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TG-09 Abstract

Stable isotopes of N2O emitted from tropical soils: causes of spatial variability and implications for constraining the global N2O budget

Plínio Barbosa de Camargo — USP - Universidade de Sao Paulo (SA-PI)
Tibisay Josefina Perez Acosta — University of California (SA-PI)
Susan E. Trumbore — University of California (US-PI)

Objectives



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.














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Last Updated: April 17, 2000

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