System Science, University of California, Irvine
Plinio B. de Camargo -- Centro de Energia Nuclear (CENA)/Universidade de São Paulo (USP)
Tibisay Josefina Perez Acosta -- Department of Earth System Science, University of California, Irvine



Stable isotopes measured in atmospheric N2O and its sources have been proposed as constraints for the global N2O budget. As yet, few measurements exist to characterize the most important natural source of N2O, tropical soils. We have developed field sampling and laboratory methods to measure stable isotopes in N2O emitted from soils. Preliminary measurements in tropical rain forest soils show considerable spatial variation in the d15N of N2O emitted during the rainy season, both regionally with soil type, and locally, within a few meters. We hypothesize that the causes of spatial variability in N2O emissions reflect differences in microbial processes and physical transport. The work proposed here will test specific hypothesis to explain the causes of spatial variability in N2O emissions.

We will investigate variability in N2O isotopes at the Tapajos National Forest sites in collaboration with investigators in the NASA LBA-ECO program. Measurements of the isotopic signature of N2O emitted from the soil surface using chambers will quantify local variability. Depth profiles of N2O concentration and isotopic signature at the same sites, combined with diffusion modeling, will test the hypothesis that the depth of maximum N2O production controls both the flux and isotopic signature of N2O. The increase of N2O in the nocturnal canopy layer will be used to determine the spatially integrated emission rate and isotopic signature of the soil source averaged over hundreds of square meters for comparison with other scaling methods. Regional variability in the isotopic signature of N2O emissions along a soil textural gradient will be related to soil physical properties and nutrient status. To test the role of NO3- in inhibiting N2O reduction, we will apply NO3- fertilizer and measure the resulting change in the isotopic composition of emitted N2O. To facilitate modeling of the microbial pathways of N2O production and consumption, we will determine 15N isotopic fractionation factors associated with denitrification (NO3- to N2O and N2O to N2) using both jar incubations and in situ addition of 15N labeled NO3- under conditions of different soil water content. Isotopic measurements of N2O in each case will be supplemented with measurements of the amount and isotopic composition of extractable nitrate, ammonium, and soil organic matter, estimates of soil gas diffusion rates based on soil water content and physical structure, and measurements of N2O and NO fluxes. The complete data sets will allow us to test models of soil N and C cycling that predict N2O emissions at regional to global scales.

Our measurements will provide (1) better characterization of the isotopic signature of this important N2O source for global budgets; (2) process-based understanding linking the pathway of N2O production (nitrification, denitrification) and the rate of loss (conversion to N2) to the isotopic signature of emitted N2O; and (3) prediction of how the isotopic signatures of tropical soil N2O emissions may change following disturbance.


Background Information

The concentration of nitrous oxide (N2O) has increased in the atmosphere since pre-industrial times, and as a greenhouse gas, it contributes significantly to anthropogenic radiative forcing. Its tropospheric mixing ratio is currently increasing annually by about 0.25% per year. The reason for this increase seems to be mostly dominated by agricultural intensification since 1500 AD, with a smaller contribution from fossil fuel combustion, biomass burning and industrial processes. Tropical forest soils are the largest natural source of N2O to the atmosphere. Conversion of tropical forest for pasture and agricultural use changes the rates of N2O emission from soils; however the net effect of changing tropical land use on the N2O budget is poorly known. The use of nitrogen fertilizers in tropical agricultural systems is increasing rapidly, which will presumably increase tropical N2O emissions in the future.

The use of stable isotopes of N and O in atmospheric N2O and its sources has been proposed as a way to better constrain the global N2O budget. In essence, the overall tropospheric 15N  and 18O value for nitrous oxide represents the balance between  N2O that is enriched in the heavy isotopes mixing down from the stratosphere and oceanic and soil sources that are depleted in the heavy isotopes.  The lightest isotopic signatures for N2O measured are from nitrogen- fertilized agricultural fields. We are hypothesizing that increased N2O sources from agriculture have resulted in a measurable decline of 15N  in atmospheric N2O over the past century (manuscript in preparation by Pérez et al.)

The majority of the available results for the isotopic signature of N2O emitted from tropical soils in were measured in our laboratory, as part of the PhD thesis of Tibisay Pérez. We have been able to explain the large spatial variability in the isotopic signature of emissions as a combination of microbial and physical processes influencing production and consumption of N2O in soils (Pérez, Trumbore, Tyler, Davidson, and Keller, and Camargo, Isotopic variability of N2O emissions from tropical forest soils, in press, Global Biogeochemical Cycles). The work proposed for the new NSF grant will expand on this previous work both looking at mechanisms of N2O production and consumption in tropical forest, pasture and agricultural soils, and attempting to do spatial averaging to estimate the global isotopic signature of N2O emitted from tropical soils.


Proposed Measurement Sites and Types of Measurements

We have proposed to work at two LBA field sites:

  1. A primary tropical forest in the FLONA Tapajós, Brazil in collaboration with Keller, Crill and de Mello, as well as pasture/agricultural fields in collaboration with E. Davidson/Sá in the Santarém and Paragominas area.

  2. A primary forest and a savanna site in Edelca station, La Gran Sabana, Bolivar State, Venezuela, in collaboration with Dr. Eugenio Sanhueza of IVIC.

Measurements will include: Isotopes in N2O emissions from soils, N2O concentrations and isotopes in soil and canopy air, amounts of N and 15N in KCl extractable ammonium, nitrate in soils and in organic matter, incubation experiments to determine isotope fractionation factors for 15N for nitrification and denitrification. We intend to collaborate with other LBA investigators to obtain access to vertical air profiles from towers. Some of the analyses will be done at CENA, others will be done at IVIC (which will be developing a stable isotope mass spectrometer facility).


Training and Education Plans

 Enir Salazar da Costa, a Ph.D. student at UC Irvine (originally from Manaus, Brazil) will do part of her Ph.D. thesis on measurements related to this proposal (she will also work on radiocarbon isotopes as part of the CD-08 proposal led by Trumbore and Camargo). We intend to include other local students in the work as well (in both Venezuela and Brazil). Dr. Pérez is a Venezuelan citizen and will be actively involved in educational efforts at IVIC.

Last Updated: April 17,