Rainer Brumme

Nitrous oxide emissions from soil during freezing and thawing periods

In a laboratory investigation, the processes of N2O emissions during freezing/thawing periods were studied. Four undisturbed soil columns from an agricultural site were subjected to two freeze/thaw cycles. Two periods of higher N2O emissions were detected, a period of elevated N2O emissions during continuous soil freezing and a period of brief peak emissions during thawing. Soil respiration indicated that microorganisms were still active in both periods. We concluded that N2O was produced by microorganisms during continuous soil freezing in an unfrozen water film on the soil matrix. This thin liquid water film was covered by a layer of frozen water. The frozen water in form of an ice layer represents a diffusion barrier which reduces oxygen supply to the microorganisms and partly prevents the release of the N2O. Peak emissions during soil thawing were explained by the physical release of trapped N2O and/or denitrification during thawing.

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%).

Nitrous oxide emissions from frozen soils under agricultural, fallow and forest land

In a field study, N2O emissions were measured in an agricultural, a fallow, and a forest system once a week from December 1995 to November 1996. Elevated N2O emissions were detected during periods of both soil freezing and soil thawing. The dynamics of the N2O winter emissions were influenced by the changes in soil temperatures. The highest emission rates were observed during soil thawing. The N2O emissions during the entire winter period (December 1995 to March 1996) amounted to 2.8, 1.3, and 0.7 kg N2O–N for the agricultural land, fallow and forest, respectively, and contributed to 58, 45 and 50% of the annual N2O emissions from these systems. Differences in the winter emissions among the three sites could not be explained by means of nitrate concentration but rather by water-filled pore space (WFPS). Additionally, the upper organic layers of the forest and the grass vegetation of the fallow site delayed the time of soil freezing and reduced the depth of frost penetration. Both WFPS and vegetation control the N2O emissions in winter.

Site variation in methane oxidation as affected by atmospheric deposition and type of temperate forest ecosystem

Factors controlling methane oxidation were analyzed along a soil acidity gradient (pH(H 2 O) 3.9 to 5.2) under beech and spruce forests in Germany Mean annual methane oxidation ranged from 0.1 to 2.5 kg CH 4 ha -1 yr -1 and was correlated with base saturation (r 2 = 0.88), soil pH (r 2 = 0.77), total nitrogen (r 2 = 0.71), amount of the organic surface horizon (r 2 = 0.49) and bulk density of the mineral soil (r 2 = 0.43). At lower pHs the formation of an organic surface horizon was promoted. This horizon did not have any methane oxidation capacity and acted like a gas diffusion barrier, which decreased the methane oxidation capacity of the soil. In contrast, on sites at the higher end of the pH range, higher burrowing activity of earthworms increased macroporosity and thereby gas diffusivity and methane oxidation. Gas diffusivity was also affected by litter shape: broad beech leaves reduced methane oxidation more than spruce needles. An increase in methane oxidation of most soil samples following sieving indicates that diffusion is the main limiting factor for methane oxidation. However, this sieving effect was less in soils with a pH below 5 than in soils with a pH above 5, which we attribute to a direct effect of soil acidity. We discuss our results using a hierarchical concept for the short-term and long-term controls on methane oxidation in forest ecosystems.

Evaluating annual nitrous oxide fluxes at the ecosystem scale

Evaluation of N2O flux has been one of the most problematic topics in environmental biogeochemistry over the last 10–15 years. Early ideas that we should be able to use the large body of existing research on terrestrial N cycling to predict patterns of N2O flux at the ecosystem scale have been hard to prove due to extreme temporal and spatial variability in flux. The vast majority of the N2O flux measurement and modeling activity that has taken place has been process level and field scale, i.e., measurement, analysis and modeling of hourly and daily fluxes with chambers deployed in field plots. It has been very difficult to establish strong predictive relationships between these hourly and daily fluxes and field-scale parameters such as temperature, soil moisture, and soil inorganic N concentrations. In this study, we addressed the question of whether we can increase our predictive understanding of N2O fluxes by examining relationships between flux and environmental parameters at larger spatial and temporal scales, i.e., to explore relationships between annual rather than hourly or daily fluxes and ecosystem-scale variables such as plant community and soil type and annual climate rather than field-scale variables such as soil moisture and temperature. We addressed this question by examining existing data on annual fluxes from temperate forest, cropland, and rangeland ecosystems, analyzing both multiyear data sets from individual sites as well as cross-site comparison of single annual flux values from multiple sites. Results suggest that there are indeed coherent patterns in annual N2O flux at the ecosystem scale in forest, cropland, and rangeland ecosystems but that these patterns vary by region and only emerge with continuous (at least daily) flux measurements over multiple years. An ecosystem approach to evaluating N2O fluxes will be useful for regional and global modeling and for computation of national N2O flux inventories for regulatory purposes but only if measurement programs are comprehensive and continuous.

The influence of soil gas transport properties on methane oxidation in a selection of northern European soils

The oxidation of atmospheric methane in soils was measured in situ at a selection of sites in northern Europe, mainly under forest but also under moorland and agricultural arable land and grassland. Our objective was to examine how land use, soil type, and location affected methane oxidation through their impact on gas diffusivity and air permeability. Gas diffusivity at the soil surface and, in some cases, after removal of any surface organic layer was measured in situ using Freon-22 tracer in a portable probe. For about half of the sites, gas diffusivity was also measured in intact topsoil core samples in the laboratory using krypton 85. Air permeability and porosity were also measured on these cores. Although the method of measurement of CH4 oxidation varied between sites, the same techniques were used to measure soil physical properties at all sites. CH4 oxidation rates ranged from 0 to 2.5 mg m−2 d−1. Diffusivity also covered a very wide range, being lowest in loam cores from wet grassland in Norway and highest in relatively dry, sandy soils in Denmark and Scotland. CH4 oxidation tended to increase with gas diffusivity measured in situ at the soil surface, though the relationship was poor at high diffusivities, presumably because CH4 oxidation was not limited by diffusion. Removal of the surface organic layer reduced in situ diffusivity at the surface and improved its relationship with CH4 oxidation rate. Sites where soils had well-developed structure and a loose and permeable organic layer at the surface tended to have the highest CH4 oxidation rates. Core measurements, particularly of air permeability, could not be obtained at some sites owing to the inability to take suitable samples. Diffusivity measured in cores generally decreased with increasing depth of sampling in the topsoil, with the 50-to 100-mm depth giving the best correlation with CH4 uptake; cores from within this layer also gave the highest CH4 oxidation during laboratory incubation. Effective comparisons between sites were hampered by the differing responses of CH4 oxidation and diffusivity to soil properties. However, multivariate cluster analysis that included the above transport variables plus others relevant to CH4 oxidation (namely, soil texture; bulk density; airfilled porosity; pH; carbon, nitrogen, and water contents; presence and depth of organic layers; and N deposition) confirmed the importance of soil water content, structure and texture in distinguishing different soil and site conditions.

Mechanisms of carbon and nutrient release and retention in beech forest gaps

Fluxes of CO2 and N2O were measured along a microclimatic gradient stretching from the centre of a gap into a mature beech stand using an automated chamber method. Simultaneously the regulating factors like soil water tensions, soil temperatures, nitrate concentrations were measured along the gradient. The daily mean values of the fluxes of CO2 and N2O were divided into classes of temperature and furthermore subdivided into classes of soil water tension to assess the significance of each regulating factor.

Soil carbon stocks decrease following conversion of secondary forests to rubber (Hevea brasiliensis) plantations.

Forest-to-rubber plantation conversion is an important land-use change in the tropical region, for which the impacts on soil carbon stocks have hardly been studied. In montane mainland southeast Asia, monoculture rubber plantations cover 1.5 million ha and the conversion from secondary forests to rubber plantations is predicted to cause a fourfold expansion by 2050. Our study, conducted in southern Yunnan province, China, aimed to quantify the changes in soil carbon stocks following the conversion from secondary forests to rubber plantations. We sampled 11 rubber plantations ranging in age from 5 to 46 years and seven secondary forest plots using a space-for-time substitution approach. We found that forest-to-rubber plantation conversion resulted in losses of soil carbon stocks by an average of 37.4±4.7 (SE) Mg C ha−1 in the entire 1.2-m depth over a time period of 46 years, which was equal to 19.3±2.7% of the initial soil carbon stocks in the secondary forests. This decline in soil carbon stocks was much larger than differences between published aboveground carbon stocks of rubber plantations and secondary forests, which range from a loss of 18 Mg C ha−1 to an increase of 8 Mg C ha−1. In the topsoil, carbon stocks declined exponentially with years since deforestation and reached a steady state at around 20 years. Although the IPCC tier 1 method assumes that soil carbon changes from forest-to-rubber plantation conversions are zero, our findings show that they need to be included to avoid errors in estimating overall ecosystem carbon fluxes.

Interception and uptake of NH4 and NO3 from wet deposition by above-ground parts of young beech (Fagus silvatica L.) trees

Uptake of NH4 and NO3 by above ground parts of beech trees was studied by spraying young trees with varying concentrations of 15N labeled solutions, different N-forms, and spray regimes over four months. Following treatment, the trees were harvested and analyzed for 15N and major element content. Throughfall was collected and analyzed in addition in order to study the interaction between nitrogen uptake and cation leaching.Significant amounts of N were taken up by the above ground plant parts in all treatments as indicated by 15N analysis of the trees as well as by throughfall measurements. NH4 uptake exceeded the uptake of NO3 if applied in the same concentration. Uptake of N increased linearly with increasing concentration in the spray solution and with spray intensity. The uptaken N was translocated within the plant. The contribution of N from uptake by above ground parts to the total N content of tissues differed and reached a maximum level of 6% in leaves. No effect of above ground N uptake on the total N content of tissues was found.Calculating atmospheric N inputs to forest ecosystems by throughfall measurements may underestimate the actual N input.

Effects of prolonged soil drought on CH4 oxidation in a temperate spruce forest

Our objective was to determine potential impacts of changes in rainfall amount and distribution on soil CH4 oxidation in a temperate forest ecosystem. We constructed a roof below the canopy of a 65-year-old Norway spruce forest (Picea abies (L.) Karst.) and simulated two climate change scenarios: (1) an extensively prolonged summer drought of 172 days followed by a rewetting period of 19 days in 1993 and (2) a less intensive summer drought of 108 days followed by a rewetting period of 33 days in 1994. CH4 oxidation, soil matric potential, and soil temperature were measured hourly to daily over a 2-year period. The results showed that annual CH4 oxidation in the drought experiment increased by 102% for the climate change scenario 1 and by 41% for the climate change scenario 2, compared to those of the ambient plot (1.33 kg CH4 ha−1 in 1993 and 1.65 kg CH4 ha−1 in 1994). We tested the relationships between CH4 oxidation rates, water-filled pore space (WFPS), soil matric potential, gas diffusivity, and soil temperature. Temporal variability in the CH4 oxidation rates corresponded most closely to soil matric potential. Employing soil matric potential and soil temperature, we developed a nonlinear model for estimating CH4 oxidation rates. Modeled results were in strong agreement with the measured CH4 oxidation for the ambient (r2 = 0.80) and drought plots (r2 = 0.89) over two experimental years, suggesting that soil matric potential is a highly reliable parameter for modeling CH4 oxidation rate.