CH 4 and N 2 O fluxes from planted forests and native Cerrado ecosystems in Brazil

: Forest soils are N 2 O sources and commonly act as CH 4 sinks. This study evaluated the dynamics of the CH 4 and N 2 O fluxes of soils under Eucalyptus plantations and native Cerrado vegetation, as well as possible interactions between environmental factors and fluxes. The study was carried out in the Distrito Federal, Brazil, during 26 months, in three areas: in two stands of the hybrid Eucalyptus urophylla × Eucalyptus grandis , planted in 2011 (E1), and in 2009 (E2) and native Cerrado vegetation (CE). Measurements to determine the fluxes in a closed static chamber were carried out from Oct 2013 to Nov 2015. Soil and climate factors were monitored. During the study period, the mean CH 4 fluxes were –22.48, –8.38 and –1.31 μg CH 4 m –2 h –1 and the mean N 2 O fluxes 5.45, 4.85 and 3.85 μg N 2 O m –2 h –1 from E1, E2 and CE, respectively. Seasonality affected plantations in the studied sites. Cumulative CH 4 influxes were calculated (year-1: –1.86 to -0.63 kg ha –1 yr –1 ; year-2: –1.85 to –1.34 kg ha –1 yr –1 ). Cumulative N 2 O fluxes in the three sites were ≤ 0.85 kg ha –1 yr –1 . The change in land use from Cerrado to Eucalyptus plantations did not significantly changed regarding greenhouse gases (GHG), compared to the native vegetation. Flux rates of both gases (N 2 O and CH 4 ) were low. Temporal variations in GHG fluxes and different ages of the stands did not cause significant differences in cumulative annual fluxes.


Introduction
Changes in land use have led tropical soils to estimated emissions of 0.2 Gt C yr -1 , accounting for 10-30 % of total C emissions from deforestation (Houghton, 1999;Achard et al., 2004). In 2010, GHG emissions were estimated at 49 × 10 9 Mg CO 2 eq, of which 21 to 24 % were generated by agriculture, forestry, and other land uses (Tubiello et al., 2013;IPCC, 2014). In the context of a low-carbon economy, forest stands can be an option to reduce pressure on the native vegetation and mitigate the effects of climate change (Bonan, 2008). In forest stands, Eucalyptus is the second most commonly planted genus, due to its high adaptability, fast growth, and economic value (Iglesias-Trabado et al., 2009). Moreover, nutrients released in forest litter may enhance stabilization mechanisms of soil organic carbon (SOC) (Huang et al., 2011).
Land-use changes alter chemical, physical, and biological properties of the soil, modifying the GHG fluxes (Kim and Kirschbaum, 2015). In addition, forest type (Masaka et al., 2014) and soil management practices play an important role (Kim et al., 2016), as well as water-filled pore space (Santos et al., 2016;Smith, 2017), soil O 2 contents, pH, plants, and N input (Hickman et al., 2015;Carvalho et al., 2017). Moreover, long-term N inputs may favor soil C stocks, mainly in litter with a high lignin content (Grandy and Neff, 2008). Microbial communities can be altered by climate warming, N fertilization, pH and the C:N ratio (Högberg et al., 2007;Lucas et al., 2007). In particular, metabolic responses of the microbial community to N enrichment are complex and highly variable (Michel and Matzner, 2003).
Native forests and commercial Eucalyptus stands of different ages have been little studied about the potential emission or consumption of GHG, but they are main drives for GHG fluxes in the Cerrado region. This study investigated GHG emissions with a two-year GHG monitoring program in natural and planted forests. Tree seedlings were planted in E1 in Dec 2011, at 3.5 m × 1.7 m spacing in a site previously covered with native Cerrado vegetation. Soil tillage consisted of disking and subsoiling in the planting row to a depth of 1.2 m.
The tree seedlings in E2 were planted in Dec 2009 (3.5 m × 1.7 m). The site had previously been used for cropping soybean (2003 to 2005), sorghum (2006), and soybean again (2007)(2008)(2009). Soil tillage consisted of heavy disk harrowing (width 1.5 m) and opening furrows in the center of the row to a depth of 25 cm.
In the Eucalyptus plantations (E1 and E2), soil acidity was corrected by incorporating dolomitic limestone (2.5 t ha -1 ) to a depth of 20 cm and with 700 kg ha -1 agricultural gypsum applied on the soil surface two months later. Fertilization at planting consisted of NPK (5-25-15), corresponding to 16.52 kg N ha -1 . After one year, a side dressing of 60 kg K 2 O ha -1 was applied in the form of potassium chloride, 50 kg N ha -1 in the form of urea, and 1 g boron per plant in the form of borax. In Jan 2014, the application of 60 kg K 2 O ha -1 was repeated.
The soil of the experimental site was classified as clayey Oxisol (Typic Haplustox) (Soil Survey Staff, 2006). The soil chemical properties and density (0-5 and 5-10 cm depth) are listed in Table 1. Table 2 shows the litter quality in E1, E2 and CE. The C:N ratio in the litter ranged from 66:1 to 75:1 and the lignin content, from 99.5 to 162.0 The regional climate is Aw (rainy tropical), according to Köppen-Geiger classification (Cardoso et al., 2014), with two well-defined seasons: dry season (May-Sept) and rainy season (Oct-Apr). The mean annual precipitation observed in the last 40 years was 1345.8 mm (Silva et al., 2017b). The precipitation in year-1 and year-2 was 1210.9 and 1305.6 mm, respectively, and > 94 % of the rain was concentrated in the rainy season. The average air temperature was 21.4 and 22.2 °C in year 1 and 2, respectively ( Figure 2).

Measurements of CH 4 and N 2 O fluxes
The CH 4 and N 2 O fluxes were measured from Oct 2013 to Nov 2015, distinguished in year-1 (Oct 2013 to Sept 2014) and year-2 (Oct 2014 to Nov 2015). The years were analyzed separately to identify the behavior of GHG fluxes and contributions of the soil-climate variables.
The closed-chamber method was used for measurements (Alves et al., 2012), with a sampling frequency of three times per month (Zanatta et al., 2014), since the sites with fully established Eucalyptus trees were not fertilized with synthetic N. Three 30 m × 30 m plots were randomly delimited in each site. To ensure representativeness of the system, four closed chambers were installed per plot, two in the Eucalyptus rows and two in-between rows, spaced 10 m. The chambers in the rows did not receive N fertilization after the first experimental year. In CE, four static gas chambers were placed randomly in each plot, resulting in 36 chambers installed in the sites. Each closed chamber consisted of a metal base (0.38 m × 0.58 m) inserted into the soil, and an upper part of PVC (height 9.5 cm), coated with a thermal aluminum blanket, which, together with the metal base, sealed the space covered by the chamber, where the gases accumulated for later collection and determination.    A hole was drilled in the center of the top part of each chamber and connected to a rubber hose and a three-way valve, by which the gas outlet could be controlled at sampling. Digital thermometers were installed to monitor the air temperature within the chambers.
The soil temperature was measured at 5 cm deep with a digital thermometer at the sampling times. The gas samples were captured between 09h00 a.m. and 11h00 a.m., following the recommendation of Alves et al. (2012), to best represent the daily average flux. The air trapped in the chambers was sampled at 0, 15, and 30 min after closing the device. A 60-mL polypropylene syringe was used, coupled with a three-way valve, in which 30 mL gas samples were collected and transferred to vials. In addition, one sample of atmospheric gas per plot was taken as reference to analyze gas samples. Before and after sampling, the vials were transported in ice-cooled thermal boxes and stored in a refrigerated environment at 16 ºC for measurements.
The CH 4 and N 2 O concentrations were determined with a gas chromatograph (Trace 1310 GC ultra) equipped with a Porapak Q column at 65 °C, an electron capture detector (ECD) and a flame ionization detector (FID). The following standards were used: 200, 600, 1000 and 1500 ppb N 2 O; and 1000, 5000, 10000 and 50000 ppb CH 4 . The calculated detection limit was 51 ppb for N 2 O and 145 ppb for CH 4 and the calculated quantification limit 154 ppb for N 2 O and 484 ppb for CH 4 . The CH 4 and N 2 O fluxes were measured by the linear variation in gas concentration in relation to the incubation time in closed chambers, and calculated by Equation (1), as proposed by Bayer et al. (2015): where in the flux (µg m -2 h -1 ); dC/dt is the change in gas concentration (nmol N 2 O and CH 4 h -1 ) in the chamber in the incubation interval (t); V and A are, respectively, the chamber volume (m 3 ) and the soil site covered by the chamber (m 2 ); m is the molecular weight of N 2 O or CH 4 (µg), and Vm is the molar volume at the sampling temperature.
The fluxes were calculated for the sampling times 0, 15, and 30 min, expressed in µg N 2 O m -2 h -1 and µg CH 4 m -2 h -1 . The average daily N 2 O and CH 4 fluxes were calculated from the average value of the four chambers installed per plot. To determine the cumulative fluxes, the area under the curve was integrated, based on the daily N 2 O and CH 4 soil fluxes (Santos et al., 2016). The amount of equivalent carbon (C eq) required to mitigate the cumulative annual fluxes of CH 4 and N 2 O was calculated by Equations 2 and 3, respectively. CH 4 = (CAI* 16/12) * GWP * F ( 2) where: CAI is the cumulative annual influx (kg CH 4 ha -1 yr -1 ); GWP is the global warming potential of CH 4 (25 kg CO 2 , IPCC (2007)); F is the factor 0.273 (used for the conversion from CO 2 to C); where: CAF is the cumulative annual flux (kg N 2 O ha -1 yr -1 ); GWP is the global warming potential of N 2 O (298 kg CO 2 , IPCC (2007)); F is the factor 0.273(used for conversion from CO 2 to C).

Environmental factors
For each gas sampling, soil samples from the 0-5 cm layer were also collected to determine mineral N in the forms of nitrate (NO 3 -) and ammonium (NH 4 + ), at eight points near the chambers, forming a composite sample. From each soil sample, a sub-sample was taken to determine gravimetric soil moisture. For the extraction of NO 3 and NH 4 + , we used 50 mL of a solution of 2 mol L -1 KCl , according to the methodology of Bremner and Mulvaney (1982). The solution was analyzed by spectrophotometry with a system of flux injection analysis (FIA) (Hambridge, 2007a, b) to determine NO 3 and NH 4 + concentrations. Soil particle density was determined by the ring and volumetric flask methods, respectively (Embrapa, 1997). Soil moisture was calculated by oven-drying a soil sub-sample of known weight at 105 ºC for 48 h. From these variables, the water-filled pore space (WFPS in %) was calculated for each gas sampling date and determined by the equation: (4) where: the gravimetric moisture is expressed in %, BD is the bulk density (g cm -3 ) and PD is the particle density (2.65 g cm -3 ).
The meteorological data were recorded using a datalogger (CR 1000) installed near the study site.

Calculations and statistical analyses
The environmental variables were subjected to the descriptive statistical analysis and applied to the normality test (Shapiro-Wilk), followed by analysis of variance (ANOVA). The daily CH 4 and N 2 O fluxes had a non-normal distribution; therefore, the nonparametric Kruskal-Wallis test of medians was performed at 5 % probability to find possible differences between the areas and years studied by comparisons.
The CH 4 influxes and accumulated N 2 O fluxes for the sampling dates were calculated by linear interpolation. To compare sites and years, the data of accumulated fluxes and equivalent carbon were subjected to analysis of variance (ANOVA) and the Tukey test (p < 0.05).
Another analysis evaluated the seasonality effect and the relationship between CH 4 and N 2 O fluxes and the environmental variables by the Pearson's correlation, using the FactoMineR package of program R (version 3.2.2). Only significant correlations are shown in the text.
The mean annual N 2 O fluxes were 4, 8, and 3 µg m -2 h -1 for year-1 and 5, 5, and 4 µg m -2 h -1 for year-2 in E1, E2, and CE, respectively ( Figure 3B). Average N 2 O fluxes in the rainy season for E1, E2, and CE were, respectively, 1.00, 4.00, and 0.07 µg m -2 h -1 (year-1) and 7, 4, and 3 µg m -2 h -1 (year-2), and in the dry season 10, 14, and 7 µg m -2 h -1 (year-1) and 3, 7, and 5 µg m -2 h -1 (year-2) ( Figure 3B). In year-1, significant differences between N 2 O fluxes were observed in the rainy season, with the highest average N 2 O fluxes in E2 (p = 0.0196). However, in year-2, differences were only significant in the rainy season between the average fluxes in E1 and CE (p = 0.0273), which were higher for E1. In all GHG evaluation, an uptake of CH 4 was observed in 44 % of the evaluations in CE, but in 19 % in the Eucalyptus stands. For N 2 O, the average fluxes were 5, 6, and 4 µg m -2 h -1 in E1, E2, CE, respectively. In this study, the results of CH 4 and N 2 O fluxes in the Eucalyptus rows and interrows were not shown, since the differences were not significant.
The environmental variables showed that the CH 4 pulse in µg m -  Figures 3A and 4A).
In year-1, the Cerrado was the only site with a correlation of 0.58 (p = 0.0018) and -0.68 (p = 0.0015) between the CH 4 flux and WFPS, respectively, in the rainy and dry seasons. Only in E2, correlations were detected between N 2 O fluxes and WFPS in the rainy and dry seasons (0.63 (p = 0.0005) and -0.59 (p = 0.0041), respectively). In year-2, WFPS ranged from 19 % to 71 % in the rainy and from 30 % to 43 % in the dry season. With regard to correlations, CH 4 and WFPS in CE positively correlated (0.84; p < 0.0001). For N 2 O, a correlation with WFPS of 0.62 (p = 0.0190) was observed in E1, both correlations occurred in the dry season.
In year-1, the soil temperature in all sites remained between 17 and 24 ºC. The highest CH 4 and N 2 O pulses occurred at soil temperatures ≥ 19 ºC. With regard to correlations, CH 4 and soil temperature in CE positively correlated (0.65; p = 0.023). In year-2, the soil temperature ranged from 16 to 26 ºC and was correlated with CH 4 flux in E2 (0.59; p = 0.024).

Soil mineral N dynamics
The highest soil NO 3 and NH 4 + concentrations were recorded in the rainy season in all sites (p < 0.0240) ( Figures  4B and 4C) and did not coincide with the GHG pulse. In the rainy season of year-1, a correlation of -0.68 between CH 4 × NO 3 -(p = 0.0004) in E1 was identified of 0.49 and 0.53 between N 2 O × NO 3 -(p = 0.0033), and N 2 O × NH 4 + (p = 0.0023) in CE, while in the dry season, the correlation was 0.55 between CH 4 and NO 3 -(p = 0.0065) in CE.
The ammoniacal N was predominant ( Figure  4C). The NO 3 concentrations were higher in the rainy season (p < 0.0093). The N 2 O soil pulse coincided with NO 3 concentrations < 0.50 mg kg -1 of soil and only in some moments, NO 3 exceeded 2.00 mg kg -1 soil ( Figure 4B). The seasonality effect on NH 4 + concentrations differed only in CE, with higher soil concentrations in the dry season (p < 0.0001), coinciding with higher N 2 O pulse in all studied sites.
In year-2, NO 3 and NH 4 + soil concentrations were very close between the sites, except for E2, where concentrations were below 1 mg kg -1 of soil, from Feb 2015 onwards (Figures 4B and 4C). With regard to correlations, E1 was the only area in which N 2 O and NO 3 concentrations negatively correlated (-0.63; p = 0.002) during the dry season.

Cumulative CH 4 and N 2 O fluxes and carbon equivalent (C eq)
In the study period, cumulative fluxes were not influenced by annual variation, age of Eucalyptus stands, or by the replacement of native vegetation for Eucalyptus. For N 2 O, variations were 0.33 to 0.85 kg ha -1 yr -1 in year-1 and 0.32 to 0.43 kg ha -1 yr -1 in year-2 ( Table 3). The CH 4 values were negative, with cumulative influxes of -1.86 to -0.63 kg ha -1 yr -1 in year-1, and -1.85 to -1.34 kg ha -1 yr -1 in year-2 ( Table 3).
The C eq ranged from 42 to 108 kg ha -1 in year-1 and from 40 to 54 kg ha -1 in year-2. However, on an annual basis, the cumulative C eq for CH 4 was negative in all sites evaluated (-6 to -17 kg ha -1 in year-1; -12 to -17 kg ha -1 in year-2) ( Table 3).
The C eq ranged from 42 to 108 kg ha -1 in year-1 and from 40 to 54 kg ha -1 in year-2. However, on an annual basis, the cumulative C eq for CH 4 was negative in all sites evaluated (-6 to -17 kg ha -1 in year-1; -12 to -17 kg ha -1 in year-2) ( Table 3).

Temporal variability of CH 4 and N 2 O fluxes and environmental variables
The CH 4 influxes were predominant in this study (81 % for E1 and E2, and 57 % for CE), reaffirming that forest soils are common sinks of this gas. The same behavior was observed in other studies (Godoi et al., 2016;Liu et al., 2017). In general, well-drained soils consume atmospheric CH 4 (Ciais et al., 2013;IPCC, 2013) and indicate CH 4 uptake from the atmosphere mediated essentially by methanotrophic bacteria due to reduced soil water content, favoring diffusion of atmospheric CH 4 into soils (Hiltbrunner et al., 2012). Other studies focused on savannas also mentioned these uptake conditions. In Eucalyptus globulus stands and Australian savanna vegetation, respectively, Livesley et al. (2009) recorded influxes of -7 and -16 µg m -2 h -1 . In areas under cerradão, Siqueira-Neto et The CH 4 pulses in the three sites occurred in the dry season. Priano et al. (2014) found an inverse relation between CH 4 emission rate and WFPS. The results of this study also indicate that wet soils can oxidize as much CH 4 as relatively dry soils, with average WFPS values near 40 % in the study period. Previous reports also demonstrated that peaks of CH 4 soil oxidation occur at some intermediate level of soil water content (Khalil and Baggs, 2005). According to Humer and Lechner (1999), the optimum saturation degree for oxidation is between 40 and 80 % (moisture content between 25 and 50 %). Moreover, measuring potential methane (CH 4 ) oxidation rates in semiarid soil, Sullivan et al. (2013) observed that CH 4 oxidation rates were higher in the wet than in the dry season.
During the assessed period, N 2 O fluxes in soil under native vegetation (CE) rarely exceeded 10 µg GHG fluxes in native and planted forests Sci. Agric. v.78, n.1, e20180355, 2021 m -2 h -1 . This is attributed to the very low amount of NO 3 and to the predominant N form (NH 4 + ) ( Figures  3B and 3C) that did not induce high N 2 O emissions. These results were similar to previous descriptions in studies on Oxisols under native Cerrado vegetation, which reported few measurements above 10 µg m -2 h -1 and average fluxes between 0.6 and 16 µg N 2 O m -2 h -1 (Siqueira- Neto et al., 2011;Santos et al., 2016;Carvalho et al., 2017).
In year-1, the N 2 O fluxes were higher in the dry than in the rainy season. Even under conditions in which fluxes are common (WFPS > 48 %, predominance of NH 4 + and soil temperature > 21 ° C), very high fluxes were not observed in the entire dry season, but only in late Aug (08/28/2014). At this time, mineral N exceeded 5.0 mg kg -1 soil (lower than in the rainy season) and the soil temperature rose from 17 to 20.3 ºC. From Sept onwards, the soil temperature was 23 ºC. In this period, there was possibly a response in the N 2 O pulses to the increase in soil temperature in E1 and CE, however low (< 59 µg m -2 h -1 ).
Forest soils in the Cerrado biome usually have low N 2 O fluxes, which can be attributed to the physical, chemical, and biological characteristics of the system (Martins et al., 2015a;Martins et al., 2015b;Santos et al., 2016). In general, N availability in tropical savanna ecosystems in Brazil is low, since 15 -37 % of N is resorbed prior to leaf dehiscence  and in the Cerrado, inorganic N is available by mineralization of SOM (Catão et al., 2016). Under undisturbed conditions, N pools are at steady state and production and consumption are equal (Booth, 2005); thus, the annually mineralized inorganic N in unburned Cerrado does not exceed 15 kg ha -1 yr -1 . With these efficient cycling and use mechanisms, little N is lost by leaching or gas transformation .
Under natural conditions, available N in Cerrado soils depends to a large extent on organic sources and litter ( Table 2). The equilibrium between mineralization and immobilization depends on the C: N ratio, which is high (> 65:1) in this study and on the material incorporated into the soil, which may induce N immobilization, showing that residue quality also influences N 2 O emissions. According to Alluvione et al. (2010), a high C:N ratio may increase N immobilization thus reducing the occurrence of denitrification and consequently of GHG emissions.
Another important factor to be considered is the cycling efficiency. If high, low values of mineral N can be established in the soil, and the N mineralized by decomposition can be absorbed quickly by the plant root system, after nitrification and denitrification (Ugalde et al., 2007). Low rates of N 2 O and CH 4 emissions were also documented in other studies on forests in Brazil, Australia, and China. Godoi et al. (2016) observed similar fluxes of CH 4 (-22.7 and -24.4 mg CH 4 m -2 h -1 ) and N 2 O (5.3 and 5.4 mg N 2 O m -2 h -1 ), for soils with Acacia and native vegetation, respectively. Werner et al. (2006) observed values near 7 mg N 2 O m -2 h -1 in reforested areas in China, under tropical conditions, while Allen et al. (2009) reported a high CH 4 uptake (-1 to -50 µg CH 4 m -2 h -1 ) and low N 2 O fluxes (-5 to 50 µg N 2 O m -2 h -1 ) in subtropical Australian soils under Eucalyptus.
In general, when good drainage conditions, which reduce WFPS (Baggs and Philippot, 2010), are combined to a low relative NO 3 production, the mineral N concentrations rarely exceed the N demand of microorganisms and plant roots (Martins et al., 2015b).
Although several studies reported correlations between GHGs and mineral N in the soil (Siqueira-Neto et al., 2011;Santos et al., 2016;Carvalho et al., 2017), and in this study, some correlations between these N forms and gas fluxes were also observed, the gas emission pulses did not occur synchronously with the highest NO 3 and NH 4 + concentrations, which may have reduced the significance of relations.
In this study, the GHG fluxes were low, regardless of the season and year studied, which was mainly associated to WFPS < 50 % and NO 3 concentrations < 8 mg kg -1 . In soils with high permeability, such as in the Oxisol of this study, both WFPS and texture are key factors in N 2 O emission. In soils with WFPS > 60 %, denitrification tends to be the predominant process (Livesley et al., 2009;Gregorich et al., 2015), while in low WFPS soils, ammonia oxidation is favored (Bateman and Baggs, 2005). These findings show that Eucalyptus stands of different ages and areas of native Cerrado vegetation probably contribute to reduce N 2 O emissions, where no external N sources are provided and the systems depend exclusively on internal nutrient cycling. Furthermore, based on the predominant form of mineral N (NH 4 + ), it is assumed that the studied N 2 O emissions may result from nitrification rather than from denitrification processes.
However, it should be considered that transformations of N (immobilization or mineralization) in an ecosystem are coupled with C transformations, especially when organic carbon molecules are converted into CO 2 by soil heterotrophic microbial populations (McGill and Cole, 1981), which can reduce the partial pressure of oxygen and favor denitrification. Significant GHG pulse emissions following the first rains after a dry season, often with a short time lag, have been reported in different seasonally dry ecosystems. The pulses are generally preceded by significant GHG emissions immediately after the soil is re-moistened, due to waterinduced activation of soil microbes (Santos et al., 2016).
Studies show that soil moisture expressed by WFPS, soil temperature, and mineral N content are the main variables that control and express GHG emissions (Bayer et al., 2015). Siqueira-Neto et al. (2011) reported divergent observations for native Cerrado vegetation, where lower N 2 O emissions were observed during the dry season, indicating that the seasonal variations occur due to the absence of rainfall and consequent reduction in soil moisture.
In this study, no environmental variable stood out as an environmental drive, despite variables with relevant effects on N 2 O emissions in agricultural systems found by our research group (Santos et al., 2016;Carvalho et al., 2017;Sato et al., 2017). Therefore, these results indicate that the lack of consistent environmental effects may be due to both the intrinsic low GHS emission and the high spatial/temporal variability of native and cultivated forest environments in the Cerrado.

Cumulative CH 4 and N 2 O fluxes
The cumulative annual CH 4 fluxes were negative for the areas and years studied ( Figures 4B and 4D). The cumulative CH 4 flux from Eucalyptus under the conditions assessed shows the crop potential of methane mitigation. These cumulative CH 4 influxes in Eucalyptus plantations and native savanna vegetation are in agreement with reports of emissions from forests and savannas (Table 4) that indicate CH 4 uptake from the atmosphere, since a greater amount was consumed by the environment than the effectively produced quantity (Bustamante et al., 2012).
Studies indicate that high soil N mineral contents may stimulate CH 4 oxidation (Bodelier and Laanbroek, 2004;Liu and Greaver, 2009). Mineral N seems to be a prerequisite for CH 4 consumption, although the identification of relationships between N availability and CH 4 consumption, as well as the bacteria involved, is still a challenge. For the time being, N must be treated as a potential inhibitor and as a beneficial factor for CH 4 consumption in soils (Bodelier and Laanbroek, 2004). Ammonium can compete with CH 4 for the enzyme methane mono-oxygenase, effectively lowering CH 4 oxidation by methanotrophs in the soil (Hanson and Hanson, 1996).
The N 2 O emissions were below 0.86 kg ha -1 yr -1 for all areas studied ( Figures 4A and 4C). Other studies also reported cumulative N 2 O fluxes below 1 kg ha -1 yr -1 (Table 4). To date, no studies have evaluated soil N 2 O fluxes for a period of two years under planted Eucalyptus systems in Brazil. In our study, mainly during the dry season in native Cerrado, N 2 O influxes and low nitrate levels were observed. The N 2 O influx under native vegetation may be associated to a low N mineral content, in predominantly ammoniacal N, a rapid drainage of soil water to the subsurface layers (Martins et al., 2015b;Santos et al., 2016;Carvalho et al., 2017;Sato et al., 2017), and low soil pH (Lopes and Cox, 1977), since nitrification tends to decrease with increasing soil acidity (Hickman et al., 2014).
In an integrated crop-livestock-forest system, Carvalho et al. (2017) observed N 2 O fluxes lower than in an integrated crop livestock system. The authors claimed that the presence of Eucalyptus plant residues in the system, which are rich in phenolic compounds and are acidic (Soumare et al., 2015), inhibit the enzyme and microbial activity in the soil (Chen et al., 2013).
Despite limitations of the static chambers in relation to the real conditions of free soil-atmosphere fluxes, these boxes are easy to construct and can be used under different conditions. Therefore, even lowmagnitude flows can be detected and information on the spatial variability of emissions is provided, although no significant differences related to the position and no emission pattern could be observed in our study. Other drawbacks refer to variations in daily measurements that are commonly wide, the chamber measurements may be underestimated or overestimated, and the values of standard deviation are generally high. Possibly, the high spatial and temporal variability of the gas emissions was due to the quantity of variables that are interrelated and drive the flow dynamics. However, cumulative N 2 O emissions from forest soils and CH 4 influxes were low (< 1.0 kg ha -1 ), although their variability should not be left unmentioned. In evaluations at 15 positions in natural forests in Germany, Jungkunst et al. (2012) also found high variability and low spatial representativeness of the chambers, attributed to the C:N ratio, N input, and water availability by the authors. However, considering the variations observed, a random distribution of the chambers is suggested, since the measurements seem to be independent from the chamber position and distance from the Eucalyptus trees.
Planting Eucalyptus in areas of former native vegetation or previous agricultural use resulted in no marked differences between cumulative GHG fluxes. From the ecological and environmental viewpoint, greater biodiversity in areas of native vegetation stands out. This similarity in the cumulative GHG fluxes may be related to the stability observed in sites with Eucalyptus stands after planting.

Conclusions
The change in land use from Cerrado to Eucalyptus plantations did not induce significant changes in GHGs, compared to the native vegetation. The flux rates were low for both gases (N 2 O and CH 4 ). The temporal variations in GHG fluxes and the different ages of stands caused  no significant differences between cumulative annual fluxes. Our results may be an important contribution to a better understanding of the dynamics of GHG fluxes in commercial forest plantations in the Cerrado region.