Effects of multiple environmental factors on CO2 emission and CH4 uptake from old-growth forest soils

To assess contribution of multiple environmental factors to carbon exchanges between the atmosphere and forest soils, four old-growth forests referred to as boreal coniferous forest, temperate needle-broadleaved mixed forest, subtropical evergreen broadleaved forest and tropical monsoon rain forest were selected along eastern China. In each old-growth forest, soil CO 2 and CH4 fluxes were measured from 2003 to 2005 applying the static opaque chamber and gas chromatography technique. Soil temperature and moisture at the 10 cm depth were simultaneously measured with the greenhouse gas measurements. Inorganic N (NH+4 -N and NO − 3 -N) in the 0–10 cm was determined monthly. From north to south, annual mean CO 2 emission ranged from 18.09± 0.22 to 35.40± 2.24 Mg CO2 ha−1 yr−1 and annual mean CH 4 uptake ranged from 0.04 ± 0.11 to 5.15± 0.96 kg CH4 ha−1 yr−1 in the four old-growth forests. Soil CO2 flux in the old-growth forests was mainly driven by soil temperature, followed by soil moisture and NO −3 -N. Temperature sensitivity ( Q10) of soil CO2 flux was lower at lower latitudes with high temperature and more precipitation, probably because of less soil organic carbon (SOC). Soil NO−3 accumulation caused by environmental change was often accompanied by an increase in soil CO 2 emission. In addition, soil CH4 uptake decreased with an increase in soil moisture. The response of soil CH 4 flux to temperature was dependent upon the optimal value of soil temperature in each forest. Soil NH+4 -N consumption tended to promote soil CH 4 uptake in the old-growth forests, whereas soil NO −3 accumuCorrespondence to: G. R. Yu (yugr@igsnrr.ac.cn) lation was not conducive to CH 4 oxidation in anaerobic condition. These results indicate that soil mineral N dynamics largely affects the soil gas fluxes of CO 2 and CH4 in the oldgrowth forests, along with climate conditions.


Introduction
It is generally thought that old-growth forests cease to accumulate carbon (C) with the equal of mean CO 2 emission from heterotrophic respiration to mean CO 2 sequestration as net primary production (Odum, 1969). Recently, some studies have suggested that old-growth forests can continue to sequester C and serve as a global CO 2 sink Luyssaert et al., 2008). Most of the sequestered CO 2 is stored as slowly decomposing organic matter in litter and soil . As an important process of C cycling, soil CO 2 and CH 4 fluxes are driven by many environmental factors, including availability and amount of C substrates, temperature, precipitation and soil water content, redox potential and aeration, diffusion, soil texture, soil pH, salinity, sodicity and acidity, ion deficiencies and toxicities and elevated CO 2 and atmospheric N deposition (Dalal and Allen, 2008). Therefore, assessing contribution of multiple environmental factors is critical to model or to predict C exchanges between the atmosphere and forest soils.
In the past two decades, studies on responses of soil C fluxes to climate change and N deposition in forests mostly focused on manipulative experiments such as warming (Melillo et al., 2002), throughfall exclusion Borken et al., 2006;Sotta et al., 2007), and N addition (Bowden et al., 2004;Micks et al., 2004;Mo et al., 2008;Zhang et al., 2008). However, manipulative experiments could be incompletely equal to natural environmental changes due to transient change of activities of plant roots and soil microorganisms, which could draw various conclusions (Corre et al., 2007;Kleja et al., 2008). For example, addition of N to forest soils may increase (Tessier and Raynal, 2003;Micks et al., 2004), decrease (Chantigny et al., 1999;Bowden et al., 2004) or have no effect on (Prescott et al., 1999;McDowell et al., 2004) soil CO 2 and CH 4 fluxes. Also, the responses of soil respiration to warming included both promotion (Bergner et al., 2004) and acclimation (Luo et al., 2001;Melillo et al., 2002). To our knowledge, only few reports are available on evaluating the combination effects of multiple environmental factors on CO 2 and CH 4 fluxes from old-growth forest soils under non-manipulation conditions. Environmental gradient method, which can deal with a gradual and continuous change in time and space, is widely applied in studying the responses of C and N processes to climate change (Corre et al., 2007;Kleja et al., 2008). In eastern China, mean annual temperature varies from −7 • C in the cold temperate continent monsoon climatic zone of the north to over 26 • C in the equatorial monsoon climatic zone of the south (Yu et al., 2008). Mean annual precipitation decreases from about 2200 mm in the south to less 230 mm in the north (Yu et al., 2008). Zonal forest ecosystems, from the tropical rain forest in the south to the boreal coniferous forest in the north, provide a unique research platform to investigate the effects of multiple environmental factors on soil CO 2 and CH 4 fluxes in old-growth forests.
In this paper, we analyzed three-year data on soil CO 2 and CH 4 fluxes, soil temperature, soil moisture, and mineral N concentrations which were measured in four old-growth forests in eastern China. Our specific aims are (Eq. 1) to compare the difference of soil CO 2 and CH 4 fluxes in different forests in eastern China; (Eq. 2) to evaluate the relationship between soil C fluxes and soil temperature, moisture and soil mineral N concentrations.

Study sites
Four old-growth forests are referred to as Daxinganling boreal coniferous forest, Changbaishan temperate needlebroadleaved mixed forest, Dinghushan subtropical evergreen broad-leaved forest, and Xishuangbanna tropic monsoon rain forest from north to south, hereafter referred to as boreal, temperate, subtropical, and tropical forest, respectively ( Fig. 1, Table 1). These forest sites expand from the mean annual temperature of −5.4 • C in the boreal forest to 21.4 • C in the tropical forest, and annual precipitation from 500 mm in cool temperate climate region to over 1600 mm in tropical and subtropical climate region. The total nitrogen deposition increases from 8.5 kg N ha −1 yr −1 in the boreal to 38.4 kg N ha −1 yr −1 in the subtropical. The boreal forest is a pure forest with Chinese larch (Larix gmelinii) as the predominant tree species (Jiang et al., 2002). The dominant vegetation species in the temperate forest are Korean pine (Pinus koraiensis), basswood (Tilia amuresis), Manchurian ash (Fraxinus mandshurica) and oak (Quercus mortgolica) in the tree layer . The major species in the subtropical forest are guger-tree (Schima superba), rose apple (Syzygium jambos), henry chinkapin (Castanopsis chinensis) in tree layers . The most abundant species in the tropical forest are downy malugay (Pometia tomentosa), bayberry waxmyrtle-fruit (Terminalia myriocarpa), Yunan nutmeg (Myristica yunnanensis), South-Yunnan horsfieldia (Horsfieldia tetratepala), glabrous homalium (Homalium laoticum) (Werner et al., 2006). The soils are Greyzems, Luvisols, Ferralsols and Lixisols in the four old-growth forests from north to south, respectively (WRB, 2006). More extensive description on the sites was given in Table 1.

Soil CO 2 and CH 4 flux measurements
At each forest site, three replicate chambers were randomly designated to measure CO 2 and CH 4 fluxes applying static opaque chamber and gas chromatography method (Wang and Wang, 2003). The chambers were made of stainless-steel and consisted of a square collar (length × width × height = 50 cm × 50 cm × 10 cm) and a removable cover chamber (length × width × height = 50 cm × 50 cm × 50 cm). The collar was inserted directly into the forest floor about 10 cm below the floor surface, and the cover was placed on top during sampling and removed afterwards. The collars A fan with a diameter of 10 cm (4200 rpm ± 10%, 12V, 0.21 A) was installed on the top wall inside each chamber to make turbulence when chamber was closed. White insulating cover was added outside of the stainless steel cover to reduce the impact of direct radiative heating during sampling.
The soil C fluxes were measured between 9:00 and 11:00 a.m. (China Standard Time, CST) by fitting the chambers to the collars for 30 min. A diurnal study demonstrated that CO 2 and CH 4 fluxes measured from 9:00 to 11:00 were close to daily means . The four gas samples were taken by 100 mL plastic syringes at intervals of 0, 10, 20 and 30 min after closing the chambers. All gas samples were analyzed within 24 h following gas collection. Soil CO 2 and CH 4 fluxes were calculated based on their rate of concentration change within the chamber, which was estimated as the slope of linear regression between concentra-tion and time (Wang and Wang, 2003). All the coefficients of determination (r 2 ) of the linear regression were more than 0.95 in our study. Positive and negative fluxes indicate forest soils function as net source and sink of CO 2 and CH 4 , respectively.
Soil temperature and soil moisture at 10 cm below soil surface were monitored at each chamber simultaneously. Soil temperature was measured applying portable temperature probes (JM624 digital thermometer, Living-Jinming Ltd., China). Volumetric soil moisture (m 3 m −3 ) was measured applying moisture probe meter (MPM160, Meridian Measurement, China). Field measurements were carried out weekly in the vegetative season (from May to October) and monthly in the non-vegetative season (from November to April next year). 398 H. J. Fang et al.: Soil CO 2 emission and CH 4 uptake in four old-growth forests.

Soil sampling and mineral N analysis
In the middle ten days of each month during the vegetative season, mineral soils nearby the gas chambers were taken from 0-10 cm depth applying an auger (5 cm in diameter) after careful removal of O-horizon. Soil sampling was conducted in the same way in the non-vegetative season at the tropical and subtropical forest sites. However, soil samples were not collected in the non-vegetative season because frozen soil occurred from November to April next year at the boreal and temperate forest sites. Four samples were collected at each site. Soils were immediately passed through a 2 mm sieve to remove roots, gravel and stones. Soil samples were extracted in 100 ml 0.2 M KCl solution and shaken for 1 h. The soil suspension was subsequently filtered through Whatman No. 40 filter papers for NH + 4 -N and NO − 3 -N determination on a continuous-flow autoanalyzer (Bran Luebbe, Germany).

Non-linear relationship between C fluxes and soil temperature
An exponential growth model was used to fit the relationship between soil CO 2 flux and soil temperature. The sensitivity of soil CO 2 flux to soil temperature at 10 cm depth (Q 10 ) was obtained from a coefficient, B, in the exponential equation (Eq. 1, 2) (Lloyd and Taylor, 1994): where, F CO 2 is the soil CO 2 flux, T is the soil temperature, F 0 is the soil CO 2 flux as soil temperature equal to zero degree Celcius, and B is a regression coefficient. A Gaussian equation (Eq. 3) was used to fit the relationship between soil CH 4 flux and soil temperature: where, a and b are the parameters of the Gausian equation, and T 0 is the optimal soil temperature as soil CH 4 oxidation is the maximum.

Statistical analysis
A repeated measures analyses of variance (ANOVA) was performed on monthly mean to test the variation of soil temperature, moisture, mineral N contents and soil C fluxes in forests and seasons. Additionally, the relationships between CO 2 and CH 4 fluxes and soil variables (temperature, moisture and mineral N) were examined in a way of linear or nonlinear regression models fitting. Mean Square Error (MSE) and R 2 of the model parameters were used to determine goodnessof-fit. All statistical analyses were performed using SAS software (SAS Institute, 2001). A p-value<0.05 was used to reject the null hypothesis that the model is not significant.

Seasonality of primary environmental variables
Soil temperature and moisture presented clear seasonal courses (Fig. 2a), being highest in summer and lowest in winter. Soil moisture was correlated strongly with soil temperature at all the forest sites, except in the subtropical forest ( Fig. 2a and Fig. 2b). Monthly average precipitation showed similar pattern to soil moisture in the four old-growth forests (Fig. 2c). In both vegetative and non-vegetative seasons, there was significant difference of soil temperature and moisture among the four old-growth forests, but no trend in soil moisture from north to south (Table 2).

Seasonality of soil mineral N
In the boreal, temperate and tropical forests, the two peaks of NH + 4 -N concentration occurred in the early vegetative season between April and May and in summer between July and August (Fig. 3a). However, the NH + 4 -N concentration remained relatively constant in the subtropical forest in the whole year Table 2. Effects of forest type and season on the mean (standard error) of soil temperature, moisture, mineral N concentrations and soilatmospheric C exchanges*.  (Table 2).
In contrast, only the subtropical forest soil showed strong seasonal variation in NO − 3 -N concentration, with peaks of soil NO − 3 -N occurring between June and August (Fig. 3b). However, in the boreal and temperate forests NO − 3 -N concentration tended to decrease in early spring and then slightly rose in the late vegetative season (Fig. 3b). From 2003 to 2005, mean NO − 3 -N concentrations in the vegetative season were significantly different among four old-growth forests. Their order was as follows: subtropical forest (16.15 ± 2.09 mg N kg −1 ) > tropical forest (7.46 ± 0.10) > temperate forest (4.57 ± 0.29) > boreal forest (2.01 ± 0.19) (p <0.001) ( Table 2).

Seasonality of soil CO 2 and CH 4 fluxes
Soil CO 2 flux showed a consistent variation with soil temperature. The seasonal difference in CO 2 flux was more pronounced in the boreal and temperate forests than in the tropical and subtropical forests (Fig. 4a). Although total soil CO 2 flux increased ranging from 15.78 to 23.19 Mg CO 2 ha −1 season −1 in the vegetative season, the difference among four forests was not significant (p >0.05, Table 2). In the non-vegetative season, total CO 2 flux in the boreal and temperature forests ranged from 2.31 to 3.28 M CO 2 , ha −1 season −1 , which were significantly lower than those in the tropical and subtropical forests ranging from 12.21 to 12.86 Mg CO 2 ha −1 season −1 (p <0.001, Table 2). Soil CH 4 flux also showed seasonal variations. The higher uptake and emission in the boreal and temperate forests were observed in summer and in winter, respectively (Fig. 4b). The subtropical forest soil behaved as a net soil CH 4 sink throughout the study period (Fig. 4b). There was a significant difference in CH 4 flux among four old-growth forests in both vegetative and non-vegetative seasons (Table 2). In the non-vegetative season, the boreal forest behaved as a CH 4 source (0.78 kg CH 4 ha −1 season −1 ) and other forests behaved as CH 4 sinks ranging from −1.18 to −3.66 kg CH 4 ha −1 season −1 (Table 2). However, in the vegetative season, only the tropical monsoon rain forest soil was a source of CH 4 of 1.18 kg CH 4 ha −1 season −1 , whereas the soil CH 4 sink occurred in other forests ranging from −0.82 to −2.36 kg CH 4 ha −1 season −1 (Fig. 4b and Table 2).

Relationships between soil temperature, moisture and C fluxes
Soil CO 2 flux was fitted with soil temperature in the equation (Eq. 1), and the results indicated that soil temperature could explain 49%-96% of CO 2 flux variation ( Fig. 5a and Table 3). The average Q 10 was significantly higher in the boreal (3.08) and temperate forests (2.61) than in the tropical (2.16) and subtropical forests (2.05) ( Table 3). Soil CO 2 flux and soil moisture had a positive linear relationship, explaining 40%-49% of variation in CO 2 flux ( Fig. 5b and Table 3). These results showed that soil CO 2 flux was mainly driven by soil temperature, and followed by soil moisture. For all forests, soil CH 4 flux was negatively correlated with soil temperature when soil temperature is less than the optimal value, while a positive relationship appeared when soil temperature is above the temperature (Fig. 5c). This optimal value increased with a decrease in latitude. For instance, the values were 7.3 • C and 7.8 • C in the boreal and temperate forests, 15.7 • C and 18.0 • C in the tropical and subtropical forests, respectively (Table 3). If forest types were not considered and all observed data were included in the model, the relationship between CH 4 flux and soil temperature was fitted well with equation (Eq. 3) and the average optimal soil temperature was 15 • C ( Fig. 5c and Table 3). A negative relationship between CH 4 flux and soil moisture in the boreal forest, and a positive relationship between them in the tropical and subtropical forests were observed, explaining 19%-42% of variation in CH 4 fluxes ( Fig. 5d and Table 3). These results suggested that the response of soil CH 4 flux to soil temperature depended upon the optimal soil temperature and the response to soil moisture varied by forest type.

Relationship between soil mineral N and soil C fluxes
Soil CO 2 fluxes in the tropical and subtropical forests were positively correlated to the concentrations of NH + 4 -N and NO − 3 -N in the 10 cm soil (Fig. 6a, b and Table 4). However, the relationships between soil CO 2 fluxes and mineral N concentrations were not statistically significant in the boreal and temperate forests (Fig. 6a, b). Additionally, a positive correlation between soil CH 4 flux and soil NH + 4 -N was observed in the tropical forest ( Fig. 6c and Table 4). In the boreal and tropical forests where CH 4 emission occurred periodically, soil CH 4 flux was positively correlated to NO − 3 -N concentration ( Fig. 6d and Table 4). Taking four forests together, soil CO 2 flux was positively related to soil NO − 3 -N concentration and soil CH 4 flux was positively related to NH + 4 -N concentration (Fig. 6b, d and Table 4). These results suggested that the accumulation of soil NO − 3 -N and NH + 4 -N were consistent with the increase of gas emission of CO 2 and CH 4 in the old-growth forests in eastern China, respectively.

Effects of soil temperature and moisture on soil C fluxes
With an increase in soil temperature, soil CO 2 flux increased, whereas the temperature sensitivity (Q 10 ) tended to decrease at warmer sites. In the boreal and temperate forests, soil warming can enhance the soil microbial activities and root growth sharply during the short summer (Zheng et al., 2009). This leads to an active decomposition of soil organic C and the enhancement of plant-derived CO 2 release from root respiration. However, the temperature limitation on root growth and soil microbial activities is low in tropical and subtropical forest ecosystems (Zheng et al., 2009). Besides, the labile pool of soil organic carbon (SOC) is an important substrate for soil respiration, and the composition of microbial community are linked to quantity/ quality of SOC (Gu et al., 2004;Knorr et al., 2005b;Fierer et al., 2005;Zheng et al., 2009). When other environmental factors are fixed, soil CO 2 flux tends to be higher under the conditions of higher SOC content. Gradient variation in SOC storage in the four oldgrowth forests also supported the above explanation (Table  1), which suggested that the acclimation of soil CO 2 flux to warming could be also induced by the lower temperature sensitivity with lower SOC content in the subtropical and tropical forests (Melillo et al., 2002). Both soil CO 2 flux and Q 10 value are closely related to soil moisture. Xu and Qi (2001) and Rey et al. (2002) found that soil CO 2 flux increased with increasing soil moisture when soil water-filled pore space (WFPS) was below 60%. In our study, soil moisture contents across four forest sites were generally less than 0.5 m 3 m −3 in the whole year (Fig. 5b,  d), which were equivalent to 55% of WFPS calculated from the equation described by Franzluebbers (1999). Additionally, the Q 10 value of forest ecosystems tended to decrease with mean annual precipitation (Table 1 and 3), which was consistent with a recent study (Peng et al., 2009). Wang et al. (2006) also suggested that the Q 10 tended to increase with soil moisture until reaching a threshold (0.45 g H 2 O g −1 soil), and then decline, which was mainly attributed to limitation of oxygen diffusion.
CH 4 oxidation in soil is controlled by CH 4 and O 2 availability in water-unsaturated forest soil (Teh et al., 2005). When the diffusion rates of CH 4 and O 2 from the atmosphere into the soil are equal to soil CH 4 and O 2 consumption, soil CH 4 oxidation rates reach the maximum values at a given temperature. Cai and Yan (1999) called this temperature as the optimal temperature. The optimal temperature for soil CH 4 oxidation varies with bioclimatic areas, about 20-30 • C in low latitude region (Boeckx and VanCleemput, 1996;Cai and Yan, 1999), 5-25 • C in middle latitude region (Castro et al., 1995), and less than 10 • C in high latitude region (van den Pol-van Dasselaar et al., 1998). Our results fall in the same ranges described above. When soil temperature is relatively low, the CH 4 and O 2 diffusion potentials from the atmosphere to soil are higher than soil CH 4 and O 2 consumption rates due to weak soil microbial activity (Nedwell and Watson, 1995). In this case, soil temperature is the limiting factor for CH 4 oxidation (Peterjohn et al., 1994;Prieme and Christensen, 1997). However, if soil temperature continually rises to superior the optimal level, the reproduction and activity of methanotrophs in soils will gradually decrease because methanotrophs fail to compete with nitrifiers and other microbes for the limited oxygen in soil air (Horz et al., 2005;Castaldi and Fierro, 2005;Borken et al., 2006).

Effects of soil mineral N on soil CO 2 flux
The positive correlation between soil CO 2 flux and soil NO − 3 -N concentration across four forests suggested that NO − 3 -N accumulation caused by environmental change could promote forest soil CO 2 emission. The ability of plants to compete available N (especially NO − 3 -N) is often stronger than soil microorganisms in N-limiting natural forest ecosystems (Jaeger et al., 1999). Soil NO − 3 -N accumulation in forest ecosystems tended to increase fine root/ biomass ratio, which was a good index of C allocation to root. Our calculated data showed that the fine root/biomass ratio (0.06) in the subtropical forest with higher soil NO − 3 -N content was significantly higher than those (0.04) in other forest ecosystems (Table 1). This could partially contribute to the higher autotrophic respiration. However, excessive NO − 3 accumulation and occurrence of ecosystem N saturation would decrease fine root biomass and soil respiration Fang et al., 2009). Soil NO − 3 -N accumulation could also increase litter decomposition rate due to the decline of its C/N ratio with more mineral N incorporated in organic matter (Berg et al., 1998;Hobbie and Gough, 2004). In addition, soil CO 2 fluxes were positively correlated to both NO − 3 -N and NH + 4 -N in N-rich tropical and subtropical forests (Fig. 6a, b). This could be partially attributed to microbial immobilization of soil available N. Soil microorganism need more available C to immobilize redundant mineral N in N-rich forests, and this would stimulate soil microbial activity, elevate organic matter decomposition and increase heterotrophic respiration (McDowell et al., 2004). All these are consistent with experimental findings that N addition promoting soil respiration in Nlimited forest ecosystems (Micks et al., 2004;McDowell et al., 2004). Based on meta analysis, Knorr et al. (2005a) also observed that litter decomposition was stimulated by N addition at field sites exposed to low ambient N deposition (<5 kg N ha −1 yr −1 ). But litter decomposition would be inhibited by N additions when fertilization rates were 2-20 times larger than the anthropogenic N-deposition level, when ambient N deposition was 5-10 kg N ha −1 yr −1 (Knorr et al., 2005a).

Effects of soil mineral N on soil CH 4 flux
The positive relationship between soil NH + 4 -N and soil CH 4 flux across four forests (Fig. 6c) suggested that soil NH + 4 -N accumulation can significantly inhibit CH 4 oxidation. This result is consistent with findings in many N addition experiments that adding N decreased CH 4 uptake by 14% to 51% relative to the control (e.g. King and Schnell, 1994;Sitaula et al., 1995;Gulledge et al., 2004;Zhang et al., 2008). High concentration of soil NH + 4 -N could significantly inhibit methanotrophic activities in soils because it stimulates the quantity of NH + 4 -oxidizer bacteria in the organic layers of forest soil (King and Schnell, 1994;Whalen and Reeburgh, 2000). This is also an acceptable explanation for the highest soil CH 4 uptake in the subtropical forest. Due to the relative high N deposition rate (more than 30 kg N ha −1 yr −1 ) and the high initial content of soil N, the subtropical forest at Dinghushan site, where soil mineral N is dominated by soil NO − 3 -N (Fig. 3) and soil N leaching and gaseous emission is very high Fang et al., 2009), has already reached N saturation . However, the tropical forest at Xishuangbanna site where economy is undeveloped and N deposition is low, soil mineral N is still dominated by soil NH + 4 -N and is not unsaturated yet (Fig. 3). On the contrary, because soil NH + 4 in the boreal and temperate forests was mainly assimilated by plants or immobilized by soil clay mineral, the relatively lower CH 4 uptake was attributed to other reasons such as lower substrate availability or lower gas diffusion rather than NH + 4 inhibition. For example, low temperature and frozen layer in winter as well as the thicker O-horizon obstruct the diffusion of CH 4 into soil, which will indirectly decrease the uptake of soil CH 4 (Elberling et al., 2008).
The positive relationships between soil NO − 3 -N and soil CH 4 flux in the tropical and boreal forests (Fig. 6d) were inconsistent with other studies that NO − 3 has either increase or no effect on CH 4 uptake (Dunfield et al., 1995;Corton et al., 2000). In general, soil nitrification consumes soil NH + 4 -N and subsequently accumulates soil NO − 3 in warm and mesic condition. Unsaturated soil and no NH + 4 -N accumulation favor soil CH 4 uptake in subtropical and temperate forests. However, no significant accumulation of soil NO − 3 -N within the study period in the boreal and tropical forests partially attributed to soil denitrification, especially during soil freezing in the boreal forest in winter and soil water-logging of the tropical forest in rain season. Hydroxylamine (NH 2 OH) or nitrite (NO − 2 ) produced via NH + 4 oxidation and NO − 3 reduction could produce a toxic inhibition on CH 4 uptake (King and Schnell, 1994).

Conclusions
Soil CO 2 emissions of old-growth forests in eastern China were mainly driven by temperature, followed by soil moisture, NO − 3 -N content. The apparent sensitivity of CO 2 flux to soil temperature (Q 10 ) tended to decrease with the increase in air temperature and moisture, indicating the acclimation of soil CO 2 flux to warming in forests with lower SOC content. Soil NO − 3 accumulation caused by warming and nitrogen deposition was often accompanied by an increase in soil CO 2 emission, which could be partially attributed to the increase of C allocation to root and decomposability of organic materials.
Soil CH 4 uptake decreased with the increase in soil moisture. The response of soil CH 4 flux to temperature was dependent upon the optimal value of soil temperature in each forest. Soil NH + 4 -N consumption tended to promote soil CH 4 uptake in the old-growth forests, whereas soil NO − 3 accumulation was not conducive to CH 4 oxidation in anaerobic condition. The mechanism by which soil mineral N affected CH 4 uptake included both a competitive inhibition of monooxygenase by NH + 4 and a toxic inhibition by hydroxylamine or nitrite. These results indicate that soil mineral N dynamics largely affects the soil gas fluxes of CO 2 and CH 4 in the old-growth forests, along with climate conditions.