Management scheme influence and nitrogen addition effects on soil CO2, CH4, and N2O fluxes in a Moso bamboo plantation

It is still not clear whether the effects of N deposition on soil greenhouse gas (GHG) emissions are influenced by plantation management schemes. A field experiment was conducted to investigate the effects of conventional management (CM) versus intensive management (IM), in combination with simulated N deposition levels of control (ambient N deposition), 30 kg N·ha− 1·year− 1 (N30, ambient + 30 kg N·ha− 1·year− 1), 60 kg N·ha− 1·year− 1 (N60, ambient + 60 kg N·ha− 1·year− 1), or 90 kg N·ha− 1·year− 1 (N90, ambient + 90 kg N·ha− 1·year− 1) on soil CO2, CH4, and N2O fluxes. For this, 24 plots were set up in a Moso bamboo (Phyllostachys edulis) plantation from January 2013 to December 2015. Gas samples were collected monthly from January 2015 to December 2015. Compared with CM, IM significantly increased soil CO2 emissions and their temperature sensitivity (Q10) but had no significant effects on soil CH4 uptake or N2O emissions. In the CM plots, N30 and N60 significantly increased soil CO2 emissions, while N60 and N90 significantly increased soil N2O emissions. In the IM plots, N30 and N60 significantly increased soil CO2 and N2O emissions, while N60 and N90 significantly decreased soil CH4 uptake. Overall, in both CM and IM plots, N30 and N60 significantly increased global warming potentials, whereas N90 did not significantly affect global warming potential. However, N addition significantly decreased the Q10 value of soil CO2 emissions under IM but not under CM. Soil microbial biomass carbon was significantly and positively correlated with soil CO2 and N2O emissions but significantly and negatively correlated with soil CH4 uptake. Our results indicate that management scheme effects should be considered when assessing the effect of atmospheric N deposition on GHG emissions in bamboo plantations.


Introduction
Extensive research on soil greenhouse gases (GHGs) has shown that CO 2 , CH 4 , and N 2 O released from soil significantly contribute to global warming (IPCC 2013;WMO 2019). Atmospheric concentrations of CO 2 , CH 4 , and N 2 O have increased considerably from 278 ppm, 722 ppb, and 270 ppb in 1750 to 408 ppm, 1869 ppb, and 331 ppb in 2018, respectively (WMO 2019). Forest ecosystems, as an important source of atmospheric CO 2 and N 2 O and an important sink of CH 4 (Liu and Greaver 2009), play a key role in regulating GHG fluxes under global climate change.
Recently, atmospheric N deposition has drastically increased in East and South Asia and is expected to continue to increase (Galloway et al. 2008;Reay et al. 2008;Tian et al. 2016), particularly in subtropical China , where N deposition has reached 30.9 kg N·ha − 1 ·year − 1 (Xie et al. 2008) and is predicted to reach 50 kg N·ha − 1 ·year − 1 by 2050 (Galloway et al. 2004). Many studies have shown that N deposition can significantly influence forest soil CO 2 , CH 4 , and N 2 O emissions, including both promotion and inhibition effects (Zhang et al. 2008a;Liu and Greaver 2009;Deng et al. 2020). For example, in temperate forests, the effect of N deposition on soil CO 2 emissions includes promotion (Bowden et al. 2004;Zheng et al. 2018) and inhibition (Geng et al. 2017), as well as no effect (Krause et al. 2013;Sun et al. 2014). N deposition reduces (Sitaula et al. 1995;Gulledge et al. 2004;Kim et al. 2012;Krause et al. 2013) or increases (Geng et al. 2017) soil CH 4 uptake and accelerates soil N 2 O emissions (Venterea et al. 2003;Kim et al. 2012;Krause et al. 2013;Sun et al. 2014;Song et al. 2017a). In tropical forests, N deposition reduces soil CO 2 emissions (Mo et al. 2008;Cusack et al. 2011;Tian et al. 2019); enhances (Zhang et al. 2008b;Wang et al. 2014;Tian et al. 2019) or has no effect (Zhang et al. 2008b;Müller et al. 2015;Tang et al. 2018) on soil N 2 O emissions; and inhibits or has no effect on soil CH 4 uptake (Zhang et al. 2012). N deposition was shown to promote soil N 2 O emissions in an N-saturated forest (Xie et al. 2018), while it increased soil CO 2 emissions in a bamboo ecosystem (Tu et al. 2013), an evergreen forest (Gao et al. 2014), and a highly P-limited forest (Liu et al. 2019). Additionally, Wang et al. (2015) found that N deposition promoted soil N 2 O emissions but reduced soil CH 4 uptake in a slash pine plantation. Li et al. (2015) also measured the effects of N deposition in a slash pine plantation and found that soil CO 2 and N 2 O emissions increased, but soil CH 4 uptake was unaffected. These inconsistent results indicate that the effects of N deposition on GHG emissions strongly depend on forest type. Moreover, most studies have only observed the fluxes of one or two GHGs in forest soil under N deposition (Jassal et al. 2010;Kim et al. 2012;Wang et al. 2015;Tian et al. 2019). Studies that simultaneously measure the fluxes of three GHGs in forest soils under increasing N depositions are scarce (Krause et al. 2013;Song et al. 2017a), especially in subtropical plantations Song et al. 2020).
An increasing number of plantations have been established in recent years to satisfy the increasing global demand for timber products (FAO 2005). As one of the forestry practices that increases productivity, intensive management (IM) is essential for meeting current and future timber needs (McEwan et al. 2020). Several studies have shown that IM significantly increases CO 2 emissions (Mori et al. 2017;Yang et al. 2017) but does not affect CH 4 or N 2 O emissions from forest soils (Mori et al. 2017). Moso bamboo (Phyllostachys edulis) plantations are one of the most important types of forests in southern China. Approximately 4.43 million hectares are under Moso bamboo cultivation in this region, comprising 84.02% of the global Moso bamboo forest area (Song et al. 2017b). In recent decades, owing to the high economic and ecological benefits of Moso bamboo, an increasing number of farmers have utilized IM practices to replace conventional management (CM) practices in Moso bamboo plantations, including fertilization, plowing, and weeding understory vegetation (Song et al. 2015). In Moso bamboo plantations, IM has been observed to significantly increase soil CO 2 emissions Tang et al. 2016), while its effects on soil N 2 O and CH 4 fluxes remain unknown. Therefore, the effect of IM on soil GHG emissions in Moso bamboo plantations is an empirical gap that needs to be addressed.
Furthermore, Moso bamboo plantations are located in subtropical China where N deposition had increased dramatically in recent years . Our previous study showed that N deposition increased soil CO 2 emissions (Li et al. 2019) and N 2 O emissions ) but decreased soil CH 4 uptake  in Moso bamboo plantations under IM. However, the comprehensive effects of management scheme combined with N deposition on soil GHG fluxes in Moso bamboo plantations remain unclear. Here, we studied the individual and combined effects of N deposition and management scheme on soil CO 2 , CH 4 , and N 2 O fluxes for one year in a Moso bamboo plantation. We hypothesized that (1) IM increases CO 2 emissions but does not affect N 2 O emissions or CH 4 uptake, because IM can promote soil respiration; (2) N addition promotes CO 2 and N 2 O emissions but inhibits CH 4 uptake under CM, because N addition can promote soil respiration, nitrification and denitrification but inhibit methane oxidation; and (3) IM intensifies the effect of N addition on soil GHG fluxes, because IM can provide more nutriment, especially N.

Study site
The details of the study site were reported previously (Song et al. 2015). Briefly, the site is located in Lin'an District (30°14′ N, 119°42′ E), Hangzhou City, Zhejiang Province, China, and is characterized by a subtropical monsoon climate with a mean annual temperature of 15.6°C and a mean annual precipitation of 1420 mm. The soil belongs to the Ferrisols derived from granite (Song et al. 2015). The landscape is hilly, with an elevation range of 100 to 300 m a.s.l. The Moso bamboo plantations were initially established in the late 1970s to replace a native evergreen broadleaf forest, with similar soil type and topography (a southwest slope of approximately 6°). Moso bamboo forests in the study area are divided into CM and IM plantations according to the management scheme to which they are subjected. Conventionally managed plantations are selectively and regularly harvested for bamboo stems and shoots according to demand, with no other management practices in place. In IM plantations, additional management practices such as plowing, weeding by herbicide spray, and fertilization are practiced, in addition to bamboo harvesting as per CM plantations. Specifically, every year in September, fertilizers (67.5 kg N·ha − 1 , 11.8 kg P·ha − 1 , and 74.7 kg K·ha − 1 ) are evenly spread on the ground and then plowed to mix with the 30-cm topsoil (Song et al. 2015). Compared with CM plantations, IM plantations have fewer understory species and lower shrub and herbal biomass. Forest stand and soil characteristics at the study site are shown in Table 1 (Song et al. 2015).

Experimental design and measurements
In November 2012, 24 plots (20 m × 20 m) with a 20-mwide buffer zone (to avoid disturbing nearby plots) were set up in the Moso bamboo plantations of the study site. According to the N-deposition simulation method reported by Mo et al. (2007) and background atmospheric N deposition data of the site (30.9 kg N·ha − 1 ·year − 1 ; Xie et al. 2008), the N addition rate was set to equal, double, and triple the local N deposition rate. There were 12 IM plots and 12 CM plots with three replications of four treatment levels: control (ambient N deposition), N30 (low N treatment, ambient + 30 kg N·ha − 1 ·year − 1 ), N60 (medium N treatment, ambient + 60 kg N·ha − 1 ·year − 1 ), and N90 (high N treatment, ambient + 90 kg N·ha − 1 · year − 1 ) (Song et al. 2015(Song et al. , 2017c. The N source for Ndeposition simulation was ammonium nitrate (NH 4 NO 3 ; Song et al. 2015). It has been reported that NH 4 + and NO 3 − account for 56.1% and 43.9% of the wet N deposition in China, respectively, and the average NH 4 + : NO 3 − ratio was 1.28 (Lei et al. 2016). From January 2013 to December 2015, the amount of NH 4 NO 3 (Xilong Chemical Co. Ltd., China) corresponding to each N treatment was dissolved in 10 L of water and uniformly sprayed on the forest floor of each N-treated plot in CM and IM plantations once a month (Song et al. 2015). Each control treatment plot was sprayed with 10 L of Nfree water to balance the effects of added water.

Measurement of soil GHG fluxes
Soil CO 2 , CH 4 , and N 2 O were collected using the static chamber method. The static chamber assembly consists of a permanently mounted base box (40 cm × 40 cm × 10 cm) with a U-shaped groove (5 cm wide and 5 cm deep) at the top and a removable cover box (40 cm × 40 cm × 40 cm). During gas sampling, the cover boxes were placed onto the base boxes and the grooves filled with water to serve as an air seal. A small fan was installed in each chamber to mix the air within the chamber during sampling. The base frame was directly inserted 5 cm into the soil in January 2013. Sampling was conducted between 9:00 am and 10:00 am to minimize the influence of variation. Four samples were taken with a 60-mL plastic syringe attached to a 3-way stopcock every 10 min for 30 min (i.e., at 0, 10, 20, and 30 min). Button thermometers (iButton DS1923; Wdsen Electronic Technology Co. Ltd., China) buried at a depth of 5 cm were used to monitor soil temperature at hourly intervals. GHG concentrations were analyzed after Li et al. (2019) using a gas chromatographer (GC-2014 Shimadzu Corp., Japan) within two days. We collected GHG emission data on a clear day once a month from January to December 2015. Gas fluxes were calculated using the following equation (Li et al. 2019): where R is gas flux (mg CO 2 -C·m − 2 ·h − 1 for CO 2 , μg CH 4 -C·m − 2 ·h − 1 for CH 4 , and μg N 2 O-N·m − 2 ·h − 1 for N 2 O), ρ is gas density under normal conditions (mg·m − 3 ), V is the volume of the static chamber (m 3 ), A is the area that the static chamber covered, Δc/Δt is the change in gas concentration (Δc) during a certain time (Δt), T is air temperature (°C), and n is the coefficient Table 1 Forest stand and soil characteristics in the Moso bamboo forests at the study site Soil pH CM 3106 ± 386a 10.08 ± 0.38a 1.06 ± 0.07a 27.8 ± 0.3a 0.9 ± 0.03b 0.06 ± 0.00b 0.4 ± 0.01a 0.002 ± 0.000a 4.53 ± 0.02a IM 3362 ± 309a 10.16 ± 0.13a 0.97 ± 0.07a 23.7 ± 0.2b 1.1 ± 0.04a 0.09 ± 0.00a 0.5 ± 0.01a 0.002 ± 0.000a 4.46 ± 0.01a CM conventional management; IM intensive management; DBH diameter at breast height; SBD soil bulk density; SOC soil organic C; TN soil total N; AN available N; TP soil total P; AP available P. Different small letters within columns indicate significant difference at the 0.05 level for converting the masses of CO 2 , CH 4 , and N 2 O to the masses of C and N (12/44 for CO 2 , 12/16 for CH 4 , and 28/44 for N 2 O).
The following equation was used to calculate cumulative soil CO 2 , CH 4 , and N 2 O fluxes : ) flux determined at each sampling time; i is the sampling number, t is the sampling time, and d is the number of days in each month.
Based on these measurements, an exponential regression model was used to describe the relationship be-tween soil CO 2 efflux and soil temperature (Li et al. 2019): where Y is soil CO 2 efflux, T is soil temperature at 5cm depth, and α and k are the model coefficients. Soil CO 2 efflux temperature sensitivity (Q 10 ) was calculated as in Li et al. (2019): The Global warming potential (GWP) metric was developed to allow comparisons of the global warming impacts of different gases. The GWP of soil GHG emissions was computed by considering the respective GWP coefficients of CH 4 and N 2 O using the following equation (Tian et al. 2015): where F CO 2 − C , F CH 4 − C , and F N 2 O − N are the annual fluxes of CO 2 , CH 4 , and N 2 O respectively based on the masses of C and N; 25 and 298 indicate the radiative forcing of CH 4 and N 2 O, respectively, in terms of a CO 2 eq unit at a 100-year time horizon (Forster et al. 2007).

Soil analysis
Ten soil cores (3.5-cm inner diameter) were collected randomly from the topsoil layer (0-20 cm) in each plot in every quarter of 2015. The soil samples were sieved through a 0.15-mm sieve and divided into two portions. One portion was air-dried for measuring soil pH and conducting soil organic carbon (SOC), soil total N (TN), and soil total phosphorus (TP) assays, and the other portion was stored in a refrigerator for measuring soil microbial biomass carbon (MBC), NH 4 + , and NO 3 − . Briefly, soil pH was measured in a soil:water ratio of 1:2.5 using a pH meter (Li et al. 2016). Soil SOC and TN concentrations were determined using an elemental analyzer (Elementar Vario EL III; Germany). Soil TP was extracted with a Bray-2 solution (Bray 1945) and determined using the molybdate blue colorimetric method. Soil MBC was estimated using the chloroform-fumigation extraction method (Bao 2000). Samples were extracted with a 2-mol·L − 1 KCl solution, and the concentrations of NH 4 + and NO 3 − were determined using a Dionex ICS 1500 ion chromatographer (Dionex Corp. Atlanta, GA).

Statistical analysis
Data analyses were performed using SPSS 22.0 (SPSS Inc., Chicago, IL, USA) for Windows. One-way analysis of variance (ANOVA) and least significant difference multiple comparisons were used to identify significant differences in Q 10 , GWP, and soil CO 2 , CH 4 , and N 2 O fluxes. Two-way ANOVA was used to test the significance of the interaction between N addition and management scheme for the variation in soil CO 2 , CH 4 , and N 2 O fluxes. All data were tested for homogeneity of variance and distribution normality before conducting the ANOVA. In addition, Pearson's correlation analyses between soil characterization and soil CO 2 , CH 4 , and N 2 O fluxes were conducted.

Soil GHG fluxes
The seasonal variation of soil CO 2 flux showed the same pattern in all treatments, peaking in summer and reaching a trough in winter (Fig. 1). Compared with CM, IM significantly promoted annual soil CO 2 emissions by 7.5%. Compared with the control treatment, N30 and N60 significantly promoted annual soil CO 2 emissions by 31.7% and 22.1% in CM plots, and by 34.0% and 20.9% in IM plots, respectively, while N90 had no significant effect on annual soil CO 2 emissions in either CM or IM plots (Fig. 1). Management scheme had no significant effect on annual soil CH 4 uptake. Compared with the control treatment, N30 had no significant effect on annual soil CH 4 uptake in either CM or IM plots, but N60 and N90 significantly inhibited annual soil CH 4 uptake by 23.6% and 27.1% in CM plots, respectively (Fig. 2). The dynamics of annual soil N 2 O emission rates were not significantly affected by management scheme, but N addition caused N 2 O emission rates to peak from March to April (Fig. 3). In CM plots, compared with the control treatment, N30 did not significantly affect annual soil N 2 O emissions, while N60 and N90 significantly promoted annual soil N 2 O emissions by 52.7% and 47.0%, respectively (Fig. 3). On the contrary, in IM plots, compared with control treatment, N30, N60, and N90 significantly promoted annual soil N 2 O emissions by 61.3%, 69.2%, and 49.3%, respectively. Two-way ANOVA showed that N addition or management scheme independently had significant effects on soil CO 2 emissions, CH 4 uptake, and N 2 O emissions, but the interactions between them did not (Table S1).
The Q 10 value of soil CO 2 efflux varied from 1.89 to 2.37 under the different treatments combining management scheme and N addition (Table S2). IM significantly increased the Q 10 value by 11.3% relative to that in CM when no N was added. N addition had no significant effect on the Q 10 value in CM plots but significantly decreased the Q 10 value in IM plots (Table S2). Furthermore, the significantly higher Q 10 value in IM than in CM plots under no N addition treatments decreased under both N60 and N90 treatments (Table S2).
Soil CO 2 flux was significantly and positively correlated with soil MBC and TP concentrations but significantly and negatively correlated with soil SOC and TN concentrations, and C/N ratio (P < 0.05, Table 2). Soil CH 4 flux significantly and negatively correlated with soil MBC, TP, and NH 4 + concentrations (P < 0.05, Table 2). Soil N 2 O flux was significantly and positively correlated  with soil MBC and NO 3 − concentrations and negatively correlated with C/N ratio and pH (P < 0.05, Table 2).

Annual GWP of soil GHG fluxes
Annual GWP owing to soil CO 2 , CH 4 , and N 2 O emissions was 34.88 ± 1.19 t CO 2 eq·ha − 1 in the Moso bamboo plantations under CM and without N addition (Fig. 4). Compared with CM, IM significantly increased the annual GWP by 7.98%. Furthermore, compared with the control treatment, N30 and N60 significantly increased the annual GWP by 31.5% and 23.7% in CM plots, and by 35.6% and 23.6% in IM plots, respectively, while N90 had no significant impact on the annual GWP under either CM or IM treatments.
We found that IM significantly increased annual CO 2 emissions, which partially supports our first hypothesis and was consistent with the results of Liu et al. (2011) in Fig. 4 Annual global warming potential (GWP) of soil greenhouse gas emissions (CO 2 , CH 4 , and N 2 O) in Phyllostachys edulis plantations (mean ± standard deviation, n = 3). Lowercase letters indicate differences in GWP under different N addition treatments in CM plots (P < 0.05). Uppercase letters indicate differences in GWP under different N addition treatments in IM plots (P < 0.05). The asterisk indicates differences in GWP under different management schemes within the same N addition treatment (P < 0.05) Moso bamboo forests. However, some studies indicate that the state box method using linear fitting will cause an underestimation of soil CO 2 efflux (Wang 2005;Luo and Zhou, 2006). Therefore, the real soil CO 2 efflux may be greater in the Moso bamboo plantation. Soil CO 2 efflux mainly comprises root respiration and microbial respiration (Coleman 1973). IM practices, such as fertilization, stimulate root respiration (Jassal et al. 2010;Mori et al. 2013;Tang et al. 2016). Concomitantly, ploughing disaggregates soil and releases protected SOC (Sainju et al. 2008;Li et al. 2013;Tivet et al. 2013), which provides more substrate for microbial respiration, and the decomposition of protected SOC can increase soil CO 2 emissions. Meanwhile, fertilization increases soil MBC by providing abundant nutrients for microbial growth (Li et al. 2016) and accelerates the decomposition of organic matter by heterotrophic microorganisms (Cleveland et al. 2002;Ilstedt et al. 2003), resulting in a decrease in SOC concentration (Ma et al. 2011) and promotion of soil CO 2 emissions (Tu et al. 2013). We observed that soil CO 2 flux was significantly and positively correlated with soil MBC and significantly and negatively correlated with SOC (Table 2). IM had no significant effect on annual CH 4 uptake or N 2 O emissions, which partially supports our first hypothesis regarding the fluxes of these two gases, i.e., IM does not affect soil N 2 O emissions or CH 4 uptake. Previous studies have also shown that management does not affect soil CH 4 uptake or N 2 O emissions (Whalen and Reeburgh 2000;Jassal et al. 2010;Zhang et al. 2015). The response of soil N 2 O emissions to external environmental factors and the influence on soil CH 4 uptake were determined to be the major reasons for the differences observed in soil GHG emissions (Yan et al. 2014). Some studies found that applying N fertilizer could stimulate soil N 2 O emissions in farmland soil (Jäger et al. 2013) and vineyard soil (Tatti et al. 2012). However, compared with CM, IM did not significantly increased soil N 2 O emissions (Fig.  3), which may be attributed to infrequent fertilization (once a year).
The Q 10 value reflects the temperature dependence of soil CO 2 efflux, calculated from a series of soil CO 2 efflux measurements over a time period while soil temperature changes (Rey et al. 2002;Ma et al. 2014). The Q 10 value of the soil CO 2 efflux in plantations under CM is 2.13, which is close to the Q 10 value of Moso bamboo forests in the Wanmulin Natural Reserve (2.08; Wang et al. 2011) and the average Q 10 of bamboo forests in China (2.10; Song et al. 2014). Compared with CM, IM significantly increased the Q 10 of soil CO 2 efflux (Table S2). Tang et al. (2016) observed the same result and concluded that the main reason might be the increase in Q 10 of soil microbial respiration. In the present study and the previous study on the same site, IM significantly increased soil MBC (Table S4; Li et al. 2016). Moreover, a significant positive correlation between soil CO 2 flux and soil MBC was found in this study (Table 2), which supports the conclusion of Tang et al. (2016). Li et al. (2020) have found that root respiration does not affect Q 10 of soil CO 2 efflux in forest ecosystems.
Compared with CM, IM significantly increased the annual GWP in Moso bamboo plantations (Fig. 4), which can be mainly attributed to the increase in CO 2 emissions (Table S5). The results suggest that IM induces greater GHG emissions from soils than CM did, although IM may enhance the productivity of Moso bamboo plantations (Zhou et al. 2010). Therefore, the C benefits of IM to Moso bamboo plantations need further comprehensive evaluation, especially in the scenarios of increasing atmospheric N deposition.
Effect of N addition on soil CO 2 , CH 4 , and N 2 O fluxes In this study, N addition enhanced CO 2 emissions in both CM and IM plots, which partially supports our second hypothesis, i.e., N addition promotes CO 2 emissions. Tu et al. (2009) observed that simulated N deposition promoted soil CO 2 emission in a bitter bamboo plantation. Some short-term simulated N deposition studies have also shown similar results (Madritch and Hunter 2003;Mo et al. 2005;Song et al. 2007). Soil CO 2 emissions are related to above-ground biomass, litter mass, underground root biomass, and soil biological factors (e.g., microorganisms and animals) (Zhang et al. 2008a).
Our previous studies showed that N input increased the amount of leaf litter , decomposition of leaf litter (Song et al. 2015), fine root litter (Song et al. 2017c), and soil microbial biomass (Li et al. 2016) in the current study site, all of which contributed to oxidizing organic C to CO 2 (Steudler et al. 1991;Emmett 1999), thus increasing CO 2 emissions. In this study, soil MBC significantly and positively correlated with CO 2 flux (Table 2), which supports the conclusion that N addition increased soil CO 2 emissions by increasing MBC. However, Li et al. (2017) found that the CO 2 emissions of the Moso bamboo forest soil did not change after N addition (40 kg N·ha − 1 , KNO 3 ) in their incubation experiment, which was different from our experimental results. This difference may be owing to the difference in N source, the external environment of the experiment, and the processing time.
N addition significantly inhibited soil CH 4 uptake in CM, which supports our second hypothesis, i.e., N addition decreases CH 4 uptake in CM. Similar results have been observed in a Douglas fir stand (Jassal et al. 2011) and a young Japanese larch plantation (Kim et al. 2012). Soil CH 4 uptake rate is usually negatively correlated with soil NH 4 + concentration (Zhang et al. 2012), as was observed in this study ( Table 2). The inhibitory effect of NH 4 + on soil CH 4 oxidation can be attributed to the production of the intermediates, hydroxylamine and NO 2 − , during the nitrification of NH 4 + , which likely inhibits the activity of methane-oxidizing bacteria, thereby extending the inhibition time (Nyerges and Stein 2009). Further, low pH can reduce the activity of methane-oxidizing bacteria (Semenov et al. 2004), because soil acidification may increase the concentration of Al 3+ in the forest soil solution, while Al 3+ ions have an obvious toxic effect on CH 4 -oxidizing bacteria (Nanba and King 2000;Tamai et al. 2007).
In this study, N addition (N60 and N90) significantly increased N 2 O emissions, which partially supports our second hypothesis, i.e., N addition increases soil N 2 O emissions. Similar results were observed in a young Japanese larch forest (Kim et al. 2012) and in incubated Moso bamboo forest soil . N input can increase soil N availability, nitrification, and denitrification and, thus increase N 2 O emissions (Repo et al. 2009). N addition decreased soil pH significantly (Table S4), while NO 2 − may have induced aerobic denitrification in acidic soils (Mørkved et al. 2007), thereby increasing N 2 O emissions. A significant negative correlation between soil pH and soil N 2 O flux was found in this study (Table 2). N addition increased soil total N content and, thus decreased the soil C/N ratio (Li et al. 2019), which is beneficial for the proportion of external N input converted to N 2 O (Zhang et al. 2008a). Similarly, a significant negative correlation between soil C/N ratio and soil N 2 O flux was found in this study (Table 2).
N addition had no effect on Q 10 of soil CO 2 efflux relative to that in the control treatment in plots under CM but significantly decreased Q 10 value in IM plots (Table S2). Similar results have been reported by Mo et al. (2007), who found three-year high-N addition (150 kg N·ha − 1 ·year − 1 ) reduced Q 10 values in a mature tropical forest. Tu et al. (2013) also observed that N addition decreased Q 10 of soil CO 2 efflux in a bamboo ecosystem in southwestern China. Karhu et al. (2014) suggested that microbial community responses increase the temperature sensitivity of soil heterotrophic respiration. Our previous studies have shown that, although N addition significantly increased soil MBC, it also decreased soil pH (Li et al. 2016), which might result in microbial activity being inhibited by soil acidity (Kunito et al. 2016). This, in turn, may hamper the microbial community responses, whereby the microbial community has no effect on the temperature sensitivity of soil CO 2 efflux. Furthermore, our previous studies have shown that increases in soil MBC are inhibited when N addition exceeds 60 kg N·ha − 1 ·yr − 1 (Li et al. 2016). We suspect that N input from N addition and N fertilizer in IM treatments inhibited any increase in soil MBC and even reduced soil MBC. This might be why N addition did not affect the temperature sensitivity of soil CO 2 efflux under CM, yet it reduced the Q 10 value in the plots under IM.
The GWP of a GHG is a measure of how much energy the emissions of 1 kg of a gas will absorb over a given period of time relative to that absorbed by the emissions of 1 kg of CO 2 (Tian et al. 2015). The larger the GWP, the more that a given gas warms the Earth compared with CO 2 over that time period. Moderate N addition significantly increased annual GWP of soil GHG fluxes in both CM and IM plots, which is mainly attributed to the increase in annual soil CO 2 emissions. Annual soil CH 4 uptake and N 2 O emissions did not significantly affect annual GWP. The reason was that the GWP values of CH 4 and N 2 O were much larger than that of CO 2 (25 and 298 times, respectively), but the annual soil CH 4 uptake and N 2 O emissions were only 0.12‰-0.18‰ and 0.16‰-0.31‰ of the annual CO 2 emissions, respectively.
In addition, IM enhanced soil CO 2 and N 2 O emissions under low N addition (N30), which partly supports our third hypothesis, i.e., IM promotes soil CO 2 and N 2 O emissions under N addition. The finding that IM provided more N input may be why IM significantly increased soil CO 2 and N 2 O emissions under low N addition. IM did not affect soil CH 4 uptake under N addition, which was consistent with the effect on plots without N addition, but it did not support our third hypothesis that IM inhibits soil CH 4 uptake under N addition. This may due to the combination of IM and N addition offsetting the inhibitory effect of N addition on soil CH 4 uptake. In summary, IM significantly increased GWP under low N addition owing to the main contribution of CO 2 emissions to GWP.

Conclusion
Compared with CM, IM significantly increased the GWP of soil GHG emissions and sensitivity of soil CO 2 efflux to soil temperature (Q 10 ), mainly owing to an increase in soil CO 2 emissions. Nitrogen deposition (≤60 kg N·ha − 1 ·year − 1 ) significantly increased soil CO 2 and N 2 O emissions but inhibited CH 4 uptake, which resulted in a significant increase in GWP. However, N addition (> 60 kg N·ha − 1 ·year − 1 ) decreased all soil CO 2 and N 2 O emissions and CH 4 uptake. Concomitantly, the Q 10 value of soil CO 2 efflux was significantly reduced after N addition in plots under IM, which indicates that N addition might mitigate the effect of future climate warming on soil CO 2 efflux in intensively managed Moso bamboo plantations. Soil MBC correlated significantly and positively with soil CO 2 and N 2 O fluxes but correlated negatively with soil CH 4 fluxes, indicating that soil microbes have a strong influence on soil GHG emissions. These results demonstrate that management scheme and N application influenced the GWP of the Moso bamboo plantation ecosystem under study.