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Gayana. Botánica

versión impresa ISSN 0016-5301versión On-line ISSN 0717-6643

Gayana Bot. v.62 n.2 Concepción  2005

http://dx.doi.org/10.4067/S0717-66432005000200003 

 

Gayana Bot. 62(2): 72-87, 2005 ISSN 0016-5301

ARTÍCULOS REGULARES

 

NO AND N2O FLUXES FROM A BELGIAN FOREST AFFECTED BY ELEVATED NITROGEN DEPOSITION

FLUJOS DE NO Y N2O EN UN BOSQUE BELGA AFECTADO POR UNA ELEVADA DEPOSITACION DE NITROGENO

 

Pascal Boeckx, Hilde Vervaet & Oswald van Cleemput

Laboratory of Applied Physical Chemistry (ISOFYS), Faculty of Bioscience Engineering, Ghent University, Coupure 653, 9000 Gent, Belgium. pascal.boeckx@ugent.be


ABSTRACT

Nitrous oxide (N2O) and nitric oxide (NO) are important atmospheric trace gases, and soils (including forest soils) are a substantial source of both gases. In forests subjected to elevated N deposition these emissions are considered as indirect emissions from agricultural N sources (mainly NH3) or combustion (e.g. NOx from traffic). However, knowledge about N2O and NO exchange between forests and the atmosphere is scarce. The aim of this study was to determine N2O and NO emission rates from a deciduous forest in Belgium, receiving a high N-deposition (ca. 40 kg N ha-1 yr-1). In April 2000, the NO emission from the forested location was 17.3 (± 1.2) and 17.05 (± 0.7) ng NO-N m-2 s-1, respectively, for the soil including the organic layer and the mineral soil only. In June 2000, the NO flux from the same location was 94.3 (± 14.1) and 53.8 (± 10.3) ng NO-N m-2 s-1, respectively, for the soil with organic layer and the mineral soil only. In April 2000, the NO emission from a clear-cut location in the same forest for the soil with organic layer was 30.5 (±1.52) ng NO-N m-2 s-1, whereas the flux from the mineral soil was 42.3 (± 1.33) ng NO-N m-2 s-1. In June 2000, however, the mineral soil emitted a significantly higher amount of NO than the clear-cut soil with organic layer (145.8 ± 7.2 ng NO-N m-2 s-1 versus 22.6 ± 3.1 ng NO-N m-2 s-1). The soil at the forested location acted as a net sink for N2O, with a mean annual flux of -0.63 ± 0.57 ng N2O-N m-2 s-1. In contrast the soil at the clear-cut site acted as a net N2O source, with a mean annual flux of 1.55 ± 0.90 ng N2O-N m-2 s-1. From these measurements it could be concluded that the forested soil was a significant secondary NO source and a N2O source or sink. However, more research is needed to investigate how N deposition, climate and forest management can affect N trace gas exchange in forest soils.

Keywords: Emission, forest soil, nitrogen cycling, nitric oxide, nitrous oxide.

RESUMEN

El óxido nitroso (N2O) y el óxido nítrico (NO) son importantes gases traza atmosféricos, y los suelos (incluyendo los suelos forestales) son una fuente sustancial de tales gases. En bosques sujetos a una elevada depositación de N, estas emisiones son consideradas como emisiones indirectas desde fuentes agrícolas (principalmente NH3) o combustión (por ejemplo, NOX desde el tráfico). Sin embargo, el conocimiento acerca del intercambio de N2O y NO entre los bosques y la atmósfera es escaso. El objetivo de este estudio fue determinar las tasas de emisión de N2O y NO desde un bosque deciduo en Bélgica, recibiendo una alta depositación de N (alrededor de 40 kg N ha-1 a-1). En abril del 2000, la emisión de NO desde el sitio forestal fue 17,3 ( ±1,2) y 17,5 (± 0,7) ng NO-N m-2 s-1, para el suelo incluyendo una capa orgánica y el suelo mineral, respectivamente. En junio del 2000, el flujo de NO desde el mismo sitio fue 94,3 (± 14,1) y 53,8 (± 10,3) ng NO-N m-2 s-1, para el suelo con capa orgánica y suelo mineral, respectivamente. En abril del 2000, la emisión de NO desde un sitio talado en el mismo bosque para el suelo con capa orgánica fue 30,5 (± 1,52) ng NO-N m-2 s-1, mientras que el flujo desde el suelo mineral fue 42,3 (± 1,33) ng NO-N m-2 s-1. En junio del 2000, sin embargo, el suelo mineral emitió una cantidad significativamente más alta de NO que el suelo talado con capa orgánica (145,8 ± 7,2 ng NO-N m-2 s-1 versus 22,6 ± 3,1 ng NO-N m-2 s-1). El suelo en el sitio forestal actuó como un sumidero neto de N2O, con un flujo promedio anual de - 0,63 ± 0,57 ng N2O-N m-2 s-1. En cambio, el suelo en el sitio talado actuó como una fuente neta de N2O, con un flujo promedio anual de 1,55 ± 0,90 ng N2O-N m-2 s-1. A partir de estas mediciones se podría concluir que el sitio forestal fue una significativa fuente secundaria de NO y una fuente o sumidero de N2O. Sin embargo, se requiere más estudio para investigar cómo la depositación de N, el clima y el manejo forestal pueden afectar el intercambio de los gases traza de N en los suelos forestales.

Palabras claves: Ciclaje de nitrógeno, emisión, óxido nitroso, óxido nítrico, suelo forestal.


INTRODUCTION

Because of their reactive nature in the atmosphere, the nitrogen oxides NO and NO2 (NOX, the sum of NO and NO2) play a central role in both stratospheric as well as tropospheric chemistry. Biogenic emissions from soils are the third important surface source of NO. This emission is the result of the natural N cycle. However, fertilizer use on agricultural soils or indirect fertilisation of natural soils has introduced an anthropogenic component to this natural source. According to present knowledge, NO is produced in soils nearly ubiquitously and constitutes a continuous background flux to the atmosphere (Ludwig et al. 2001).

Davidson & Kingerlee (1997) concluded that the global NO emission may be bounded by a modelled estimate of 10 Tg N yr-1 (Delmas et al. 1997) and an inventory estimate of 21 Tg N yr-1. Cultivated land (5.5 Tg N yr-1), forests (1.8 Tg N yr-1) and grassland/woodland (13.7 Tg N yr-1) are the most dominant terrestrial NO sources. Temperate forests and N affected forests account for 0.1 and 0.3 Tg N yr-1, respectively, to the 1.8 Tg N yr-1 from forest. Gasche & Papen (1999) estimated the contribution of N affected coniferous forest soils to the global NOX emission from soils at 0.3 Tg N yr-1, whereas the contribution of N affected deciduous forests would be negligible (< 0.02 Tg N yr-1).

Nitrous oxide (N2O) is one of the major greenhouse gases, accounting for 6-8% of the present enhanced greenhouse effect (IPCC 2001). Secondly, N2O is the major natural regulator of stratospheric O3.

The IPCC 2001 presented an overview of the different sources and budgets of N2O. The anthropogenic sub-total of 8.1 Tg N yr-1 is comprised of industrial sources (1.3 Tg N yr-1), biomass burning (0.5 Tg N yr-1), cattle and feedlots (Tg N yr-1) and agricultural soils (4.2 Tg N yr-1). The natural sub-total is 9.6 Tg N yr-1 comprised of open oceans (3 Tg N yr-1), atmospheric NH3 oxidation (0.6 Tg N yr-1), temperate forests and grasslands (1.0 Tg N yr-1, each) and tropical wet forests and dry savannas (3.0 and 1.0 Tg N yr-1, respectively). Thus, the total sources are 17.7 (range = 6.7-36.6) Tg N yr-1. The stratospheric sinks are estimated at about 12.3 Tg N yr-1.

Through a variety of biochemical processes, numerous groups of micro-organisms contribute to the production and consumption of NO and N2O in terrestrial ecosystems. The involvement of nitrification, denitrification - the major and biotic sources - and chemical decomposition of HNO2 (chemodenitrification), the decomposition of NH2OH and the reaction of NO2- with phenolic constituents of soil organic matter - the abiotic sources - is well established (Firestone & Davidson 1989; Williams et al. 1992b; Hutchinson & Davidson 1993; Bremner 1997). Nitrifying and denitrifying microorganisms produce NO and N2O as a byproduct or an intermediate of their metabolism (Firestone & Davidson 1989; Papke & Papen 1998). Only denitrification is recognised as a significant biological consumer of NO. However, abiotic oxidation processes are also identified to be NO consuming (Firestone & Davidson 1989; Gödde & Conrad 1998).

The three mostly reported environmental variables that control NO and N2O fluxes from soil are soil moisture content, soil temperature and soil N availability (Goodroad & Keeney 1984; Williams et al. 1992a; Sullivan et al. 1996; Mogge et al. 1998; Skiba et al. 1999). Other chemical (soil pH, concentration and composition of organic matter), physical (soil texture), and biological (plant cover) parameters might be of importance under more specific environmental conditions (Goodroad & Keeney 1984; Nägele & Conrad 1990; Meixner 1994; Ambus & Christensen 1995). Differences in climate, soil types and vegetation of similar sites make comparison of fluxes difficult.

Nitric and nitrous oxide emissions are highly variable, both temporally and spatially. This variability is a consequence of spatial heterogeneity and temporal changes in the underlying processes and the environmental factors controlling these processes. Soil can alternately act as a net source as well as a net sink of NO and N2O, depending on ambient concentrations (Ambus & Christensen 1995; Schiller & Hastie 1996; Butterbach-Bahl et al. 1997; Henrich & Hasselwandter 1997; Papke & Papen 1998; Skiba et al. 1999; Ludwig et al. 2001). Furthermore, the source/sink strenght of forest soils for N2O and NO may be significantly altered as a result of increased N deposition to natural ecosystems from anthropogenic activities (Butterbach-Bahl et al. 1998). Emissions of NO and N2O from temperate forest soils are generally very low. In Table I an overview of measured NO and N2O fluxes from forest soils in the literature is presented. For uniformity all fluxes were brought to the same unit.


In this paper we present data on in situ NO and N2O fluxes from a temperate forest in Belgium affected by elevated N deposition, mainly due to agricultural activities. The objective of the measurements was to assess whether the forest soil acted as a source or a sink of NO and N2O. In addition the effect of forest management (clear cut vs. forest areas), organic soil layer and seasonal fluctuations were assessed.

MATERIALS AND METHODS

Both NO and N2O fluxes were measured in situ at two locations in the study area "de Gulke Putten" (51°04'42'' N, 3°20'27'' E). The forest mainly consists of oak (Quercus robur L., Quercus rubra L.) and birch (Betula pendula Roth.), mixed with some serviceberry (Amelanchier laevis Wieg.), hornbeam (Carpinus betulus L.) and alder (Alnus incana L.). Further details concerning the forest area can be found in Vervaet (2003) and Vervaet et al. (2002).

Continuous NO measurements were conducted at two locations and during 2 periods. At the forested location (F), NO measurements were conducted from 11 to 18 April 2000 and from the 15 to 23 June 2000, with an interruption due to technical problems from 18 to the 21 June. At the clear-cut location (C) NO was measured continuously from 18 to 25 April 2000 and from the 26 June to 3 July 2000.

The NO emission was measured using a dynamic flow-through chamber (Fig.1). A Perspex chamber with a removable lid (length × width × height = 25 × 25 × 29.5 cm), covering 0.0625 m², was attached to a stainless steel frame. The frame was inserted into the soil to a depth of 0.05 m and was provided with a water-lock to prevent air leakage into or out of the chamber. The chamber inlet had an internal diameter of 6.2 cm (central point at ± 9 cm). The chamber inlet was equipped with an adjustable fan (150mA - DC, Jamicon) forcing air through the chamber. In the corners of the chamber opposite to the inlet two smaller fans (90 mA - DC, Jamicon) were installed to assure proper mixing of the headspace. The air left the chamber through an outlet (inner diameter = 2.5 cm with the central point at 27.5 cm). In order to prevent NO accumulation or suction of NO from the soil, the air speed of the inlet fans varied between 0.3 and 1.0 m s-1 to obtain an airflow rate ranging between 1.16 and 3.85 L s-1 through each chamber. This corresponded with a residence time between 5 and 16 seconds. Since no separate speed controllers were present for each fan, the air speed at the inlet of each chamber (after the fan) was determined at the start of each measurement occasion with a hot wire wind speed probe (Type 0635, Testo), connected to a logger (Model 454, Testo). An 8-channel sampling system, controlled by a T-step programming unit, allowed measurement of emissions from several chambers at the same time. Six sampler ports were reserved for the sampling of the chamber outlets; the two other ports were reserved for the sampling of incoming ambient air. The programming unit was set to switch between channels every 7.5 minutes. The sampler was connected to a NOx Chemiluminescence analyser (Model 42, Thermo Environmental Instruments), which was calibrated just before the start of each experiment. A calibration curve was made using at least 5 data points using a dilution of NO standard gas of 9.57 ppm in N2 (e.g. NO (ppb) = 0.9999441 × measured value (ppb); R² = 0.9995). A data logger (1200 series Squirrel meter/logger, Grant) registered data every 50 seconds. The NO flux (NO, ng NO-N m-2 s-1) was calculated from the concentration difference between the ambient air (Cair, ppb) and the outlet (Cout, ppb), the flow rate of air through the chamber (Q, L s-1) and the surface covered by the chamber (A, m²) and a conversion factor a (ppb to ng N L-1), as can be seen in formula (1):


FIGURE 1. Scheme of a dynamic chamber for NO flux measurements. The arrows show the gas flow in and out of the chamber; gas sampling occurred at the inlet and outlet of the chamber.

FIGURA 1. Esquema de una cámara dinámica para mediciones de flujos de NO. Las flechas indican el flujo de gas dentro y fuera de la cámara, la colecta de gas ocurre en la entrada y salida de la cámara.

At each measuring location, NO fluxes were measured at six plots (six dynamic chambers, three with the organic and three without the organic layer). Every hour during the measuring periods all six plots were measured and for every hour one average flux could be calculated with a standard deviation for the three plots with and the three plots without the organic layer. An integrated flux over the total measuring period was calculated as the surface below this average curve (NO flux versus time) divided by time. The standard deviation of the integrated flux was calculated based on the hourly standard deviations.

Monthly in situ N2O measurements were conducted from March 1999 until May 2000 (with the exception of June 1999, November 1999 and January 2000) at the forested location F and from July 1999 until June 2000 (with the exception of November 1999 and January 2000) at the clear-cut location C. For the in situ measurement of the N2O flux the vented closed box method was used (six replicates). Therefore, six bottomless boxes were pressed into the soil, and the gas flux was calculated from the measured accumulation or depletion of N2O in the headspace of the closed box. Perforated PVC cylinders, acting as a frame were pushed ± 10 cm into the soil, leaving ± 3 cm above ground level and remained there throughout the whole year during which monthly measurements were conducted in order to minimise soil disturbance. When a measurement was conducted, PVC cylinders were fitted airtight over these frames (height = 16.5 cm and cross-sectional area of 177 cm²), leaving an enclosed volume of 2.9 litres. Only during the measurement boxes were closed. The removable lid of each chamber was equipped with a vent (inner diameter = 0.4 cm, length = 2 m) to ensure equilibrium of air pressure inside and outside the chamber. Each chamber was connected to a CBISS Intelligent 6-channel sampler (CBISS Ltd., England), with two 5 m long Teflon tubes (inner diameter = 0.4 cm). The sampler was set to automatically switch from one box to the next every three minutes. Each box was sampled 5 times over a total period of 72 min. The collected air sample was automatically transferred to a Multi-Gas Monitor Type 1302 (Brüel & Kjær, Denmark), equipped with optical filters allowing the measurement of N2O, CH4, CO2 and water vapor. Gas concentration measurements were based on the photo-acoustic infrared detection method. The chambers, sampler and Multi-Gas Monitor were connected to each other in a closed loop. Traditionally, fluxes are assumed to be constant during the total measurement and therefore a linear regression can be used to quantify the N2O flux (Matthias et al. 1980). Because the flux can decrease during the measurement, due to gas accumulation in the headspace, Hutchinson & Mosier (1981) proposed an improvement that accounted for this decrease. De Visscher (2001) proposed a refinement of this method. By means of a simple statistical test the choice between a linear regression and a nonlinear regression based on the Hutchinson and Mosier model was automated. This computer model was used to calculate fluxes. In most cases the linear regression was chosen by the model, which was expected because measurements were conducted in a natural and not in an agricultural ecosystem. However, in some cases the model used the "Hutchinson and Mosier model" because the linear model would underestimate the real flux. Contamination between the boxes was corrected for with the method of De Visscher et al. (2000).


FIGURE 2. Setup-of the photo-acoustic gas analyser and the multipoint sampler connected to six closed chambers for N2O emissions measurements.

FIGURA 2. Estructura del analizador foto-acústico de gases y el colector múltiple conectado hacia seis cámaras cerradas para mediciones de las emisiones de N2O.

Fluxes were measured only during daytime. The fluxes from the six boxes at each location were averaged and a standard deviation was calculated. Because fluxes were only measured once a month, the yearly flux was estimated as the average monthly flux. However, this flux is only indicative. More intensive monitoring is needed to obtain a reliable annual flux.

Together with the N2O flux, also the CO2 flux was measured simultaneously in the closed boxes using the same method.

Soil and air temperature, moisture content, NO3- and NH4+ content were also measured at the same day of the trace gas flux. Soil mineral N (NH4+-N and NO3--N) and the gravimetric moisture content (GMC) have been determined by classical methods (ISO 11465 1993; ISO/DIS 14256-1 1997). Soil temperature was determined at 5 cm depth. For the determination of the soil mineral N and the gravimetric moisture content a composite sample of 10 sub-samples was taken of the 0-10 cm organic and mineral layer together and of the mineral 0-10 cm separately. Therefore no error bars could be calculated for the temperature, moisture and inorganic N content. The water-filled pore space (WFPS) was calculated from the gravimetric moisture content of the mineral layer. This parameter can only be used as an indication, since the organic layer is not taken into account in this parameter. Although, it plays an important role in the gas emissions.

Total C, total N, pHH2O, texture, bulk and particle density were determined by classical methods (Walkley & Black 1934; Springer & Klee 1953, 1955; De Leenheer 1966; Blake & Hartge 1986a, 1986b; Gee & Bauder 1986; Soil Survey Staff 1990; ISO 11465 1993; ISO 11261 1995; ISO/DIS 14256-1 1997). Statistical differences of N trace gas fluxes were tested using a paired sample t-test.

RESULTS

The general soil properties of the two locations are presented in Table II. Only the organic layer and the 0-10 cm layer of the mineral soil were taken into account. For the mineral N content, a sample was taken from the 0-10 cm layer containing the organic layer and part of the mineral layer.


NO FLUX

In April 2000, this 0-10 cm layer contained 10 kg ha-1 NH4+-N and 4 kg ha-1 NO3- -N for location F and 6 kg ha-1 NH4+-N and 3 kg ha-1 NO3- -N for location C. In June, these contents were 15 kg ha-1 NH4+-N and 10 kg ha-1 NO3- -N for location F and 5 kg ha-1 NH4+-N and 3 kg ha-1 NO3- -N for location C. The results of the monitoring are presented in Fig. 3 and Fig. 4 for location F and in Fig. 5 and Fig. 6 for location C, respectively. In April, samples were collected from the mineral 0-10 cm layer, the organic layer and the mixed organic/mineral layer at the beginning, the end and in between the measurements in order to determine the gravimetric moisture content and/or the WFPS. In June, technical problems occurred and only the samples of the mixed organic/mineral layer could be recovered. In Table III the moisture contents are presented. Only the moisture content of the mixed organic/mineral layer was measured during June 2000, due to technical problems (Table III).


A pronounced diurnal effect of the air temperature could not be observed for the NO emissions at location F in April 2000. Soil temperature at 5 cm depth showed a flattened diurnal pattern. Over the total period, the air temperature averaged 8.5 (± 5.1) °C whereas the soil temperature averaged 7.9 (± 1.1) °C. Within this measuring period, the flux of the soil with the organic layer did not differ significantly (P<0.05) from that of the mineral layer. The flux, calculated over the total period, was 17.3 (± 1.2) ng NO-N m-2 s-1 for the measurements with organic layer and 17.05 (± 0.7) ng NO-N m-2 s-1 for the mineral soil only. During this measuring period no NO uptake has been observed.


FIGURE 3. NO flux (filled diamonds show the NO flux from the spot with organic layer as moving average, empty diamonds show the NO flux from the spot without organic layer as moving average), air temperature (filled triangles) and soil temperature (horizontal lines) from location F in April 2000.

FIGURA 3. Flujo de NO (rombos achuradas indican el flujo de NO desde el punto con horizonte orgánico como promedio móvil, rombos vacíos indican el flujo de NO desde el punto sin horizonte orgánico como promedio móvil), temperatura del aire (triángulos achurados) y temperatura del suelo (líneas horizontales) en la localidad F en abril del año 2000.

In June 2000, the NO emissions were much more variable (Fig. 4), changing from uptake to emission. The air and soil temperature averaged 15.9 (± 3.5) and 14.5 (± 0.9) °C, respectively. Within this period, the flux from the soil with the organic layer was 94.3 (± 14.1) ng NO-N m-2 s-1 and significantly different (P>0.05) from the mineral soil flux of 53.8 (± 10.3) ng NO-N m-2 s-1. The fluxes observed in June 2000 were significantly different (P<0.05) from those in April 2000.


FIGURE 4. NO flux, air and soil temperature from the location F in June 2000 (legend see Fig. 3).

FIGURA 4. Flujo de NO, temperatura del aire y del suelo en la localidad F en Junio del año 2000 (ver leyenda Fig. 3)

In April, the average air and soil temperature at location C was 11 (± 1.62) °C and 10.5 (± 0.8) °C, respectively. The NO flux from the soil with the organic layer was 30.5 (±1.52) ng NO-N m-2 s-1, whereas the flux from the mineral soil was 42.3 (± 1.33) ng NO-N m-2 s-1 (Fig. 5). Fluxes were significantly different (P<0.05) from each other.


FIGURE 5. NO flux, air and soil temperature from the location C in April 2000 (legend see Fig. 3).

FIGURA 5. Flujo de NO, temperatura del aire y del suelo en la localidad C en abril del año 2000 (ver leyenda Fig. 3).

In June at location C the mineral soil emitted a significantly higher amount of NO (Fig. 6) than the soil with the organic layer (145.8 ± 7.2 ng NO-N m-2 s-1 versus 22.6 ± 3.1 ng NO-N m-2 s-1). No specific trend of the NO flux could be observed during this measuring period. The air temperature averaged 18.3 (±9.7) °C and the soil temperature 15.2 (± 1.4) °C.


FIGURE 6. NO flux, air and soil temperature from the location C in June 2000 (legend see Fig. 3).

FIGURA 6. Flujo de NO, temperatura del aire y del suelo en la localidad C en junio del año 2000 (ver leyenda Fig. 3).

N2O FLUX

The results of the year-round monthly measurements are presented in Fig. 7 and Fig. 8 for location F and C, respectively. The monitoring was too extensive to study statistically significant correlations between the different soil parameters. However, some trends could be observed and will be discussed.


Figure 7. Measurements conducted at location F from March 1999 - May 2000; from top to bottom: soil and air temperature and gravimetric moisture content, N2O flux, CO2 flux, and mineral N content.

Figura 7. Mediciones realizadas en la localidad F desde marzo de 1999 - mayo 2000; desde arriba hacia abajo: temperatura del suelo y del aire y contenido de humedad gravimétrica, flujo de N2O y CO2, y contenido de N mineral.


Figure 8. Measurements conducted at location C from july 1999 - june 2000; from top to bottom: soil and air temperature and gravimetric moisture content, N2O flux, CO2 flux, and mineral N content.

Figura 8. Mediciones realizadas en la localidad C desde julio de 1999 - junio 2000; desde arriba hacia abajo: temperatura del suelo y del aire y contenido de humedad gravimétrica, flujo de N2O y CO2, y contenido de N mineral.

At location F, the N2O flux increased from March 1999 to May 1999, which coincided with increasing temperature, inorganic N content and decreasing moisture content. The maximum N2O flux (3.5 ng N2O-N m-2 s-1) was observed in May 1999 at an air temperature of 21 °C, soil temperature of 13°C, a moisture content of 69% GMC (gravimetric moisture content) (or ± 63% WFPS) and an inorganic N content of 1.4 g N m-2. From May 1999 to August 1999 the N2O emission decreased, changing from emission towards uptake (-2.9 ng N2O-Nm-2s-1), although temperature and N content were still increasing, but the soil moisture content reached a minimum of 43% GMC (or ± 39% WFPS). From August 1999 to April 2000 the N2O uptake continued, while the moisture content increased and the inorganic N content and temperature decreased and remained rather constant during the winter period. In May 2000, N2O emission was observed again (1.2 ng N2O-N m-2 s-1). The moisture content remained at the same level as observed in wintertime (> 60% GMC), but the temperature and the inorganic N rose again. The CO2 flux followed the same trend as the soil and air temperature. The annual N2O flux was estimated at -0.63 ± 0.57 ng N2O-N m-2 s-1.

At location C (Fig. 8), the N2O flux decreased from 5.6 ng N2O-N m-2 s-1 in July 1999 to -0.3 ng N2O-N m-2 s-1 in October 2000. Temperature and inorganic N content followed the same decreasing trend, while soil moisture content remained almost constant at 40% GMC (± 70% WFPS). The N2O uptake continued till April 2000 under conditions of low temperature and inorganic N content and moisture content with only minor variation. The maximum flux at location C was observed in May 2000 (5.8 ng N2O-N m-2 s-1) at 14°C soil temperature, 21°C air temperature, 60% GMC (or ± 100% WFPS) and an inorganic N content of 900 mg m-2. The CO2 and N2O flux, temperature and inorganic N content at the clear-cut location followed the same trend. The annual N2O flux was estimated at 1.55 ± 0.89 ng N2O-N m-2 s-1.

DISCUSSION

NO FLUX

Too little data on moisture content were available to study correlations between the NO flux, temperature and moisture content. Because the data on moisture content were minimal, it is difficult to exactly explain differences in NO fluxes by differences in moisture content during both periods and at both locations. However, the results suggest that the moisture content was a driving parameter at location F. At this location, the vegetation prevents a direct influence of temperature compared to location C. The NO emissions - and more in particular the spots without the organic layer or during the measuring campaign of June 2000 - seemed to show a lag-phase with respect to temperature. This means that maximum NO emissions were observed after maximum soil temperatures. Soil temperature was the main parameter influencing the NO flux of the mineral layer at location C, which could be expected due to the direct impact of light and temperature. In the literature, it was also reported that the driving influence of both moisture content and temperature was site-specific and circumstantial (Sullivan et al. 1996; Pilegaard et al. 1999; Ludwig et al. 2001). Year round measurements of NO fluxes, temperature and moisture content are needed to obtain a better insight in the temperature dependence and the moisture content determining the NO flux.

Comparing the results of both measuring periods at both locations with the data summarized in Table I, it could be concluded that the fluxes were of the same order of magnitude as fluxes from the N affected forests. However, it should be taken into account that the fluxes in this study were measured during the period in which the highest fluxes were expected according to Papke & Papen (1998) and Williams et al. (1992a). Therefore, the fluxes cannot be extrapolated towards an annual basis. The forest received on annual basis 34 kg N ha-1 yr-1 in 1999-2000 and the open area received 19 kg N ha-1 yr-1 in the same year (Vervaet 2003). According to Gasche & Papen (1999) the NO flux can amount to up to 15% of the N input. Because of the short measuring period, the monthly average deposition on the forested and clear-cut location was considered and the measured flux was extrapolated to a monthly flux to obtain a rough idea of the proportion of NO-N flux versus N-deposition level. For this study, the NO-N flux was between 15 % and 85% of the average monthly N input. This value cannot be extrapolated toward a yearly basis because 85% of the annual NO flux usually occurs in spring and summer (Williams et al. 1992a). However, these values could indicate that the percentage of 2.5% of the N input, proposed by Skiba et al. (1998, 1999), would be exceeded by the NO flux solely. According to this indicator, both the clear-cut and the forested area could be N saturated. However, more extensive long-term research is needed to confirm this hypothesis (Butterbach-Bahl et al. 2004).

From the results, a varying role of the organic layer was observed. At the forested location, the organic layer is replenished every year and played an active role in the NO emission. In April 2000 (Fig. 3) no difference between the plots with and without organic layer could be observed, whereas the organic layer contributed significantly to the NO flux in June 2000 (Fig. 4). This finding was in agreement with the results of Papke & Papen (1998) and Gasche & Papen (1999). Pilegaard et al. (1999) reported a NO flux from a beech forest floor of 0.31 ng NO-N m-2 s-1 in April/May and a flux of 3.40 ng NO-N m-2 s-1 in June. At the clear-cut location, the organic layer was not replenished from February 1999 on and as a consequence has been decomposing and became more compact. The mineral soil emitted a large amount of NO in June (Fig. 6). However, the organic layer formed a barrier and prevented NO efflux as proposed by Ball et al. (1997) and Williams & Fehsenfeld (1991). It should be further investigated whether the reduction of the NO flux by the organic layer was due to a barrier to gaseous diffusion or due to consumption by the organic layer.

N2O FLUX

The soil acted as a source or sink for N2O at both locations. This was also observed by Ryden (1981), Ambus & Christensen (1995), Schiller & Hastie (1996), Butterbach-Bahl et al. (1997), Henrich & Hasselwandter (1997) and Skiba et al. (1999). For both locations, N2O uptake was consistent during wintertime. However, at the forested location, the uptake remained for a longer period of time. The overall flux for the forested area resulted in N2O uptake, whereas the overall flux at the clear-cut location resulted in N2O emission. The interaction of temperature, moisture content and inorganic N content influenced the N2O flux. When temperature was optimal the flux was defined by the moisture content and/or the inorganic N content and vice versa. Maximum N2O fluxes were measured at a moisture content > 60% WFPS; however, optimal moisture contents differed. This was in agreement with data from the literature (Ambus & Christensen 1995; Schiller & Hastie 1996; Ambus 1998; Castaldi & Smith 1998; Mogge et al. 1998).

The fluxes measured at the two locations fitted within the ranges reported from other ecosystems (Table I). Since ranges of the N unaffected and the N affected forests overlap, it is not obvious to which group the locations belong. In spite of high N deposition on the forest (34 kg N ha-1 yr-1), the forested area was still acting as an overall sink. It is possible that the N deposition reduced the N2O uptake capacity of the forest soil and changed location F into a N2O source as a result of elevated N deposition. Location C was clear-cut in February 1999. It is possible that the clearing has induced increased N2O emission and that this is a more pronounced steering parameter than N deposition. At location C, 3% of the N that was deposited in the open field (19 kg N ha-1 yr-1), was emitted again.

 

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Received 18/02/05
Accepted 07/06/05

 

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