Klaus Butterbach-Bahl

METHODS FOR MEASURING DENITRIFICATION: DIVERSE APPROACHES TO A DIFFICULT PROBLEM

Denitrification, the reduction of the nitrogen (N) oxides, nitrate (NO3-) and nitrite (NO2-), to the gases nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2), is important to primary production, water quality, and the chemistry and physics of the atmosphere at ecosystem, landscape, regional, and global scales. Unfortunately, this process is very difficult to measure, and existing methods are problematic for different reasons in different places at different times. In this paper, we review the major approaches that have been taken to measure denitrification in terrestrial and aquatic environments and discuss the strengths, weaknesses, and future prospects for the different methods. Methodological approaches covered include (1) acetylene-based methods, (2) 15N tracers, (3) direct N2 quantification, (4) N2:Ar ratio quantification, (5) mass balance approaches, (6) stoichiometric approaches, (7) methods based on stable isotopes, (8) in situ gradients with atmospheric environmental tracers, and (9) molecular approaches. Our review makes it clear that the prospects for improved quantification of denitrification vary greatly in different environments and at different scales. While current methodology allows for the production of accurate estimates of denitrification at scales relevant to water and air quality and ecosystem fertility questions in some systems (e.g., aquatic sediments, well-defined aquifers), methodology for other systems, especially upland terrestrial areas, still needs development. Comparison of mass balance and stoichiometric approaches that constrain estimates of denitrification at large scales with point measurements (made using multiple methods), in multiple systems, is likely to propel more improvement in denitrification methods over the next few years.

Nitrous oxide emissions from soils: how well do we understand the processes and their controls?

Although it is well established that soils are the dominating source for atmospheric nitrous oxide (N2O), we are still struggling to fully understand the complexity of the underlying microbial production and consumption processes and the links to biotic (e.g. inter- and intraspecies competition, food webs, plant–microbe interaction) and abiotic (e.g. soil climate, physics and chemistry) factors. Recent work shows that a better understanding of the composition and diversity of the microbial community across a variety of soils in different climates and under different land use, as well as plant–microbe interactions in the rhizosphere, may provide a key to better understand the variability of N2O fluxes at the soil–atmosphere interface. Moreover, recent insights into the regulation of the reduction of N2O to dinitrogen (N2) have increased our understanding of N2O exchange. This improved process understanding, building on the increased use of isotope tracing techniques and metagenomics, needs to go along with improvements in measurement techniques for N2O (and N2) emission in order to obtain robust field and laboratory datasets for different ecosystem types. Advances in both fields are currently used to improve process descriptions in biogeochemical models, which may eventually be used not only to test our current process understanding from the microsite to the field level, but also used as tools for up-scaling emissions to landscapes and regions and to explore feedbacks of soil N2O emissions to changes in environmental conditions, land management and land use.

The global nitrogen cycle in the twenty- first century

Global nitrogen fixation contributes 413 Tg of reactive nitrogen (Nr) to terrestrial and marine ecosystems annually of which anthropogenic activities are responsible for half, 210 Tg N. The majority of the transformations of anthropogenic Nr are on land (240 Tg N yr−1) within soils and vegetation where reduced Nr contributes most of the input through the use of fertilizer nitrogen in agriculture. Leakages from the use of fertilizer Nr contribute to nitrate (NO3−) in drainage waters from agricultural land and emissions of trace Nr compounds to the atmosphere. Emissions, mainly of ammonia (NH3) from land together with combustion related emissions of nitrogen oxides (NOx), contribute 100 Tg N yr−1 to the atmosphere, which are transported between countries and processed within the atmosphere, generating secondary pollutants, including ozone and other photochemical oxidants and aerosols, especially ammonium nitrate (NH4NO3) and ammonium sulfate (NH4)2SO4. Leaching and riverine transport of NO3 contribute 40–70 Tg N yr−1 to coastal waters and the open ocean, which together with the 30 Tg input to oceans from atmospheric deposition combine with marine biological nitrogen fixation (140 Tg N yr−1) to double the ocean processing of Nr. Some of the marine Nr is buried in sediments, the remainder being denitrified back to the atmosphere as N2 or N2O. The marine processing is of a similar magnitude to that in terrestrial soils and vegetation, but has a larger fraction of natural origin. The lifetime of Nr in the atmosphere, with the exception of N2O, is only a few weeks, while in terrestrial ecosystems, with the exception of peatlands (where it can be 102–103 years), the lifetime is a few decades. In the ocean, the lifetime of Nr is less well known but seems to be longer than in terrestrial ecosystems and may represent an important long-term source of N2O that will respond very slowly to control measures on the sources of Nr from which it is produced.

MODELING DENITRIFICATION IN TERRESTRIAL AND AQUATIC ECOSYSTEMS AT REGIONAL SCALES

Quantifying where, when, and how much denitrification occurs on the basis of measurements alone remains particularly vexing at virtually all spatial scales. As a result, models have become essential tools for integrating current understanding of the processes that control denitrification with measurements of rate-controlling properties so that the permanent losses of N within landscapes can be quantified at watershed and regional scales. In this paper, we describe commonly used approaches for modeling denitrification and N cycling processes in terrestrial and aquatic ecosystems based on selected examples from the literature. We highlight future needs for developing complementary measurements and models of denitrification. Most of the approaches described here do not explicitly simulate microbial dynamics, but make predictions by representing the environmental conditions where denitrification is expected to occur, based on conceptualizations of the N cycle and empirical data from field and laboratory investigations of the dominant process controls. Models of denitrification in terrestrial ecosystems include generally similar rate-controlling variables, but vary in their complexity of the descriptions of natural and human-related properties of the landscape, reflecting a range of scientific and management perspectives. Models of denitrification in aquatic ecosystems range in complexity from highly detailed mechanistic simulations of the N cycle to simpler source-transport models of aggregate N removal processes estimated with empirical functions, though all estimate aquatic N removal using first-order reaction rate or mass-transfer rate expressions. Both the terrestrial and aquatic modeling approaches considered here generally indicate that denitrification is an important and highly substantial component of the N cycle over large spatial scales. However, the uncertainties of model predictions are large. Future progress will be linked to advances in field measurements, spatial databases, and model structures.

Fluxes of NO and N2O from temperate forest soils: impact of forest type, N deposition and of liming on the NO and N2O emissions

Annual cycles of NO, NO2 and N2O emission rates from soil were determined with high temporal resolution at a spruce (control and limed plot) and beech forest site (“Höglwald”) in Southern Germany (Bavaria) by use of fully automated measuring systems. The fully automated measuring system used for the determination of NO and NO2 flux rates is described in detail. In addition, NO, NO2 and N2O emission rates from soils of different pine forest ecosystems of Northeastern Germany (Brandenburg) were determined during 2 measuring campaigns in 1995. Mean monthly NO and N2O emission rates (July 1994–June 1995) of the untreated spruce plot at the Höglwald site were in the range of 20–130 µg NO-N m-2 h-1 and 3.5–16.4 µg N2O-N m-2 h-1, respectively. Generally, NO emission exceeded N2O emission. Liming of a spruce plot resulted in a reduction of NO emission rates (monthly means: 15–140 µg NO-N m-2 h-1) by 25-30% as compared to the control spruce plot. On the other hand, liming of a spruce plot significantly enhanced over the entire observation period N2O emission rates (monthly means: 6.2–22.1 µg N2O-N m-2 h-1). Contrary to the spruce stand, mean monthly N2O emission rates from soil of the beech plot (range: 7.9–102 µg N2O-N m-2 h-1) were generally significantly higher than NO emission rates (range: 6.1–47.0 µg NO-N m-2 h-1). Results obtained from measuring campaigns in three different pine forest ecosystems revealed mean N2O emission rates between 6.0 and 53.0 µg N2O-N m-2 h-1 and mean NO emission rates between 2.6 and 31.1 µg NO-N m-2 h-1. The NO and N2O flux rates reported here for the different measuring sites are high compared to other reported fluxes from temperate forests. Ratios of NO/N2O emission rates were >> 1 for the spruce control and limed plot of the Höglwald site and << 1 for the beech plot. The pine forest ecosystems showed ratios of NO/N2O emission rates of 0.9 ± 0.4. These results indicate a strong differentiating impact of tree species on the ratio of NO to N2O emitted from soil.

N2O emission from tropical forest soils of Australia

Three different tropical rain forest sites (Kauri Creek, Lake Eacham, and Massey Creek) on the Atherton Tablelands, Queensland, Australia, were investigated for the magnitude of N 2 O emission from soils during different seasons, that is, wet season, dry season, and transition periods. Highest mean N 2 O emission rates were observed for soils derived from granite at the Kauri Creek site with 74.5 ± 25.2 μg N 2 O-N m -2 h -1 , whereas for soils derived from Metamorphics (Lake Eacham site) mean N 2 O emission rates were much lower (13.1 ± 1.1 μg N 2 O-N m -2 h -1 ). For the Massey Creek site, with soils derived from Rhyolite, a mean annual N 2 O emission rate of 46.2 ± 1.1 μg N 2 O-N m -2 h -1 was calculated. The mean annual N 2 O emission rate calculated for all three sites over the entire observation period was 39.0 μg N 2 O-N m -2 h -1 and thus at the high end of reports from tropical rain forest soils. N 2 O emission rates showed at all sites pronounced temporal as well as spatial variability. The magnitude of N 2 O emissions was strongly linked to rainfall events; that is, N 2 O emissions strongly increased approximately 6-8 hours after precipitation. Correlation analysis confirmed the strong dependency of N 2 O emissions on changes in soil moisture, whereas changes in soil temperature did not mediate considerable changes in N 2 O fluxes. Spatial variability of N 2 O fluxes on a site scale could be explained best by differences in water-filled pore space, CO 2 emission, and C/N ratio of the soil. On the basis of all published N 2 O flux rates from tropical rain forest soils we recalculated the contribution of such forests to the global atmospheric N 2 O budget and come up with a figure of 3.55 Tg N 2 O-N yr -1 , which is approximately 50% higher than reported by others.

General CH4 oxidation model and comparisons of CH4 Oxidation in natural and managed systems

Fluxes of methane from field observations of native and cropped grassland soils in Colorado and Nebraska were used to model CH 4 oxidation as a function of soil water content, temperature, porosity, and field capacity (FC). A beta function is used to characterize the effect of soil water on the physical limitation of gas diffusivity when water is high and biological limitation when water is low. Optimum soil volumetric water content (W opt ) increases with PC. The site specific maximum CH 4 oxidation rate (CH 4max ) varies directly with soil gas diffusivity (D opt ) as a function of soil bulk density and FC. Although soil water content and physical properties are the primary controls on CH 4 uptake, the potential for soil temperature to affect CH 4 uptake rates increases as soils become less limited by gas diffusivity, Daily CH 4 oxidation rate is calculated as the product of CH 4max , the normalized (0-100%) beta function to account for water effects, a temperature multiplier, and an adjustment factor to account for the effects of agriculture on methane flux. The model developed with grassland soils also worked well in coniferous and tropical forest soils. However, soil gas diffusivity as a function of field capacity, and bulk density did not reliably predict maximum CH 4 oxidation rates in deciduous forest soils, so a submodel for these systems was developed assuming that CH 4max is a function of mineral soil bulk density. The overall model performed well with the data used for model development (r 2 = 0.76) and with independent data from grasslands, cultivated lands, and coniferous, deciduous, and tropical forests (r 2 = 0.73, mean error < 6%).

Grazing-induced reduction of natural nitrous oxide release from continental steppe

Atmospheric concentrations of the greenhouse gas nitrous oxide (N2O) have increased significantly since pre-industrial times owing to anthropogenic perturbation of the global nitrogen cycle, with animal production being one of the main contributors. Grasslands cover about 20 per cent of the temperate land surface of the Earth and are widely used as pasture. It has been suggested that high animal stocking rates and the resulting elevated nitrogen input increase N2O emissions. Internationally agreed methods to upscale the effect of increased livestock numbers on N2O emissions are based directly on per capita nitrogen inputs. However, measurements of grassland N2O fluxes are often performed over short time periods, with low time resolution and mostly during the growing season. In consequence, our understanding of the daily and seasonal dynamics of grassland N2O fluxes remains limited. Here we report year-round N2O flux measurements with high and low temporal resolution at ten steppe grassland sites in Inner Mongolia, China. We show that short-lived pulses of N2O emission during spring thaw dominate the annual N2O budget at our study sites. The N2O emission pulses are highest in ungrazed steppe and decrease with increasing stocking rate, suggesting that grazing decreases rather than increases N2O emissions. Our results show that the stimulatory effect of higher stocking rates on nitrogen cycling and, hence, on N2O emission is more than offset by the effects of a parallel reduction in microbial biomass, inorganic nitrogen production and wintertime water retention. By neglecting these freeze–thaw interactions, existing approaches may have systematically overestimated N2O emissions over the last century for semi-arid, cool temperate grasslands by up to 72 per cent.

A process‐oriented model of N2O and NO emissions from forest soils: 2. Sensitivity analysis and validation

The process-oriented model PnET-N-DNDC describing biogeochemical cycling of C- and N and N-trace gas fluxes (N 2 O and NO) in forest ecosystems was tested for its sensitivity to changes in environmental factors (e.g., temperature, precipitation, solar radiation, atmospheric N-deposition, soil characteristics). Sensitivity analyses revealed that predicted N-cycling and N-trace gas emissions varied within measured ranges. For model validation, data sets of N-trace gas emissions from seven different temperate forest ecosystems in the United States, Denmark, Austria, and Germany were used. Simulations of N 2 O emissions revealed that field observations and model predictions agreed well for both flux magnitude and its seasonal pattern. Differences between predicted and measured mean N 2 O fluxes were <27%. An exception to this was the N-limited pine stand at Harvard Forest, where predictions of fluxes deviated by 380% from field measurements. This difference is most likely due to a missing mechanism in PnET-N-DNDC describing uptake of atmospheric N 2 O by soils. PnET-N-DNDC was also validated for its capability to predict NO emission from soils. Predicted and measured mean NO fluxes at three different field sites agreed within a range of ± 13%. The correlation between modeled and predicted NO emissions from the spruce and beech stand at the Hoglwald Forest was r 2 = 0.24 (spruce) and r 2 = 0.35 (beech), respectively. The results obtained from both sensitivity analyses and validations with field data sets from temperate forest soils indicate that PnET-N-DNDC can be successfully used to predict N 2 O and NO emissions from a broad range of temperate forest sites.

Impact of N-input by wet deposition on N-trace gas fluxes and CH4-oxidation in spruce forest ecosystems of the temperate zone in Europe

Abstract In an effort to elucidate the impact of N-deposition from the atmosphere on trace gas fluxes (N2O, NO, CH4) from soils of temperate coniferous forests, two spruce forest sites in Germany and Ireland with comparable edaphic and climatic conditions, but with pronounced differences in the amounts of N-input from the atmosphere were compared at different seasons. At the site in Germany trace gas fluxes as well as wet deposition of NH+4 and NO-3 were recorded continuously over the entire year 1994. Correlation analysis between fluxes and N-input data were performed, in order to elucidate if a direct effect between flux and N-deposition could be demonstrated. At all sampling dates N2O fluxes at the site receiving high atmospheric N-input (Hoglwald, Germany) were significantly 1.5–5 fold higher than N2O emission rates at the site receiving low N-input by wet deposition from the atmosphere (Ballyhooly, Ireland). In contrast to the Hoglwald site, at which only emission of N2O to the atmosphere was observed, at certain periods the Ballyhooly soil functioned as a sink for atmospheric N2O. Methane oxidation rates were significantly lower at the Hoglwald site compared to the Ballyhooly site. At the Hoglwald site it could be demonstrated by correlation analysis that the input of NH+4 and NO-3 by wet deposition significantly altered emissions of N2O and NO (stimulation) and of CH4 oxidation (reduction). The coefficient of determination was better for NH+4 than for NO-3 for all trace gases studied and was best for the relation between NO emission rates and NH+4-input.