Responses of CH 4 uptake to the experimental N and P additions in an old-growth tropical forest , Southern China

Responses of CH4 uptake to the experimental N and P additions in an old-growth tropical forest, Southern China T. Zhang, W. Zhu, J. Mo, L. Liu, and S. Dong Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China Department of Biological Science, State University of New York – Binghamton, Binghamton NY, 13902, USA Graduate University of Chinese Academy of Sciences, Beijing 100039, China

Soils as biological sinks of CH 4 are subjected to many abiotic and biotic controls, including temperature and moisture (Steudler et al., 1989;Davidson and Nepstad, 2004), acidity (Saari et al., 2004;Xu and Inubushi, 2009), gas diffusivity in relation to soil bulk density and texture (Tate et al., 2007), vegetation and land-use (Menyailo et al., 2008;Pendall et al., 2010), as well as soil fauna (Bignell et al., 1997).Furthermore, nutrient availability, such as ammonium, nitrate, and phosphorus (P), could affect microbial activities and methane-mono-oxygenase (MMO) activity in soil and, consequently, CH 4 flux from the soil (Le Mer and Roger, 2001).Among these factors, the effects of nitrogen (N) deposition on CH 4 oxidation have received increasing attention (Steudler et al., 1989;Bodelier and Laanbroek, 2004;Tate et al., 2007).Nitrogen addition alters the fluxes of greenhouse gases (GHGs, including CH 4 ) through regulating plant and microbial activities that are directly associated with GHGs production and consumption (Liu and Greaver, 2009).Steudler et al. (1989) first reported in a temperate forest that nitrogen (N) fertilization reduced soil CH 4 uptake by 33 %.Extensive research has been conducted to investigate the relationship between CH 4 consumption and N input and it has been generally accepted that CH 4 uptake is inhibited by nitrogenous fertilization (Bodelier and Laanbroek, 2004;Chan et al., 2005;Zhang et al., 2008).

Conclusions References
Tables Figures

Back Close
Full considered the major cause of methanotroph inhibition (King and Schnell, 1994;Dunfield and Knowles, 1995;Steinkamp et al., 2001).Aluminium toxicity after extensive N input may also inhibit soil CH 4 uptake (Bradford et al., 2001;Zhang et al., 2008).On the other hand, positive effects of increased nitrogen availability on CH 4 uptake rates were found in severely N-limited forests (B örjesson and Nohrstedt, 2000;Steinkamp et al., 2001).Soil available phosphorus also affects CH 4 flux by regulating changes in soil physico-chemical properties, plant root activities, and soil microbial activities that are directly involved in CH 4 consumption and emission.Furthermore, phosphorus availability could affect litter-decomposing fauna in tropical forests (McGlynn et al., 2007), which may influence CH 4 flux.
While N is often the primary limiting nutrient in temperate and boreal forests, P usually limits ecological processes in tropical and subtropical forests (Vitousek and Sanford, 1986;Cleveland et al., 2002;Vitousek et al., 2010).P-limited forests growing on highly weathered soils (where N cycles quickly) may respond differently to N deposition than N-limited forests (Hall and Matson, 2003).For example, chronic N deposition could lead to soil acidification and then the acidic soils, rich in iron and aluminum oxides, react with labile inorganic P, fixing P into insoluble forms (Matson et al., 1999).Furthermore, P deficiency is likely to result in less carbon (C) fixation and storage in soil, and less microbial growth (Matson et al., 1999).Low P availability would therefore affect the fluxes of major GHGs including CH 4 through the change of soil microbial activity.
We studied the relationships between soil P availability and CH 4 flux in an old-growth tropical forest located in Southern China where elevated N deposition has been well documented (Mo et al., 2006;Fang et al., 2009Fang et al., , 2011)).Previous studies have shown this old-growth forest is N saturated due to long-term atmospheric N input and age (Mo et al., 2006).Studying forest CH 4 uptake and its relationship with soil available P and elevated N deposition is very important for evaluating the contribution of tropical forests to global climate change.Due to limited research in tropical forests, it is not clear how P availability would affect soil CH 4 uptake, and how P addition may interact Introduction

Conclusions References
Tables Figures

Back Close
Full with N deposition.This 33 months study experimentally tested the effects of P and N availabilities on soil CH 4 uptake.We hypothesized that: (a) P addition would increase CH 4 uptake; (b) N addition would decrease CH 4 uptake; and (c) P addition would mitigate the inhibitive effect of N addition on soil CH 4 uptake.

Site description
This study was conducted in the 1200 ha Dinghushan Biosphere Reserve (DHSBR), which is located in the middle of Guangdong Province, Southern China ( 112• 10 E, 23 • 10 N).There are three major forest types in the reserve, a pine forest, a mixed pine and broadleaf forest (mixed forest), and an old-growth evergreen broadleaf forest (mature forest) (Zhou et al., 1986).The average annual precipitation of 1927 mm in the reserve has a distinct seasonal pattern, with 75 % falling from March to August and only 6 % falling from December to February (Huang and Fan, 1982).The mean annual temperature is 21 • C with the January mean temperature of 12.6 • C and July mean temperature of 28.0 • C (Huang and Fan, 1982).Annual mean relative humidity is 80 % (Huang and Fan, 1982).The wet N deposition was 36-38 kg N ha −1 in the 1990s (Zhou and Yan, 2001).In the year of 2004 and 2005, N deposition was measured as 34 kg N ha −1 and 32 kg N ha −1 , respectively (Fang et al., 2006), with roughly 1 : 1 NH + 4 to NO − 3 molar ratio (Fang et al., 2007).Monthly precipitation and temperature during the study period are displayed in Fig. 1.
The old-growth mature forest has been well protected from human activities for more than 400 yr (Wang et al., 1982;Mo et al., 2003;Tang et al., 2006) Bl. and Hemigramma decurrens (Hook.)Copel. in the understory layer (Wang et al., 1982).The mean annual litter biomass production was 8.3 Mg ha −1 yr −1 (Zhang et al., 2000).Stem density, tree height and mean diameter at breast height are summarized in Table 1.Soil in the reserve is oxisols from shale formation (Wu et al., 1982).The soil depth in the old-growth forest is more than 60 cm to the top of the C horizon (Mo et al., 2003).The forest in this experiment is situated on mountain slopes about 30 • -35 • .General soil properties are listed in Table 2.

Experimental treatment
Four treatments were established (each with five replicates) in 2007: control, N-addition (15 g N m −2 yr −1 ), P-addition (15 g P m −2 yr −1 ), and NP-addition (15 g N m −2 yr −1 plus 15 g P m −2 yr −1 ).A total of 20 plots of 5 m × 5 m were established and each plot was surrounded by a 5-m-wide buffer strip.Plots size and fertilizer level were referenced to the experiment in Costa Rica by Cleveland and Townsend (2006).Field plots and treatments were laid out randomly.NH 4 NO 3 and NaH 2 PO 4 solutions were sprayed once every other month to the forest floor with a backpack sprayer starting from February 2007 and continued through October 2009.Fertilizer was weighed and mixed with 5 l of water for each plot.Each control plot received 5 l of water without fertilizer.

Field sampling and measurements
CH 4 , CO 2 and nitrous oxide (N 2 O) flux were measured from January 2007 before the first fertilizer application.Static chambers were installed in each plot in November 2006, two months before the gas sampling.Gas fluxes were monitored once every month using the static chamber and a gas chromatograph (Agilent 4890D).The static chamber was a 25-cm-diameter by 16-cm-tall PVC pipe permanently anchored 8 cm into the soil.During gas collection, a 30-cm-tall removable cover chamber was attached tightly to the anchor ring with a rubber band.Gas samples were collected from Introduction

Conclusions References
Tables Figures

Back Close
Full each chamber from 09:00-10:00 LT.Diurnal studies in the adjacent forests found that greenhouse gas fluxes measured during the mid-morning (09:00-10:00 LT) were closer to the daily means (Tang et al., 2006).The GHG concentrations remained linear for up to 100 min after the chamber was closed in our study.Gas samples were taken with a 60 ml plastic syringe at 0 and 30 min after the chamber closure.Before each sampling, syringes were flushed three times with chamber gas to mix the headspace.
Laboratory tests showed that chambers and syringes were inert to N 2 O, CO 2 , and CH 4 (Steudler et al., 1989;Bowden et al., 1990) The calculation of GHG flux followed that described in Zhang et al. (2008), based on a linear regression of chamber gas concentration versus time (IAEA, 1992;Holland et al., 1999).Atmospheric pressure was measured at the sampling site using an air pressure gauge (Model THOMMEN 2000, Switzerland).Air temperature (enclosure), soil temperature (at 5 cm depth) and moisture (0-10 cm depth) were measured during each sampling.Soil moisture content was detected using a TDR-probe (Time Domain Reflectometry, Model Top TZS-I, China).Soil moisture (0-10 cm depth) values were converted to WFPS (Water Filled Pore Space) according to the following formula: Where SBD is soil bulk density, Vol is volumetric water moisture and 2.65 is the density of quartz.
Soil samples were collected in February 2007 (before the first fertilizer application) and February 2009 (after two years of fertilization) for the quantification of soil chemistry and biology.Five soil cores (2.5 cm inner diameter) were collected randomly from each of the 20 plots at 0-10 cm soil depths and combined to one composite sample.Introduction

Conclusions References
Tables Figures

Back Close
Full The litter layer was carefully removed before the soil sampling.The pH of the soil sample was measured in a 1 : 2.5 soil/water suspension.Soil microbial biomass carbon (MBC) was estimated by the chloroform fumigation-extraction method (Vance et al., 1987).Soil organic carbon (SOC) was determined by dchromate oxidation and titration with ferrous ammonium sulfate.Dissolved organic carbon (DOC) was extracted with 0.5 M K 2 SO 4 and analyzed using a total carbon analyzer (Shimadzu model TOC-500, Kyoto, Japan).Total N concentration was determined by the micro-Kjeldahl digestion followed by the analysis of ammonium on a Wescan ammonia analyzer, while total P concentration was analyzed colorimetrically after acidified ammonium persulfate digestion (Anderson and Ingram, 1989).Available P was extracted with 0.03 M ammonium fluoride and 0.025 M hydrochloric acid and analyzed colorimetrically.

Climate and soil microclimate in the 33 months study period
Our study site located in Southern China has a distinct dry-wet climate, with warmwet seasons of spring (April to June) and summer (July to September) and cool-dry seasons of fall (October to December) and winter (January to March) (Fig. 1).Annual precipitation from 2007 to 2009 averaged 1947.7 mm, close to the long-term mean of 1927 mm (Wu et al., 1982). in air temperature was observed (p = 0.64) between years (Fig. 1).
Soil temperature (at 5 cm depth) followed the air temperature in all plots, with temperatures increased from spring to summer and decreased from fall to winter (Fig. 2a).
There was no treatment effect on soil temperature during the study period.Soil WFPS (0-10 cm depth) rose following the increased precipitation from dry winters to wet springs but decreased in summer, possibly due to plant uptake and higher evaporation, despite the high amount of precipitation in summer (Fig. 2b).Repeated measurement ANOVA showed that soil WFPS was significantly lower in the P-addition and NP-addition plots (p = 0.011 and p = 0.018, respectively) whereas N-addition had no effect (p = 0.165).

Other soil properties
Nitrogen and P treatments significantly changed soil nutrient conditions (Table 3).After 24 months of the treatment, a six fold of soil available P was observed in P-addition plots (p = 0.0003) and four fold in NP-addition plots (p = 0.012), compared to the Introduction

Conclusions References
Tables Figures

Back Close
Full controls.Furthermore, total P was significant lower in N-addition plots compared to control plots (p = 0.001).Soil NH + 4 concentrations did not show any significant change in any treatment plots (N-addition, P-addition and NP-addition) compared to the controls (p = 0.163, 0.152 and 0.368, respectively).However, soil NO − 3 concentrations were significantly lower in P-addition and NP-addition plots (p = 0.0031 and 0.0029, respectively) while no change in N-addition plots (p = 0.339).Soil pH increased significantly in P-addition plots (p = 0.021) and was a little higher in NP-addition plots (p = 0.062) while no change in N-addition plots (p = 0.933).On the other hand, MBC, SOC, DOC, and total N were not affected by any treatments.

Soil CH 4 fluxes
Pre-treatment gas measurement (January 2007) showed no difference in different treatment plots compared to control plots (p = 0.807, 0.559 and 0.961, respectively in N, P, NP treatment plots).After fertilization treatment, repeated measurement ANOVA showed that P addition significantly increased soil CH 4 uptake while N addition significantly decreased CH 4 uptake (Fig. 3).The mean soil CH 4 uptake rate was 31.2 ± 1.1 µg CH 4 -C m −2 h −1 in the control plots during the 33 months study period.Soil CH 4 uptake in the P-addition plots was significantly higher (mean CH 4 uptake rate was 38.8±1.3 µg CH 4 -C m −2 h −1 , p = 0.0068).On the contrary, N-addition significantly inhibited soil CH 4 uptake (mean CH 4 uptake rate was 23.6 ± 0.9 µg CH 4 -C m −2 h −1 , p = 0.007).No significant difference was observed between NP-addition plots (33.6 ± ) and the controls (p = 0.177).
CH 4 uptake was higher in summer and fall (when the soil was low in water content) and was lower in spring (when the soil was wet) and winter (when the soil was cold).Two-way ANOVA showed a significant positive P effect on CH 4 uptake in the summers of 2007, 2008, and 2009, fall 2007, and  CH 4 fluxes and soil WFPS were positively correlated in both the control plots and N and P addition plots (Fig. 7).Under similar WFPS conditions, CH 4 uptake was the highest in P-addition plots and lowest in N-addition plots.CH 4 flux was not correlated to soil temperature.

N 2 O and CO 2 fluxes
Nitrogen and P additions also significantly altered N 2 O and CO 2 fluxes from forest soils, but fluxes of N 2 O and CO 2 had very different seasonal patterns (and when treatments had significant effects) compared to the CH 4 flux (Figs.3-6).The mean soil N 2 O flux was 14.0 ± 0.7 µg N 2 O-N m −2 h −1 in the control plots over the 33 months study period.
Repeated measurement ANOVA showed that N-addition significantly increased N 2 O emission (p = 0.030) while P-additions had no effect (Fig. 3).The mean soil N 2 O flux were 17.4 ± 1.1, 14.0 ± 0.8 and 15.9 ± 0.9 µg N 2 O-N m −2 h −1 in the N-addition, Paddition and NP-addition plots, respectively.Soil N 2 O fluxes were the highest in spring and summer (when the soil was wet and warm) and the lowest in winter (when the soil was cold and dry) (Fig. 5).Two-way ANOVA showed a significant positive N effect in spring 2007 and a positive P effect in summer 2009.Interactive effect of N and P was detected in winter 2008.The mean soil CO 2 flux in the control plots was 71.2 ± 2.8 mg CO 2 -C m −2 h −1 over the 33 months study period.Repeated measurement ANOVA showed no significant negative effect from N addition (mean soil CO 2 flux was 69.9 ± 2.9 mg CO 2 -C m the highest in spring and summer (when soil was warm and wet) and the lowest in fall and winter (when soil was cold and dry) (Fig. 6).Two-way ANOVA showed a significant positive P effect on CO 2 flux in the summers of 2007, 2009 and spring 2009 (Fig. 6), while no N effect was detected.Net GWP from CH 4 and N 2 O were altered by fertilizer application.Compared to the controls, net GWP of P-addition plots decreased by 4.6 % while N-addition plots increased by 34.0 %, over the 33 months of measurements.The combined N plus P addition increased net GWP by 15 %, mainly due to increased N 2 O emission.

Discussion
The annual CH 4 uptake rates in our old-growth tropical forest site in Southern China ranged from 2.3 to 3.4 kg CH 4 -C ha −1 yr −1 (11 months in 2007, 12 months in 2008, 10 month in 2009), and were higher in the dry year (2007) than in the wet year ( 2008).
The CH 4 fluxes quantified in this study are similar to the previous reported CH 4 fluxes measured at an adjacent forest (Tang et al., 2006;Zhang et al., 2008) and other parts of tropical Southwest China (Werner et al., 2006;Yan et al., 2008;Wang et al., 2010).
The rates of CH 4 uptake in other tropical forests are also similar, ranging from 0.8 to 4.73 kg CH 4 -C ha −1 yr −1 (Steudler et al., 1991;Kiese et al., 2003;Davidson and Nepstad, 2004;Davidson et al., 2008).Soil CH 4 uptake was negatively correlated with the soil WFPS (a better measurement of soil moisture) in this study (Fig. 7), which was consistent with previous publications (Born et al., 1990;Castro et al., 1995;Kiese et al., 2003).Higher WFPS often lead to lower soil aeration and reduced CH 4 oxidation.Steinkamp et al. (2001) similarly reported that soil moisture was the dominant factor controlling CH 4 uptake when soil temperature was > 10 • C. Temperature variation has a minor impact on soil CH 4 uptake in the tropical forest we studied, which is consistent with previous results (Tang et al., 2006).In temperate or boreal regions, in contrast, soil CH 4 oxidation was generally positively correlated with soil temperature (Crill, 1991;King, 1997).Castro et al. (1995) Introduction

Conclusions References
Tables Figures

Back Close
Full observed that methanotrophy was affected by soil temperature between −5 and 10 • C, but in our site, almost all of soil temperatures were between 10 and 30 • C (Fig. 2a).
We found P addition significantly increased CH 4 uptake, N addition decreased CH 4 uptake, and P addition mitigated the negative N effect in this 33 months field experiment, as hypothesized.The treatment effects were particularly strong in the summers and falls, when soil uptakes of CH 4 were the highest (Fig. 4).We believe this is the first experimental testing of N and P limitation on soil CH 4 flux in tropical and subtropical forests.Since phosphorus is a common limiting nutrient in tropical forests (Herbert et al., 2003;Wardle et al., 2004), microbial usage of soil C could be strongly P limited (Cleveland et al., 2002).Increased soil CH 4 uptake in this study may be due to stimulated methanotroph activities after increased P availability in soil.A companion study we found that microbial biomass carbon was increased significantly by P addition both in dry and wet seasons when adjusted for pre-treatment differences (Liu et al., 2011).Treseder and Allen (2002) observed that biomass of arbuscular mycorrhizal (AM) fungi was increased by P addition in a P-limited Hawaiian rain forest.
Increased phosphorus concentration may also increase soil fauna activity (McGlynn et al., 2007).Phosphorus addition could also affect belowground C flow and indirectly regulate soil physico-chemical properties and plant roots activities.Root growth is a significant C source for microbial organisms and renders an additional fraction of soil C available to microbial utilization (Helal and Sauerbeck, 1986), that could positively affect soil microbial processes involved in CH 4 consumption and emission.Low P availability due to strong sorption is often the main constraint on plant growth and primary production on highly weathered tropical soils (Vitousek, 1984).In 4.1 million year-old forests in Hawaii, P-fertilizer leads to a greater belowground net primary productivity and root turnover (Ostertag, 2001).Therefore, microbial activity could be stimulated by the increased belowground carbon flow due to increased available P. That is in line with our observed higher soil CO 2 emission (from soil respiration) after P-addition (Fig. 3).Finally, phosphorus fertilizer may increase the mineralization of organic P (Ofori-Frimpong and Rowell, 1999) and reduce Al 3+ toxicity.Increased soil P Introduction

Conclusions References
Tables Figures

Back Close
Full level might stabilize soil aluminum and iron through geochemical reaction of adsorption (Frossard et al., 1995).This coupling effect decreased aluminum toxicity to methanotrophs (Nanba and King, 2000).However, in our experiment we found no treatment effect on soil exchangeable Al 3+ (unpublished data).On the other hand, in this study we did find that P fertilizer decreased soil WFPS (Fig. 2b), which would increase soil aeration and potentially increase CH 4 consumption.This decreasing of soil WFPS was likely due to increased plant water consumption after P addition.
The inhibitive effect of N input on CH 4 uptake has been reported extensively.Zhang et al. (2008) suggested that the response of forest soil CH 4 uptake on N fertilization possibly depends on soil N status.They compared the CH 4 uptake under various N additions in three subtropical forests with very different N conditions and found N addition in N-saturated old-growth forest significantly decreased soil CH 4 uptake while in N-limited young forests, N addition had a limited effect.Singh et al. (1997) observed in a natural gradient of tropical forests, sites with higher soil mineral nitrogen concentrations had lower CH 4 uptake.In a Puerto Rican wet forest, Steudler et al. (1991) found that maximum reduction in CH 4 uptake coincided with the highest soil NH + 4 , suggesting that nitrogen-CH 4 linkage observed in temperate forests (Steudler et al., 1989) may also function in tropical forests.
Nitrogen inhibition on soil CH 4 uptake would be strengthened due to continuous nitrogen deposition and phosphorus limitation in Southern China tropical forests and other parts of the world.However, P addition could reverse such trends by stimulating forest soil CH 4 uptake.In this 33 months field experiment, we showed that increased P availability might mitigate the inhibitive effect of N addition on soil CH 4 uptake in this N-saturated old-growth tropical forest.While average annual consumption of CH 4 was enhanced after P addition and decreased after N addition, there was no difference between the NP-addition plots and the controls (i.e., the negative effect of N addition on CH 4 uptake was reduced by P addition) (Fig. 4).We are not aware of any other study that experimentally tested the P-addition on soil CH 4 uptake in P-limited tropical forests.Given the importance of tropical forests in global terrestrial ecosystems and

Conclusions References
Tables Figures

Back Close
Full CH 4 as an important greenhouse gas, further studies of N vs. P controls on soil fluxes of GHGs are urgently needed.Net GWP (from CH 4 uptake and N 2 O emission) was decreased by 2.23 CO 2 equivalents (g CO 2 m −2 yr −1 ) under P addition, and increased by 16.4 CO 2 equivalents under N addition.However, both were minor compared to the net GWP from soil CO 2 emission (548.2 g CO 2 m −2 yr −1 ).P addition contributed less to the system's net GWP change due to a neutral effect on N 2 O, and a limited positive effect on CH 4 uptake.
Adding N contributed positively to the system's GWP due to an enhanced N 2 O emission.

Conclusions
In this 33 months field experiment, we found that P addition significantly increased soil CH 4 uptake and potentially mitigated the inhibitive effect of N deposition on soil CH 4 uptake in an old-growth tropical forest of Southern China.Soil CH 4 uptake was negatively affected by N addition, likely due to increased nitrification, which would compete with CH 4 oxidation for MMO.Increased nitrification and denitrification also contributed  Full  Full  Full Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . Major species in the old-growth forest are Castanopsis chinensis Hance, Schima superba Chardn.and Champ., Cryptocarya chinensis (Hance) Hemsl., Cryptocarya concinna Hance, Machilus chinensis (Champ.Ex Benth.)Hemsl., Syzygium rehderianum Merr.and Perry in the tree layer and Calamus rhabdicladus Burret, Ardisia quinquegona Introduction Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Analysis of Variance (ANOVA, PROC MIXED with AR(1) from SAS (SAS Institute Inc., Cary NC, USA)) was used to examine the effect of fertilizer treatments on soil GHG fluxes from February 2007 to October 2009.Two-way ANOVA (PROC GLM from SAS) was used to examine the effects of N and P addition.One-way ANOVA was used to examine the difference in soil pH, NH + 4 , NO − 3 , and available P among treatments.Linear regression analysis was performed by Origin 8.0 (OriginLab Corporation, Northampton, MA USA) to examine the relationship between CH 4 fluxes and soil WFPS contents and soil temperature.Out of 680 observations three were identified as outliers, which were probably caused by chamber leaks, abnormally high WFPS, and other unknown factors, and were removed from the data analyses.Statistical significant differences were set at p values < 0.05 unless otherwise stated.Mean values ±1 standard error are given in the text.Discussion Paper | Discussion Paper | Discussion Paper | 3 Results Discussion Paper | Discussion Paper | Discussion Paper | the springs of 2008 and 2009 (Fig. 4), and a negative N effect in the springs of 2008 and 2009, and summer 2009 (Fig. 4).In summer 2007, CH 4 uptake in the P-addition and NP-addition plots was 26.6 % and Discussion Paper | Discussion Paper | Discussion Paper |27.3 % higher than in the control plots (p = 0.049 and 0.051, respectively).CH 4 uptake in N-addition plots was reduced by 30.3 % in fall 2007 (p = 0.052), 39.1 % in spring 2008 (p = 0.034) and 36.7 % in spring 2009 (p = 0.027).In addition, CH 4 uptake was increased by 30.6 % in P-addition plots in summer 2009 (p = 0.038).
808).However, soil CO 2 flux was significantly increased by both P addition (mean CO 2 flux was 88.4 ± 4.1 mg CO 2 -C m −2 h −1 , p = 0.033) and N plus P addition (mean CO 2 flux was 86.5 ± 3.5 mg CO 2 -C m −2 h −1 , p = 0.048) (Fig. 3).Soil CO 2 fluxes were Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | to higher N 2 O emission, another potent greenhouse gas.In this N-saturated but Plimited forest, P addition likely increased belowground C flow, soil microbial activity, and consequently CH 4 uptake.We believe this is the first field study experimentally testing the N vs. P controls on soil CH 4 uptake.Considering the global increases of CO 2 , CH 4 , and N 2 O and the global alteration of N and P biogeochemistry, the critical role of soil phosphorus under N deposition needs to be taken seriously to assess forest function as sinks and sources of GHGs.Supplementary material related to this article is available online at: http://www.biogeosciences-discuss.net/8/4953/2011/ bgd-8-4953-2011-supplement.pdf.Discussion Paper | Discussion Paper | Discussion Paper | sequences of nitrogen fertilization on soil methane consumption in a productive temperate deciduous forest, Biol.Fert.Soils, 41, 182-189, 2005.Cleveland, C. C. and Townsend, A. R.: Nutrient additions to a tropical rain forest drive substantial soil carbon dioxide losses to the atmosphere, P. Natl.Acad.Sci.USA, 103, 10316Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Figure 1 .
Figure 1.Monthly precipitation (bar) and average air temperature (line and open
. Gas samples were analyzed within 12 h in a gas chromatograph (Agilent 4890D) fitted with a flame ionization detector (FID) for CH 4 and CO 2 , and an electron capture detector (ECD) for N 2 O. CO 2 was transformed into CH 4 via (Ni)H 2 before the FID analysis.Calibration gases (CH 4 at 1.87 ppm, CO 2 at 418 ppm, N 2 O at 0.321 ppm, bottle's No. 070811) were obtained from the Institute of Atmospheric Physics, Chinese Academy of Sciences.

Table 1 .
Indices of an old-growth tropical forest at DHSBR (Dinghushan Biosphere Reserve).Survey was conducted at the beginning of February 2007 (before the start of N and P fertilization).

Table 2 .
General characteristics of the 0-10 cm mineral soils, measured at the beginning of February 2007, before the initiation of fertilization.Values are means with SE in parentheses (N = 5).Values followed by different letters are significantly different among treatments with p < 0.05.

Table 3 .
Soil properties after 24 months of fertilization treatment, measured in February 2009.Values are means with SE in parentheses (N = 5).Values followed by different letters are significantly different among treatments with p < 0.05.