Hans Papen

The complete nitrogen cycle of an N-saturated spruce forest ecosystem.

Long-term nitrogen deposition into forest ecosystems has turned many forests in Central Europe and North America from N-limited to N-saturated systems, with consequences for climate as well as air and groundwater quality. However, complete quantification of processes that convert the N deposited and contributed to ecosystem N cycling is scarce. In this study, we provide the first complete quantification of external and internal N fluxes in an old-growth spruce forest, the Höglwald, Bavaria, Germany, exposed to high chronic N deposition. In this forest, N cycling is dominated by high rates of mineralisation of soil organic matter, nitrification and immobilisation of ammonium and nitrate into microbial biomass. The amount of ammonium available is sufficient to cover the entire N demand of the spruce trees. The data demonstrate the existence of a highly dynamic internal N cycle within the soil, driven by growth and death of the microbial biomass, which turns over approximately seven times each year. Although input and output fluxes are of high environmental significance, they are low compared to the internal fluxes mediated by microbial activity.

Long-term effects of clear-cutting and selective cutting on soil methane fluxes in a temperate spruce forest in southern Germany

Based on multi-year measurements of CH(4) exchange in sub-daily resolution we show that clear-cutting of a forest in Southern Germany increased soil temperature and moisture and decreased CH(4) uptake. CH(4) uptake in the first year after clear-cutting (-4.5 ± 0.2 μg C m(-2) h(-1)) was three times lower than during the pre-harvest period (-14.2 ± 1.3 μg C m(-2) h(-1)). In contrast, selective cutting did not significantly reduce CH(4) uptake. Annual mean uptake rates were -1.18 kg C ha(-1) yr(-1) (spruce control), -1.16 kg C ha(-1) yr(-1) (selective cut site) and -0.44 kg C ha(-1) yr(-1) (clear-cut site), respectively. Substantial seasonal and inter-annual variations in CH(4) fluxes were observed as a result of significant variability of weather conditions, demonstrating the need for long-term measurements. Our findings imply that a stepwise selective cutting instead of clear-cutting may contribute to mitigating global warming by maintaining a high CH(4) uptake capacity of the soil.

Development and application of a method for determination of net nitrification rates

A laboratory method was developed that allows determination of in situ net nitrification with high sensitivity and at high temporal resolution. Nitrate in soils is quantitatively converted into nitrous oxide under strictly anaerobic conditions in the presence of 10 kPa acetylene by the soil endogenous denitrifier population, with the N2O detected by a gas chromatograph equipped with a 63Ni electron capture detector. Thus, even low net nitrification rates, i.e. small net increases in soil nitrate concentrations can easily be detected. Comparison of results using this method with results obtained using the classical in situ incubation method (buried bag soil incubation) revealed excellent agreement. Application of the new method allowed both determination of the seasonal pattern of net nitrification as well as correlation analysis between in situ NO and N2O flux rates and in situ net nitrification rates of the forest soils studied. Regardless of the forest site studied (spruce, spruce limed, beech), and during each year of a 3 years period (1995–1997), net nitrification varied strongly with season and was least during winter and greatest during summer. The long-term annual, mean rate of net nitrification for the untreated spruce site, the limed spruce site and the beech site were 1.54 ± 0.27 mg N kg−1 sdw d−1, 1.92 ± 0.23 mg N kg−1 sdw d−1 and 1.31 ± 0.23 mg N kg−1 sdw d−1, respectively. In situ rates of nitrification and NO and N2O emission were strongly correlated for all sites suggesting that nitrification was the dominate source of NO as well as N2O.

Production of NO and N2O by the Heterotrophic Nitrifier Alcaligenes faecalis parafaecalis under Varying Conditions of Oxygen Saturation

Production of NO and N 2 O by the heterotrophic nitrifier Alcaligenes faecalis subsp. parafaecalis was studied during growth in batch and continuous culture on peptone-meat extract medium. Depending on oxygen saturation level, medium redox status and amount of substrate supplied, the microorganisms produced 0.002–0.25 mg NO-N h- 1 (g protein)- 1 and 0.16–2.4 mg N 2 O-N h- 1 (g protein)- 1 . Maximum rates of nitrogen oxides production were observed during peak events initiated by sudden changes of oxygen supply in the medium and were due to combined nitrification/denitrification taking place simultaneously within the cells. Based on model simulations of enzymatic kinetics of denitrification, possible mechanisms of increased nitrogen oxides production during periods of changes in oxygen supply are suggested.

The small-scale pattern of seepage water nitrate concentration in an N saturated spruce forest is regulated by net N mineralization in the organic layer

Soil net nitrogen (N) mineralization and nitrification as well as gross nitrification rates were studied in a forest soil within a 30 × 18m homogeneous plot located in an N saturated mature spruce stand at the Höglwald Forest (Bavaria, Germany) in order to explain the small-scale variation in nitrate (NO3−) concentration in seepage water. Seepage water was sampled below the main rooting zone in 40cm depth with suction cups over two periods at 20 measuring spots respectively. The sampling spots were uniformly distributed over the plot for both sampling periods, and represented the whole concentration range of seepage water NO3−concentrations measured within a close mesh of 121 suction cups. At each measuring spot soil net N mineralization, gross and net nitrification, heterotrophic soil respiration, extractable soil ammonium (NH4+) and NO3−, and additional physical and chemical soil parameters were measured in the organic layer and correlated with the NO3− concentrations in seepage water. Furthermore, the effects of environmental parameters on N conversion processes were evaluated using multiple linear regression analysis. We found that the small-scaled variations in seepage water NO3− concentration were related to similar small-scaled variations in key processes of microbial N turnover rates in the organic layer. Within this study net N mineralization in the organic layer could explain 51–59% of the corresponding small-scale variation of nitrate concentrations in seepage water below the main rooting zone using a multiple linear regression model with stepwise procedure. In addition, we found that small-scale patterns of N turnover in the organic layer were strongly influenced by water content in the organic layer and the dry mass of organic matter.

Inundation strongly stimulates nitrous oxide emissions from stems of the upland tree Fagus sylvatica and the riparian tree Alnus glutinosa

Background and aimsNitrous oxide (N2O) and methane (CH4) can be emitted from surfaces of riparian plants. Data on the emission of these greenhouse gases by upland trees are scarce. We quantified CH4 and N2O emissions from stems of Fagus sylvatica, an upland tree, and Alnus glutinosa, a riparian tree.MethodsThe gas fluxes were investigated in mesocosms under non-flooded control conditions and during a flooding period using static chamber systems and gas chromatographic analyses.ResultsDespite differences in the presence of an aerenchyma system, both tree species emitted N2O and CH4 from the stems. Flooding caused a dramatic transient increase of N2O stem emissions by factors of 740 (A. glutinosa) and even 14,230 (F. sylvatica). Stem emissions of CH4 were low and even deposition was determined (F. sylvatica controls). The results suggest that CH4 was transported mainly through the aerenchyma, whereas N2O transport occurred in the xylem sap.ConclusionsFor the first time it has been demonstrated that upland trees such as F. sylvatica clearly significantly emit N2O from their stems despite lacking an aerenchyma. If this result is confirmed in adult trees, upland forests may constitute a new and significant source of atmospheric N2O.

The Apoplast of Norway Spruce (Picea Abies) Needles as Habitat and Reaction Compartment for Autotrophic Nitrifiers

By the combination of both, the molecular biological fluorescence in-situ hybridization-technique with the technique of confocal laser scanning-microscopy (cLSM) it could be unequivocally demonstrated for the first time at the genetic level that autotrophic nitrifiers are present inside spruce needles of a spruce forest ecosystem (The “Hoglwald”) exposed to high levels of atmospheric nitrogen deposition and that they are located within the apoplastic space of the needle leaves (sub-stomatal cavity). In contrast, autotrophic nitrifers could not be detected in needles of adult trees at a spruce forest site (“Villingen”) exposed to low levels of atmospheric N input. When needles from adult spruce trees at the Hogwald site were exposed to 10 Pa acetylene – an inhibitor of ammonia monooxigenase of chemolithoautotrophic ammonia oxidisers (CAO) – the sink strength of the needles for NH3 decreased significantly. Since the reduction of NH3 deposition due to acetylene-induced inhibition of the ammonia monooxigenase was greatest when stomata were open and only minute when stomata were closed, it is concluded that physiologically active CAO are located inside the needles rather than on the needle surface. On the other hand, a reduction of NH3 uptake when applying acetylene was not observed with adult spruce trees from the nitrogen limited stand at Villingen. From the results obtained it is concluded that the observed NH3 flux from the atmosphere into the needle leaves in N-polluted forests is not exclusively a plant physiological process as has been assumed in the past, but is the result of both plant physiological plus microbial processes. A seasonal pattern of the colonization of the needles by nitrifiers at the Hoglwald site could not be demonstrated. For gaining first insights into the pathway by which needles might be colonized by autotrophic nitrifiers, sterile spruce seedlings fumigated with both ammonia and air were inoculated in the laboratory with nitrifier cultures. A successful establishment of the autotrophic nitrifiers in the phyllosphere of the spruce seedlings could not be achieved by a single inoculation event, however, was successful after multiple inoculations. It is concluded that autotrophic nitrifiers obviously are only able to colonize spruce needles in a later stage of spruce development.