Stable isotopes in nitrous oxide emitted from tropical rain forest soils and agricultural fields: Implications for the global atmospheric nitrous oxide budget.

摘要

Nitrous oxide (N₂O) is an important greenhouse gas and is the primary source of NOx in the stratosphere. Large uncertainties exist in the global N₂O budget, mainly due to the high uncertainty associated with source estimates. Recently, stable isotopes of 15N and 18O have been proposed as a tool to better constrain the N₂O global budget. This thesis develops analytical methods for constraining and measuring stable isotopes in N₂O emitted from soils and reports initial investigations of N₂O isotopes from the largest sources in the global N₂O budget: tropical rain forest soils and agricultural fields.; We found significant differences in the isotopic composition of N ₂O emitted from tropical rain forest soils and fertilized agricultural fields. Differences were largest for 15N. Emission-weighted δ 15N-N₂O were –26 ± 2.5‰ s.d., n = 3 (Costa Rican forest), –6.6 ± 11.3‰ s.d. n = 14 (Brazilian forest) and –36.7 ± 9.2‰ s.d. n = 19 (Mexican agricultural field and Costa Rican Papaya plantation). We attribute the large range in δ 15N from tropical rain forests, where denitrification is the main source of N₂O, to differences in the degree of N₂O to N₂ reduction. We attribute the very light δ15N values in fertilized agricultural fields to the enhanced nitrogen availability in the soils which facilitates higher fractionation between substrates and products. Similarly, in the Brazilian tropical forest lighter δ 15N-N₂O from a local area of enhanced emission is attributed to locally more abundant N-substrate in that particular soil site. If the increase of N₂O in the troposphere over the past 100 years is attributable to increased use of N fertilizer, and assuming that light δ 15N-N₂O isotopic values are associated with agricultural practices, we expect the δ15N-N₂O in the troposphere to have decreased since pre-industrial times.; Theoretically, comparison of 15N and 18O signature of emitted N₂O with precursors species (NO₃ –, NH₄+, H₂O and O ₂) should uniquely determine (a) the fraction of N₂O produced by nitrification versus denitrification and (b) the fractionation of N₂O reduced to N₂. However, for this approach to be useful, in situ determinations of fractionation factors associated with nitrification and denitrification in soils are required.