In situ methane and nitrous oxide fluxes in soil from a transect in Hennequin Point, King George Island, Antarctic

Chemosphere 90 (2013) 497–504

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In situ methane and nitrous oxide fluxes in soil from a transect in Hennequin Point, King George Island, Antarctic Frederico Costa Beber Vieira a,⇑, Antônio Batista Pereira a, Cimélio Bayer b, Adriano Luis Schünemann a, Filipe de Carvalho Victoria a, Margéli Pereira de Albuquerque a, Cássio Strassburger de Oliveira a a Universidade Federal do Pampa, Campus São Gabriel, Instituto Nacional de Ciência e Tecnologia Antártico de Pesquisas Ambientais, INCT-APA, Avenida Antônio Trilha, 1847, Centro 97300-000, São Gabriel, RS, Brazil b Departamento de Solos, Universidade Federal do Rio Grande do Sul, P.O. Box 15100, 91540-000 Porto Alegre, RS, Brazil

h i g h l i g h t s " The effects of vegetal cover and sea birds on soil greenhouse gases were evaluated. " Birds and vegetation increased the soil methane uptake and nitrous oxide emission. " Nitrous oxide was the main component of the global warming potential. " Poor-drainage effect on soil gases fluxes was alleviated by its coarse texture.

a r t i c l e

i n f o

Article history: Received 23 April 2012 Received in revised form 31 July 2012 Accepted 3 August 2012 Available online 11 September 2012 Keywords: Methane Nitrous oxide Skua colonies Vegetal cover Moss

a b s t r a c t The study aimed at to determine the magnitude of the methane (CH4) and nitrous oxide (N2O) flux rates in soils at Hennequin Point, King George Island, Antarctic, under different slope positions, vegetal covers and presence of skuas, as well as to evaluate the main soil and climate factors that are involved with the flux of such gases. In situ gas sampling (closed chamber method) was performed in four sites along a transect involving a skua nesting field in a moraine with 5% and 100% of surface covered by vegetal, and two poor-drained soils in the toeslope (a bare alluvium soil and a poor-drained moss field with 100% soil cover). Flux rates ranged from 0.86 ± 0.45 to 2.75 ± 1.52 lg N2O–N m2 h1 and 12.26 ± 3.05 to 1.42 ± 1.31 lg CH4–C m2 h1. The soil totally covered by vegetal in the skua field had the largest CH4 influx rates. However, this benefic effect was counterbalanced by the greatest N2O efflux rates from this soil, resulting in the largest contribution to the global warming potential among the soils evaluated. Flux rates were closely related to soil temperature, but no significant relation was observed with mineral N contents and water-filled pore space. In turn, accumulated CH4 and N2O emissions were closely related to the total N and total organic C stocks in the soil. Net CH4 influx predominated even in the poor-drained soils, suggesting that the coarse soil texture avoided critical anaerobic conditions. No significant changes in flux rates were observed for sampling time along the day. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Soils can act as source or sink of the three main greenhouse gases (GHG) – carbonic dioxide, methane and nitrous oxide (CO2, CH4 and N2O, respectively), depending on its type of use and management practices (IPCC, 2007). However, in natural austral ecosystems as the ice-free areas of the Maritime Antarctica, studies evaluating the soil CH4 and N2O fluxes and their driving factors regarding soil and climate characteristics are scarce. ⇑ Corresponding author. Tel./fax: +55 55 3232 6075. E-mail address: [email protected] (F.C.B. Vieira). 0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2012.08.013

In the last decade, pioneer studies in the region have demonstrated that sea animals are crucial to the GHG fluxes in ice-free Antarctic soils. Larger emission rates of CH4 and N2O were observed in ornithogenic soils with penguin guano (Sun et al., 2002; Zhu et al., 2009a), penguin fresh drop addition (Zhu and Sun, 2005; Zhu et al., 2005) and seal colonies (Zhu et al., 2008a,b,2009a) than non-ornithogenic tundra soils covered by moss or lichens. However, few studies have investigated the fluxes of CH4 and N2O in soils of nesting/breeding fields of the flying bird skua (Zhu et al., 2008a,b).

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The increase in GHG emission rates by sea birds is directly related to their effect on nutrient cycling in such areas, contributing to transfer C and N from the marine to the terrestrial environment (Tatur, 2002; Park et al., 2007) and increasing their microbial activity (Zdanowski et al., 2005). The ocurrence of vegetal communities, in turn, increases the potential of the soils in the maritime Antarctic to act as sink of atmospheric C, through its retention as organic C in the soil profile (Michel et al., 2006; Park et al., 2007; Simas et al., 2008). Sites with more developed vegetal communities tend to have greater soil total organic C (TOC) and total N (TN) stocks (Simas et al., 2008). The occurrence of vegetated fields in the icefree regions is frequently accompanied by the presence of sea animals, in a mutual interaction. The incipient ornithogenic influence can increase the vegetation development, which would consequently increase the TOC contents in the soil (Simas et al., 2008). The TOC and TN contents of these soils may range from near to null in bare soils up to more than 12% and 4%, respectively (Simas et al., 2008; Zhu et al., 2009a), in ornithogenic soils covered with vegetation. However, few attempts have tried to quantify the effect of vegetal and animal communities on GHG soil fluxes as separated factors. The type of vegetal species seem to imply different CO2 emission, with largest emission from soils with flowering plant species than in moss carpets (Mendonça et al., 2010), but very few is known regarding the vegetal composition effect on the soil fluxes of CH4 and N2O. In addition to C and N availability, the GHG fluxes in maritime Antarctic soils may be affected by the soil temperature and precipitation (Zhu and Sun, 2005; Zhu et al., 2005; Bokhorst et al., 2007; Park and Day, 2007). In tundra wetlands, the water table level also seems to be a decisive factor (Zhu et al., 2008c). Both soil temperature and water table level are affected by the landscape position and, in spite of the studies cited above, more information is necessary to better understand the interaction and the effectiveness of these factors in regulating the CH4 and N2O fluxes in soils of the region. This is particularly important when the contrasting soil chemical, physical and biological attributes are taken into account (Navas et al., 2008; Simas et al., 2008; Ugolini and Bockheim, 2008; Francelino et al., 2011). The objectives of this study were to: (1) determine the magnitude of CH4 and N2O fluxes in soils of Hennequin Point, Maritime Antarctica, under different topographical positions, skua presence and vegetal covers, (2) assess the main soil and climate factors that are driving the fluxes of such gases and (3) evaluate the daily oscillation of the soil gas fluxes.

2. Materials and methods 2.1. Local climate and soil characteristics The study was carried out on February of 2011, during the 29th Brazilian Antarctic Operation. Soil GHG gases fluxes were evaluated in Hennequin Point (58°230 W; 62°070 S), Admiralty Bay, King George Island, Antartic, in the Equatorian Refugee surroundings (Fig. 1). Four sites were selected in a transect containing different soil characteristics and colonizations. The first two points, with about 50 m distant of each other, were placed in a lower moraine inside of a nesting/breeding field of the seabird skua (Catharacta maccormicki) where the hair grass Deschampsia antarctica Desv. (Poaceae) and the pearlwort Colobanthus quitensis (Kunth) Bartl. (Caryophyllaceae) predominated. The sampling sites were established on different soil coverage by plants (100% and <5%) inside the same field and are herein called DC100 and DC5, respectively. In addition to the two species above cited, Usnea antarctica (Du Rietz) I. M. Lamb, Buellia anisomera Vain, Leptogium puberulum

Hue, Syntrichia magellanica (Mont) R. H Zander, Psoroma sp. were found in the low cover site, while Sanionia uncinata (Hedw.) Loeske, Ochrolechia frigida (Sw.) Lynge, Prasiola crispa (Lightfoot) Menegh., Polytrichastrum alpinum (Hedw) G. L. Smith and Andreae gainii Card. were found in the 100% soil cover site. The third point was located about 50 m below of the first two, in a place characterized by sedimentation of eroded soils from the ice smelting (alluvium soil – AS). At this place, no lichens or plants were present. The fourth site was located about 100 m below the third, inside a S. uncinata carpet with 100% cover (SU100), near to a lake shore. No other plant species was present in the sampling area. Some soil physical and chemical attributes for the four sites evaluated in the transect are depicted in Table 1. Water table level was not detected in DC100 and DC5 up to 80 cm depth, but was kept at about 40 cm depth in AS and SU soils (data not shown). 2.2. Air sampling and greenhouse gases analysis Soil fluxes of CH4 and N2O were evaluated by using the static chambers method. In each one of the four sites, three bases (constituting three replicates) made of stainless steel and with an inner diameter of 0.20 m were inserted into the soil at 5 cm depth. On each base, PVC chambers of 0.25 m diameter  0.1 m height were allocated on a canal fulfilled with water for hermetically close the chamber. In order to avoid chambers movements by wind, two iron-made poles were fixed into the soil, in opposite positions and at about 10 cm out of each base, in which a rubber band was stretched over the chamber during the sampling events. Air sampling events occurred at 20/02/11, 23/02/11, 26/02/11 and 01/03/11. The soils with DC100 and DC5 were sampled concomitantly, starting at 8 a.m., while AS and US100 were also sampled concomitantly, starting at 10 a.m. Additionally, in order to evaluate the oscillations in soil GHG fluxes along the day, the areas with DC100 and AS were sampled at 0, 6, 12 and 18 h of the 21/02/ 11. In each chamber, air samplings were taken at 0, 30, 60 and 90 min after the chamber closing, using a 20 ml polypropylene syringe with triple Luer Lock valve, and injected in a recipient Exetainer 12 mL (Labco Ltd., High Wycombe, United Kingdom) submitted to manual vacuum immediately before the sampling. Air temperature inside the chamber and soil temperature at 5 cm depth near to the chambers were monitored during the sampling events, through digital thermometers allocated in one replicate of each treatment. Air samples were kept at low temperature (<10 °C) and were taken to Brazil. The concentration of CH4 and N2O were determined by gas chromatography (Shimadzu GC2014 ‘‘Greenhouse’’ model) at the lab of Environmental Biogeochemistry (UFRGS) in up to 25 d after the sampling moment. The chromatograph was equipped with three packed columns working at 70 °C, N2 as a carrier gas at a flow of 26 mL min1, injector with loop for direct sampling of 1 mL and temperature set at 250 °C, electron capture detector (ECD) at 325 °C for N2O detection and flame ionization detector (FID) at 250 °C for CO2 and CH4. The gas fluxes were estimated based on the following equation:

f ¼

DQ PV 1 Dt RT A

ð1Þ

where f is the nitrous oxide or methane flux (lg m2 h1 N2O or CH4), Q is the quantity of gas (lg N2O or CH4) in the chamber headspace at the sampling moment, P is the atmospheric pressure (atm) in the inner chamber-assumed as 1 atm, V is the chamber volume (L), R is the constant for ideal gases (0.08205 atm L mol1 K1), T is the temperature inside the chamber at the sampling moment (K) and A is the base area of the chamber (m2). Rates of increase in gas concentration inside the chamber were estimated by using the

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Fig. 1. Location of the investigation area and sampling sites in Hennequin Point, Admiralty Bay, King George Island, Antarctica, in the Equatorian Refugee surroundings.

linear regression adjusted from the relation between time and gas concentrations. From the rates of CH4 and N2O fluxes, daily emissions were estimated and emission for the total period (9 d) was calculated by integrating the daily emissions (Gomes et al., 2009). If we consider that the C input to the soil–plant systems of the locals evaluated in the present study are in equilibrium with the decomposition rates, we can infer that the contribution of each evaluated local to the global warming potential (GWP) basically depends on the net balance of N2O and CH4 fluxes. Therefore, based on the accumulated CH4 and N2O emission and taken into account the global warming potential of each gas in comparison to the carbon dioxide–CO2 (25 times for CH4 and 298 times for N2O), the emissions in CO2 equivalent were calculated in order to get the Global Warming Potential (GWP), assuming that C input/output by photosynthesis and decomposition, respectively, was stable in the evaluated soils. 2.3. Soil sampling and analysis In each GHG sampling event, soil samples were taken for analþ ysis of mineral nitrogen (nitrate – NO 3 and ammonium – NH4 by Kjeldahl distillation) and gravimetric soil moisture (oven-dried at 105 °C), determined according to Tedesco et al. (1995). Near to each chamber, soil samples were taken at three soil layers (0.00–0.10, 0.10–0.20 and 0.20–0.40 m) at three replicates for analysis of soil physical and chemical attributes. Soil bulk density

was determined by the core method and was employed to estimate the water-filled pore space (WFPS), in addition to the gravimetric moisture above described, assuming a particle density of 2.65 g cm3. Clay and silt contents were determined after dispersion with NaOH 1 M by separating the sand fraction by sieve and determining the clay content by the densimeter method (Tedesco et al., 1995). Soil pH was determined in a pH meter in a 2:1 ratio. Total organic C (TOC) contents were determined by dry combustion using a total organic C analyzer (Shimadzu TOC-VCSH, Shimadzu Corp., Kyoto, Japan). Total N contents were determined by acid digestion followed by Kjeldahl distillation (Tedesco et al., 1995). 2.4. Statistical analysis Accumulated GHG emission and soil chemical attributes were submitted to one-way ANOVA, with Tukey test (P < 0.05) to separate means. Greenhouse gases flux rates in the soil are presented with their standard deviation values (n = 3). Relationships among GHG flux and soil and climate variables were evaluated by the significance of Pearson correlation coefficients. 3. Results The N2O emission rates (Fig. 2a) in the soil under presence of vegetation and birds (DC100) was larger than those for the other

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Table 1 Soil bulk density, clay and silt contents, and pH for the four sites of air sampling in a transect in Hennequin Point, Antarctica. Soil attribute

Soil layer (cm)

Site DC5a

Soil bulk density (g cm

a b

3

)

0–10 10–20 20–40

1.34 ± 0.11b 1.51 ± 0.09 1.55 ± 0.02

DC100 1.27 ± 0.14 1.29 ± 0.02 1.47 ± 0.14

AS 1.43 ± 0.06 1.42 ± 0.02 1.47 ± 1.47

SU100 1.49 ± 0.02 1.49 ± 0.05 1.54 ± 0.05

Clay content (g kg1)

0–10 10–20 20–40

17.2 ± 4.5 13.0 ± 2.8 4.2 ± 1.8

14.2 ± 1.3 11.9 ± 3.7 13.4 ± 7.3

8.6 ± 3.6 9.6 ± 4.0 7.6 ± 3.0

11.8 ± 0.9 4.9 ± 2.3 3.5 ± 0.9

Silt content (g kg1)

0–10 10–20 20–40

162.2 ± 53.4 198.6 ± 31.2 266.1 ± 18.1

225.9 ± 103 173.7 ± 38.7 231.3 ± 38.2

149.3 ± 30.1 72.9 ± 8.9 199.2 ± 54.2

55.9 ± 16.1 3.1 ± 2.7 0.0 ± 0.0

pH–H2O

0–10 10–20 20–40

6.50 ± 0.44 6.67 ± 0.29 6.40 ± 0.26

5.47 ± 0.25 5.37 ± 0.21 5.50 ± 0.00

6.03 ± 0.15 6.23 ± 0.12 6.17 ± 0.12

5.87 ± 0.06 5.93 ± 0.06 6.00 ± 0.00

DC5: 5% soil cover with Deschampsia + Colobanthus; DC100: 100% soil cover with Deschampsia + Colobanthus; AS: alluvium soil; SU100: 100% soil cover with Sanionia. Values are means (n = 3) ± standard deviations.

soils in all sampling events, ranging from 0.21 ± 0.39 to 4.32 ± 2.39 lg N2O m2 h1, followed by the soil under presence of birds with low cover by vegetation (DC5). The soil under moss cover and the bare alluvium soil had the smallest N2O rates, with predominance of slightly negative values or close to zero. The contents of TOC and TN followed the same tendency, with larger values in the soils under DC100 and DC5 than for the other two places (Fig. 3). At the alluvium soil, the TN and TOC contents were extremely low (mean of 0.14 g TN dm3 and 0.13 g TOC dm3 at the 0–10 cm soil layer), corroborating with the smallest NH4–N contents (Fig. 4) and with the absence of N2O efflux from this soil. The soil TOC stocks (0–40 cm) for DC5, DC100, AS and SU100 were 9.03, 43.08, 0.51 and 15.13 Mg ha1, respectively, while soil NT stocks were 2.60, 6.54, 0.71 and 3.51 Mg ha1, respectively. Influx rates of CH4 in soil predominated during the evaluated period, ranging from 1.42 ± 1.30 to 12.3 ± 3.0 lg C m2 h1 for AS and DC100 soil, respectively (Fig. 2b). The soils that had greater CH4 consumption were the DC100 and DC5, while the fluxes for AS and SU100 were kept close to zero (Fig. 2c). Although TN contents were significantly different among the four sampling sites (Fig. 3), differences in mineral N contents (Fig. 4) were not so evident. Nitrate and ammonium contents in soil were low, with values smaller than 7 and 1 mg N dm3, respectively. No significant correlation between soil mineral N content and soil flux rates of N2O and CH4 (P > 0.05; Table 2) was observed. On the opposite, soil temperature at 5 cm depth had a close relation with flux rates of both gases (r = 0.584, P = 0.018 and r = 0.653, P = 0.006 for N2O and CH4, respectively), demonstrating a consistent difference in temperature for each local of the transect, with greater values in the soil of the moraine (DC5 and DC100; mean of 2.7 °C) than for AS and SU100 (mean of 1.6– 2.1 °C, respectively). Results of Fig. 2c represent the accumulated emission of each gas for the period of evaluation, converted to equivalent C (EC) by taking into account their warming potential. Nitrous oxide emission had larger relative contribution to the net balance of gases to GWP than CH4 for the four sites of the transect. Therefore, although sites DC5 and DC100 had net CH4 uptake, the N2O accumulated emission in these locals imparted a positive net balance for global warming, while AS and SU100, on the opposite, are contributing to mitigate the GWP. The accumulated N2O and CH4 emissions had close relation to the soil TOC at the 0–40 cm layer (r = 0.794, P = 0.002 and r = 0.609, P = 0.036, respectively) and TN stocks (r = 0.836, P < 0.001 and r = 0.643, P = 0.024, respectively) for the same soil layer (Table 2). These relations were less significant when the

stocks from the soil surface layer (0–10 cm) were employed. Apart from the relation between N2O accumulated emission and TOC stock (r = 0.670, P = 0.017), the other relations were not significant (P > 0.05), suggesting that deeper layers contributed effectively to the fluxes of these gases. Flux rates of N2O and CH4 emission were not affected by the sampling time along the day for both the soils evaluated (Fig. 5). The contrasting soil cover (completely covered in the DC100 against bare soil in the AS) demonstrated no clear interaction with the N2O and CH4 fluxes. A larger oscillation in daily temperature was observed in the bare alluvium (3.4 °C) soil than in the DC100 soil (2.5 °C).

4. Discussion 4.1. Effect of skuas and vegetal covers on soil N2O and CH4 fluxes Despite the relatively low flux rates of greenhouse gases in the soil evaluated in this study, we demonstrated that there may be marked difference in such fluxes according to the presence of skuas, flowering plants and moss species covering the soil. Such presence, in our study, seems to be more important than the topographic position, although the topographic position is closely related to the water table depth and temperature in the soil. The additional effect of flowering plants + skuas promoted to the DC100 the greatest N2O emission and also the greatest CH4 oxidation capacity in comparison to the other soils. If we assume that the skuas influence was similar for DC100 and DC5, the basic difference between these two near-sites is the abundance of vegetation, which in turn seemed to intensify the birds effect on the CH4 and N2O fluxes in the soil. The decisive factors for the fluxes seem to be the soil TOC and TN contents, which are effectively affected by the presence of birds and vegetal species (Barrett et al., 2006; Hopkins et al., 2006; Zhu et al., 2009a). It is worth to mentioning that the soil TOC and NT stocks in the DC100 reached values similar to those found in ornithogenic soils with penguin rookeries (Simas et al., 2008). The mineral N contents, however, had no effect on the fluxes of N2O and CH4 in our study. The sites with the smallest TOC and NT contents in the present study were coincidently the coldest soils. Therefore, the significant relations between soil temperature and CH4 and N2O emission rates (Table 2) were probably dissembled by this coincidence, although the soil temperature (with its large spatial variation) seems to play a crucial role in the greenhouse gas emission in the maritime Antarctic (Zhu and Sun, 2005; Zhu et al., 2008c).

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0,9 0,6

2

0,3 0,0

0

-0,3 -2

-0,6 1. DC5 2. DC100 3. AS 4. SU100

-4

-1,2

(b)

2

5

1

0

0

-5

-1 -2

-10 -3 -15 Feb/20/2011 Feb/23/2011 Feb/26/2011

CH4 flux rate (g C ha-1 d-1)

Mar/1/2011

Date

Accumulated emission for 9 days (g CE ha -1)

800

5

10

15

20

25

(a)

10

20 DC5 DC100 AS SU100 MSD Tukey (P<0.05)

40

Total nitrogen (g dm-3) 0

1

2

3

4

0

(b)

10

(c)

N2O CH4

A

600

0 0

20

40

Net balance (N2O+CH4

Carbon:nitrogen ratio

400 0

200 0 B

b

B

-400 DC5

DC100

AS

4

6

8

10

(c)

a

a

ab

-200

2

0

AB

SU100

Site Fig. 2. Rates of nitrous oxide (N2O, a) and methane (CH4, b) flux in soils of four sites in a transect in Hennequin Point, Antarctic, and their accumulated emissions (c) for 9 d of evaluation. DC5: 5% soil cover with Deschampsia + Colobanthus; DC100: 100% soil cover with Deschampsia + Colobanthus; AS: alluvium soil; SU100: 100% soil cover with Sanionia carpet. Vertical bars (figs. a and b) mean the standard deviation of the mean (n = 3). Means followed by the same letters in (c) (uppercase for N2O and lowercase for CH4) do not differ by Tukey test at P < 0.05.

10

Depth (cm)

CH4 flux rate (µg C m-2 h-1)

-6 10

-0,9

Total organic carbon (g dm-3)

Depth (cm)

4

1,2

Depth (cm)

N2O flux rate (µg N m-2 h-1)

(a)

N2O flux rate (g N ha-1 d-1)

6

20

ns

40

4.2. Effect of landscape position on soil N2O and CH4 fluxes

Fig. 3. Soil contents of total organic carbon (a), total nitrogen (b) and carbon:nitrogen ratio (c) in four sites in a transect in Hennequin Point, Antarctica. DC5: 5% soil cover with Deschampsia + Colobanthus; DC100: 100% soil cover with Deschampsia + Colobanthus; AS: alluvium soil; SU100: 100% soil cover with Sanionia carpet. Horizontal bars mean the minimum significant difference (MSD) by Tukey test at P < 0.05 (n = 3).

The greatest capacity of CH4 oxidation in the soils under presence of birds and vegetation of this study is probably linked to the better soil conditions to the methanotrophic bacteria communities. The soils with DC100 and DC5 had better soil drainage (mainly imparted by the landscape position), larger clay and silt contents and larger inputs of C and N to the soil, resulting in greater stocks of these elements and in greater CH4 oxidation capacity. The more anoxic conditions imparted by the proximity of the water table level (about 40 cm) has probably restricted the methanotrophy in the soils of AS and SU100 sites, but such restriction did not promote net CH4 emission as it would be expected (Fig. 3). We conceive that the coarse texture of these soils holds up the emergence of anoxic conditions in the more biologically active soil layer in the surface (Skiba and Ball, 2002). Similarly to CH4, the presence of the water table seemed to promote no significant increases in N2O emission. Actually, the poordrained AS and SU100 soils had net N2O uptake, independently of the TOC and NT contents – note that SU100 had soil TOC and

TN contents relatively similar to the well-drained DC5. This is in agreement with the findings of Dobbie and Smith (2006), whose modeling results suggested a decrease of 50% in soil N2O emission if water table depth would be kept below 35 cm from the surface. In addition, the water table was considered the major factor controlling the spatial variations of N2O fluxes from soils of two tundra wetlands in eastern Antarctica (Zhu et al., 2008c). The relation between WFPS and N2O and CH4 flux rates was not as close as with soil temperature, but it was significant at P = 0.10 level (Table 2). However, it is interesting to note in Fig. 4 that WFPS surpassed 70% – which is considered a threshold value for the N2O emission (Bateman and Baggs, 2005) – in two occasions (DC100 in the first sampling event and DC100 + SU100 in the third event). Coincidently, the largest N2O emission rates for DC100 were found at these two occasions, suggesting that the anaerobic conditions imparted by the greater WFPS probably reached threshold values in which, above it, denitrification was intensified, increasing the

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Feb-20-2011

Feb-23-2011

Feb-26-2011

Mar-01-2011

90

WFPS (%)

80 70 60 50 40 1. DC5 2. DC100 3. AS 4. SU100

1.0 0.8

4.3. Magnitude of N2O and CH4 fluxes rates

0.6 0.4 0.2 0.0 8 7 6 5 4 3 0 4

o

Soil temperature ( C)

N-NO3 (mg dm-3)

N-NH4 (mg dm-3)

0 1.2

et al., 2006). In addition, the water table proximity might help to explain the reason why the accumulated GHG fluxes had better relation to the TOC and NT stocks of the 0–40 cm soil layer than to the surface 0–10 cm layer. Such behavior is presumably linked to the more anaerobic conditions at subsurface layers (Hutsch, 1998; Le Mer and Roger, 2001), although the greatest organic matter concentration in surface soil layers tends to foster the largest contribution to the GHG fluxes in some soils. In our results, though the deeper soil layer was important for both gases, it seemed particularly critical to CH4.

3 2 1 0 Feb-20-2011

Feb-23-2011

Feb-26-2011

Mar-01-2011

Date Fig. 4. Water-filled pore space (WFPS), mineral N contents (NO3 and NH4) and temperature in soil in four sites in a transect in Hennequin Point, Antarctica. DC5: 5% soil cover with Deschampsia + Colobanthus; DC100: 100% soil cover with Deschampsia + Colobanthus; AS: alluvium soil; SU100: 100% soil cover with Sanionia. Horizontal bars mean the minimum significant difference (MSD) by Tukey test at P < 0.05 (n = 3).

N2O emission. For the SU100, although the WFPS was also >70% in the third sampling event, the N2O flux rates were negligible. The fact that WFPS >70% was critical for increasing the N2O emission in DC100 soil but was not for SU100 is primarily due to the different contents of total and mineral N (Figs. 3 and 4, respectively), but a more intense biological activity expected in DC100 than SU100 probably exacerbated the difference on the anaerobiosis (Hopkins

The N2O and CH4 flux rates in the soils of the transect were similar to those obtained in previous studies performed in ice-free areas of maritime Antarctic ecosystems. In our results, the average N2O flux rates were 0.33 ± 1.32, 1.56 ± 0.88, 0.42 ± 0.79 and 0.49 ± 2.47 lg N m2 h1 for the DC5, DC100, AS and SU100, respectively. In soils of two sites under upland tundra at the Fildes Peninsula, Zhu et al. (2005) found average N2O flux rates of 0.7 ± 1.4 and 0.38 ± 1.08 lg N m2 h1, but the rates increased to 2.35 ± 1.27 lg N m2 h1 in tundra soil site with penguin dropping addition. Our N2O flux rates, however, are smaller than those reported for ornithogenic soil of a penguin colony at Ardley Island, Fildes Peninsula (5.92–143.56 lg N m2 h1; Sun et al., 2002) and for lakeshore soils at Garwood Valley (26.4–132.0 lg N m2 h1; Gregorich et al., 2006) In a skua colony at the Fildes Peninsula, Zhu et al. (2008b) reported a peak of 1324.3 lg N m2 h1, although in most of sampling events the flux rates did not surpass 10 lg N m2 h1. The poor-drained soil under moss (SU100) had rates that fit the range of 13.1–54.5 lg N m2 h1 found in wet tundra sites under algae and moss at Wolong Marsh (Zhu et al., 2008c). The range of CH4 flux rates in the present study (12.26– 1.42 lg C m2 h1) is slightly smaller than those found for moss soils (42.45–98.85 lg C m2 h1) reported by Sun et al. (2002). However, in the present study, the presence of skuas and plant cover in 100% of soil surface (DC100) favored the increase of the influx in comparison to the DC5, AS and SU100, evidencing that the presence of birds and vegetation improved the methane oxidation capacity of these soils, while, oppositely, Sun et al. (2002) found larger CH4 emission rates in ornithogenic (penguin) soil than in soils with moss and lichen covering. Possibly, the larger N contents (16.48%) in the ornithogenic soil by the authors than in the present study (<0.3%) promoted NH4 contents high enough to restrict the CH4 oxidation by methanotrophic bacteria (Hutsch, 1998; Suwanwaree and Robertson, 2005; Acton and Baggs, 2011) and,

Table 2 Correlations among greenhouse gases flux rates and soil and climate variables (n = 16) and correlations among accumulated gases emissions and total organic C and total N stocks (n = 12). WFPSa

Gases

NH4

NO3

N2O flux rates

r P

0.429 0.097

0.230 0.471

0.081 0.803

0.584 0.018

CH4 flux rates

r P

0.439 0.088

0.246 0.441

0.305 0.335

0.653 0.006

TOC stock

a b

Soil temperature

TN stock 0–10 cm

0–40 cm

N2O accum.b

r P

0.670 0.017

0.794 0.002

0.552 0.063

0.836 <0.001

CH4 accum.b

r P

0.409 0.187

0.609 0.036

0.227 0.478

0.643 0.024

Water-filled pore space. Accum.: accumulated emission in 9 d of evaluation.

0–10 cm

0–40 cm

5 4 3 2 1 10

0

-5

-1

-10 5

-2

CH4 flux rate (µg CH4 m h )

2. DC100 3. AS

5

0

503

soil quality attributes in general, with emphasis on the most advanced soil formation observed in the profile and the greatest COT and NT contents. Such characteristics were probably decisive to imply the largest CH4 consumption rates among the soils tested, but such characteristics also promoted the greatest N2O emission accumulated in the period. On the other side, the bare alluvium soil (AS) had the smallest TOC and TN contents, in addition to a negligible value of CH4 oxidation accumulated in the period. However, this soil demonstrated the greater influx of N2O, promoting the most negative values of GWP or, in other words, the most mitigating value. This is particularly important if we take into account that the percentage of areas from the ice-free maritime Antarctic without animals and flowering plants is large (Peat et al., 2007; Victoria et al., 2009). Occasional influx rates of N2O have been previously documented for Antarctic soils (Sun et al., 2002; Zhu et al., 2008b,c).

6

-2

-1

N2O flux rate (µg N2O m h )

o

Soil temperature ( C)

F.C.B. Vieira et al. / Chemosphere 90 (2013) 497–504

4.5. Daily oscillation on N2O and CH4 fluxes

-5 -10 -15 -20 -25 0

6

12

18

Time (h) Fig. 5. Soil temperature (a), rates of nitrous oxide (N2O, b) and methane (CH4, c) flux in soils of two sites in a transect in Hennequin Point, Antarctic, along 24 h. DC100: 100% soil cover with Deschampsia + Colobanthus; AS: alluvium soil. Vertical bars mean the standard deviation of the mean (n = 3).

in addition to a likely greater biological activity and surface soil compaction, contributed to the largest CH4 emission rates. 4.4. Relative contribution of N2O and CH4 fluxes to Global warming potential The fact that all soils had negative accumulated emission of CH4 (including those soils where water table was not deep-AS and SU100) is an indicative that the methanotrophic microorganisms are well adapted to the Antarctic climate conditions. In addition, the predominance of methanotrophy instead of methanogenesis corroborates with the fact that those soils have suffered negligible influence of anthropic effects (Hutsch, 1998; Suwanwaree and Robertson, 2005; Omonode et al., 2007). However, in equivalent C, the contribution of N2O fluxes in the soils of the transect (Fig. 5) is relatively much more important for the net global warming potential than that from CH4 (mean N2O:CH4 ratio of 5.5 times). Therefore, taking into account only the N2O and CH4 fluxes in these areas, the negative or positive contribution to the global warming potential was basically determined by the N2O flux rates, despite net CH4 influx was observed in all soils. The magnitude of the GWP values was relatively low – the greatest value, verified for DC100, is equivalent to 0.5 kg of equivalent-C per hectare. Supposing a similar contribution during the ice-free period in a year, such value would reach about 5 kg EC ha1, which is still a low value. However, peaks of emission rates are expected in the thawing period at the beginning of summer and in the freezing-thawing cycles during the summer (Zhu et al., 2009b). In a holistic view, one can perceive that the soil with 100% cover with vegetation and with presence of skuas (DC100) had the best

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