Stanley C. Tyler

Identifying the agricultural imprint on the global N2O budget using stable isotopes

Agricultural soils are the most important anthropogenic source of nitrous oxide to the atmosphere. We observed large shifts with time in the emission rate (from 170 to 16 ng N cm−2 h−1) and in δ15N of N2O emitted (from −46‰ to +5‰ relative to atmospheric N2) from a urea-fertilized and irrigated agricultural field in Mexico. We calculated overall instantaneous enrichment factors for the sampling period, which suggest that the microbial N2O production shifts from nitrification (week 1) to denitrification (week 2). Isotopic signatures of N2O emissions were not always in accord with other proxies (such as NO/N2O emission ratio or water-filled pore space) used to estimate the relative importance of nitrification and denitrification as N2O sources. These observations strongly suggest that the soil surface emissions integrate processes occurring at different depths in the soil and a decoupling of NO and N2O production in this system. Further clues as to the source of N2O come from the positional dependence of 15N in the emitted N2O, reported here for the first time in soil emissions. Enrichment at the central N position increased relative to the terminal N position by 9.3‰ during the first 4 days after irrigation, implying that nitrification preferentially enriches the central N position compared to denitrification. The overall δ15N signature we measured for N2O emitted from N-fertilized agricultural systems is more depleted than observed δ15N values for N2O emitted from more N-limited forest soils. Assuming that one half of the total agricultural N2O emissions associated with the global increase in soil nitrogen fertilizer use have an isotopic composition comparable to those of the agricultural fields reported here, we predict a decline in the isotopic signature of tropospheric N2O during this century of as much as 3‰ for 15N. Although many uncertainties remain, we suggest that measurements of δ15N-N2O in firn air will provide constraints on how the N2O budget has changed during the past century.

13C12C Fractionation of methane during oxidation in a temperate forested soil

We have made measurements of the 13C12C fractionation of methane (CH4) during microbial oxidation by an upland temperate soil from College Woods, New Hampshire, using both in situ and laboratory incubation measurements. Uptake rates of 1–4.8 mg CH4/m2/d were measured during the active season in New Hampshire while rates of uptake were 2.6–6.8 mg CH4/m2/d in jars used for incubation studies. The fractionation factor, calculated from field measurements, was α = 0.978 ± .004. This corresponds to a kinetic isotope effect (KIE) of ki2k13 = 1.022 ± .004. Only a small dependence on temperature was noted for air temperatures between 281 and 296 K. Our results indicate that the KIE of soil CH4 oxidation is controlled by physical parameters based on gaseous diffusion into the soil. The implications of these results are discussed with respect to the global CH4 budget and balancing CH4 sources and sinks through the use of δ13CH4 measurements.

Methane oxidation and pathways of production in a Texas paddy field deduced from measurements of flux, δl3C, and δD of CH4

Irrigated rice paddies are one of the few methane (CH4) sources where the management of its emissions may be possible. Before that can be initiated, however, the relationship between production, oxidation, and emission of CH4 and the processes controlling them must be better known. To that end we have made measurements of concentration and stable carbon and hydrogen isotopes of CH4 and CO2 in paddy fields along the Gulf Coast of Texas. Although only small differences in total CH4 flux (∼46.5 g m−2 clayey and ∼43 g m−2 sandy) and average δ13CH4 (seasonal averages of −56.11±1.21‰ clayey and −53.57±0.97‰ sandy) from emitted CH4 were observed in two plots with different soil textures, by making additional measurements of belowground CH4 and CO2 we learned much about processes occurring in the paddy field. We estimated that roughly 98% of the CH4 released was transported through the plant and that residence times for belowground CH4 were from about 1 to 5 hours during most of the season, indicating fast processing of both organic carbon and current photosynthesized carbon to make CH4. The percentage of CH4 made from acetate fermentation calculated using isotope data was strongly dependent on the value of the fractionation factor (α) associated with the CO2/H2 reduction pathway for CH4 formation. Using a range of reasonable values for α, we calculated that acetate fermentation was from 67 to 80% early in the season to 29 to 60% late in the season (generally decreasing as the season progressed). Most importantly, we have strong evidence that rhizospheric CH4 oxidation occurs in paddy fields. We have developed a semiempirical equation and used it to calculate the percent of CH4 oxidized as a function of total CH4 produced from field measurements of δ13CH4 under natural conditions. Because most emitted CH4 is transported by the rice plant, it was necessary to determine the isotopic fractionation CH4 underwent during its transport through the plant. This value, 12±l‰, was used to calculate oxidation percent using belowground and emitted δ13CH4 values. In Texas, oxidation of CH4 in the soil increased from ∼20 to ∼60% over the 6 week period just prior to harvest.

NITROUS OXIDE NITRIFICATION AND DENITRIFICATION 15N ENRICHMENT FACTORS FROM AMAZON FOREST SOILS

The isotopic signatures of 15N and 18O in N2O emitted from tropical soils vary both spatially and temporally, leading to large uncertainty in the overall tropical source signature and thereby limiting the utility of isotopes in constraining the global N2O budget. Determining the reasons for spatial and temporal variations in isotope signatures requires that we know the isotope enrichment factors for nitrification and denitrification, the two processes that produce N2O in soils. We have devised a method for measuring these enrichment factors using soil incubation experiments and report results from this method for three rain forest soils collected in the Brazilian Amazon: soil with differing sand and clay content from the Tapajos National Forest (TNF) near Santarém, Pará, and Nova Vida Farm, Rondônia. The 15N enrichment factors for nitrification and denitrification differ with soil texture and site: -111 per thousand +/- 12 per thousand and -31 per thousand +/- 11 per thousand for a clay-rich Oxisol (TNF), -102 per thousand +/- 5 per thousand and -45 per thousand +/- 5 per thousand for a sandier Ultisol (TNF), and -10.4 per thousand +/- 3.5 per thousand (enrichment factor for denitrification) for another Ultisol (Nova Vida) soil, respectively. We also show that the isotopomer site preference (delta15Nalpha - delta15Nbeta, where alpha indicates the central nitrogen atom and beta the terminal nitrogen atom in N2O) may allow differentiation between processes of production and consumption of N2O and can potentially be used to determine the contributions of nitrification and denitrification. The site preferences for nitrification and denitrification from the TNF-Ultisol incubated soils are: 4.2 per thousand +/- 8.4 per thousand and 31.6 per thousand +/- 8.1 per thousand, respectively. Thus, nitrifying and denitrifying bacteria populations under the conditions of our study exhibit significantly different 15N site preference fingerprints. Our data set strongly suggests that N2O isotopomers can be used in concert with traditional N2O stable isotope measurements as constraints to differentiate microbial N2O processes in soil and will contribute to interpretations of the isotopic site preference N2O values found in the free troposphere.

Determination of the isotopic composition of atmospheric methane and its application in the Antarctic

A procedure for the determination of the 13C/12C ratio and the 14C abundance in atmospheric methane is presented. The method is based on the collection of air samples in stainless steel tanks at a pressure of 7 bar. The air is processed in the laboratory by cryogenic removal of condensibles, followed by oxidation of the methane content after which the resulting CO2 is collected. Also CO is removed prior to oxidation. The 13C/12C ratio is determined on the CO2 sample by stable isotope ratio mass spectrometry. The 14C content is determined by means of accelerator mass spectrometry. The overall precision of the technique is 0.1‰ for δ13C and 1.5 pMC for 14C. The method has been used to determine the carbon isotopic composition of methane in air collected at Baring Head, New Zealand, and also in air collected on aircraft flights between New Zealand and Antarctica. No gradient in the carbon isotopic composition between Baring Head and South Pole station was detected. The annual mean δ13C value at Baring Head was −47.13±0.20‰ for 1989 which includes seasonal effects probably due to OH variations and local meteorology. The annual mean 14C value at Baring Head in 1989 was 118.3 percent modern (pMC).

Modeling atmospheric δ13CH4 and the causes of recent changes in atmospheric CH4 amounts

Inclusion of kinetic isotope effects (KIEs) of methane (CH4) sinks (gaseous OH and Cl, and soil microbes) has a significant effect on modeled distributions of δ13C of atmospheric CH4. For a given scenario of surface sources and corresponding δ13C values of individual CH4 sources, the KIE due to soil uptake enriched δ13C by 1.18‰ in the models northern hemisphere (NH) (40°N) and 1.16‰ in the southern hemisphere (SH) (40°S) under steady state conditions in January. The KIE due to CH4 oxidation by stratospheric Cl radicals further enriched these δ13C values at the surface by 0.99‰ and 1.03‰, respectively. In the vertical direction, during January at 50°N, inclusion of a KIE due to Cl enriched δ13C at 18 km by 0.95‰ compared to the corresponding surface value, whereas the enrichment was only 0.31‰ when this KIE was omitted. These results suggest that modeling of δ13C distributions should include KIEs due to CH4 oxidation by soil and stratospheric chlorine radicals. It is shown that possible oxidation of CH4 in marine boundary layer by Cl radicals can significantly enrich δ13C. However, if a recent theoretical value for the KIE of the Cl and CH4 reaction is correct, then the impact of this reaction is less than the figures quoted above. In the model, monthly variations in OH concentration and interhemispheric exchange transport cannot reproduce the observed seasonal amplitude variation of δ13C in either the NH or SH. It is argued that seasonal variations in individual CH4 fluxes are primarily responsible for this discrepancy. We show that increasing Cl radical concentrations due to continued release of anthropogenic chlorocarbons enrich the δ13C values. The effects of an increase in tropospheric OH concentration due to stratospheric ozone depletion and a cooling of the troposphere due to the eruption of Mt. Pinatubo, with a lowering of water vapor concentration and reduction in isoprene emissions, on δ13C and surface CH4 mixing ratios are investigated. Other model simulations with adjusted surface CH4 fluxes have been performed to study the postulated explanations for recent changes in CH4 surface mixing ratios and δ13C values. A modified version of the Oslo two-dimensional global tropospheric photochemical model was used for all simulations.

Differences in CH4 oxidation and pathways of production between rice cultivars deduced from measurements of CH4 flux and δ13C of CH4 and CO2

We report measurements of CH4 flux and δ13C and δD values of emitted CH4 and sediment CH4 and CO2 during the 1995 rice growing season in Beaumont, Texas. Four rice plant cultivars, Lemont, Mars, Cypress, and Della, and an unplanted plot were studied to provide possible explanations for the differences in CH4 emissions between cultivars. Using the measured isotope values, along with data of CH4 and CO2 concentrations and other ecosystem data, we determined differences between cultivars in the processes of oxidation and production throughout the growing season. For instance, rhizospheric CH4 oxidation increased as the season progressed in both Mars and Lemont cultivars. Late in the season, however, 71±10% of CH4 produced in the Mars plot was oxidized compared to only 39±10% in the Lemont plot. The contribution of acetate fermentation to methanogenesis at specific times during the season was calculated using measured isotopic values and assuming identical isotopic fractionation factors in methanogenic pathways for the cultivars. In these calculations a range of values for the contribution to CH4 production from acetate fermentation and CO2 reduction with H2 was estimated by considering different fractionation factors for the methanogenic CO2 reduction pathway and the possibility of a 10% contribution to CH4 production from acetate produced by homoacetogenesis. In general, a steady increase in the CH4 portion produced by acetate fermentation was noted in the Lemont cultivar, while an increase followed by a decrease near the end of the season was observed for the Mars cultivar.

Can N2O stable isotopes and isotopomers be useful tools to characterize sources and microbial pathways of N2O production and consumption in tropical soils

Nitrous oxide (N2O) is an important greenhouse gas in which the main sources are tropical rainforest and agricultural soils. N2O is produced in soils by microbial processes, which are enhanced by the application of nitrogenous fertilizers. The soil N2O bulk isotopic composition (δ 15Nbulk and δ 18O) and the “site-specific,” or intramolecular, 15N isotopic composition, i.e., the 15N/14N ratio at the cenral (α) or terminal (β) nitrogen position, expressed in this study as δ 15N α and δ 15N β could help identify both the sources (natural and anthropogenic) and microbial pathways of N2O production and consumption prior to emission.We report new isotope measurements of soil N2O emissions and from soil air collected during the rainy season in a mature tropical forest (Tapajos National Forest, Para, Brazil) and in a tropical agricultural corn field (“Fundo Tierra Nueva,” Guarico State, Venezuela). The statistically different δ 15Nbulk emission weighted average between the mature forest (−18.0‰ ± 4.0‰, n = 6) and agricultural corn field (−34.3‰ ± 12.4‰, n = 17) suggest that theδ 15Nbulk data are useful for distinguishing N2O fluxes from fertilized agricultural and natural “background” soils. They also demonstrate that the site-specific δ 15N measurements have the potential to provide a new tool to differentiate between the production and consumption N2O microbiological processes in soils. This study further demonstrates that the observed correlations (or lack thereof) between δ 15N α ,δ 15N β , and δ 18O can be used to estimate the relative proportion of N2O that would have been emitted to the air but was consumed via reduction of N2O to N2 within the soil.

Experimentally determined kinetic isotope effects in the reaction of CH4 with Cl: Implications for atmospheric CH4

We report experimental values for the carbon and hydrogen kinetic isotope effects (KIEs) in the reaction of CH4 with Cl at temperatures between 273 and 350 K. Isotope ratio mass spectrometry was utilized to measure 13CH4/12CH4 and CH3D/CH4 ratios on samples taken from a 5870- L reaction chamber. At 299 K, kc12/kc13 = 1.0621±0.0001 (2σ) and kh/kd = 1.474±0.020 (2σ). For both KIEs, the ratio decreased with increasing temperature over the range studied. These results agree well with experimental studies using tunable diode laser absorption spectroscopy and FTIR absorption spectroscopy and with a recent theoretically-calculated set of values.

Seasonal variations in methane flux andδl3CH4 values for rice paddies in Japan and their implications

We have made measurements of the methane (CH4) flux and δ13C value in CH4 from rice paddies in Ryugasaki, Japan. This study is the first we are aware of in which a significant change in the δ13C signature of emitted CH4 has been documented over the rice growing season. Nutrient treatments studied were of two kinds: compound mineral fertilizer either with or without rice straw from the previous growing season incorporated into the inorganic fertilizer. The calculated annual emission rates during the 1990 growing season were 43.1 g/m2 (straw) and 40.6 g/m2 (no straw) for the two treatments. In both treatments, CH4 started out relatively enriched in 13C, became lighter in 13C, and then became more enriched again during the latter part of rice growth. The 1991 growing season showed a lower integrated flux in both nutrient treatments than for 1990 but plots of the fluxes versus time had the same general shape as the flux curves in 1990 and a similar although less pronounced trend in δ13CH4 signal. Seasonal changes in δ13C are probably related to changes in CH4 production and oxidation and plant-mediated transport. The likelihood of each process occurring and its effect on δ13C values is discussed. The range of δ13CH4 values from seasonal effects was ∼12‰ in 1991 for both nutrient treatments. The δ13CH4 range for 1991 was ∼10‰ (straw) and ∼5‰ (no straw). Our data indicate that when using flux-weighted isotopic signatures to put constraints on the tropospheric CH4 budget, attention should be paid to seasonal changes in isotopic signatures from rice paddy CH4 in a manner similar to that previously suggested from measurements in natural wetlands.