## Doctoral Dissertations

#### Title

Nitrous oxide emissions from tropical soils

Fall 1994

Dissertation

Earth Sciences

#### Degree Name

Doctor of Philosophy

Robert Harriss

#### Abstract

Nitrous oxide ($\rm N\sb2O$), the third most important greenhouse trace gas in the troposphere and one contributor to ozone destruction in the stratosphere, has its concentration in the atmosphere increasing steadily over the last few decades. This study measured sources from tropical soils, potentially the highest and least studied $\rm N\sb2O$ production areas in the world.

For two tropical volcanic soils in Costa Rica the effect of water, nitrate and glucose additions on episodic emissions of nitrous oxide were studied. Magnitudes of episodic $\rm N\sb2O$ pulses, as well as overall $\rm N\sb2O$ emissions, varied considerably and consistently, depending on soil texture, soil moisture, and kind and availability of substrates. Emission pulses began within 30 minutes, peaking no later than 8 hours after wetting. Production in the soil occurred mainly in the layer between 5 and 20 cm deep, but depended directly on the temporal dynamics of the water profile. Soil inorganic nitrogen was associated with soil $\rm N\sb2O$ concentration changes. Depending on the treatment, one episodic $\rm N\sb2O$ production event driven by one moderate rain could account for less than 15% to more than 90% of the total weekly production. Previous survey studies may have underestimated the contribution of gas emissions from tropical soils to the global budget of $\rm N\sb2O$, and better budgets will demand a detailed knowledge of both background emissions, and episodic emissions driven by rain events.

The seasonally burned cerrados of Brazil are the largest savanna-type ecosystem of South America and their contribution to the global atmospheric $\rm N\sb2O$ budget is unknown. Results showed that $\rm N\sb2O$ consumption/emission for four fire-scarred savanna ecosystems, for nitrogen and carbon fertilization and for agriculture/pasture ranged from $-$0.3 to +0.7, 1.8 to 9.1, and 0.5 to 3.7 g $\rm N\sb2O$-$\rm N\cdot ha\sp{-1}\cdot d\sp{-1},$ respectively. During the wet season the cerrado biome does not appear to be a major source of $\rm N\sb2O$ to the troposphere, even following fire events. However, conversion of the cerrado to high input agriculture, with liming and fertilization, can increase $\rm N\sb2O$ emissions more than ten fold.

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