Mangrove ecosystems are an import element in the global carbon cycle (Mcleod, Chmura, Bouillon, Salm, Bjork, Duarte, Lovelock, Schlesinger, & Silliman, 2011), and they have a direct impact on CO2 emissions, a greenhouse gas. It is estimated that 5–20% of CO2 and 15–30% of CH4 originate from wetland soils each year (Heimann & Reichstein, 2008; Nellemann C, 2009). The CO2 and N2 removed during the N-DAMO process has a high C to N ratio. Therefore, the N-DAMO process is critical to balance the C to N ratio in mangrove soils. The N-DAMO process has been detected in many different natural ecosystems, including forests (Meng, Wang, Chan, Wu, & Gu, 2016), dry lands (Zhu, Wang, Li, Zhuang, Zhao, Wang, Kuypers, Jetten, & Zhu, 2018), peatlands (Zhu, van Dijk, Fritz, Smolders, Pol, Jetten, & Ettwig, 2012; Zhong, Xue, Chen, Liu, He, Zhu, & He, 2020), reservoirs (Wang, Huang, Ye, Jiang, Song, Op den Camp, Zhu, & Wu, 2016; Naqvi, Lam, Narvenkar, Sarkar, Naik, Pratihary, Shenoy, Gauns, Kurian, Damare, Duret, Lavik, & Kuypers, 2018), rivers (Shen, Ouyang, Zhu, & Trimmer, 2019b; a), lakes (Deutzmann, Stief, Brandes, & Schink, 2014; Zhang, Liu, Li, Jiao, Dvornyk, & Gu, 2019), wetlands (Zhu, Zhou, Wang, Wang, Guo, Long, Sun, Jiang, Hou, Jetten, & Yin, 2015; Chen, Chen, Chang, Yi, Huang, Xie, & Guo, 2018; Zhang, Luo, Lin, Lin, Hetharua, Zhao, Zhou, Zhan, Xu, Zheng, & Tian, 2018), intertidal zones (Wang, Shen, He, Hu, Cai, Zheng, & Hu, 2017; Wang, Cai, Li, Hua, Wang, Yang, Zheng, & Hu, 2019; Zheng, Hou, Chen, Zhou, Liu, Yin, Gao, & Han, 2020b), estuaries (Yan, Li, Wei, Li, & Gao, 2015; Shen, Hu, Liu, Chai, He, Ren, Liu, Geng, Wang, Tang, Wang, Lou, Xu, & Zheng, 2016; Niu, Zheng, Hou, Gao, Chen, Pei, Dong, Liang, & Liu, 2022), and offshore ecosystems (Chen, Jiang, & Gu, 2015; He, Wang, Hu, Yu, Jetten, Liu, Cai, Liu, Ren, Zhang, Hua, Xu, Zheng, & Hu, 2019) (Fig. 5). Owing to the limited study in this area (three sampling sites with 3✕3 replicas ((Zhang et al., 2018))), the role of N-DAMO process in mangrove soils remain unclear.
This study explored the N-DAMO processes in fifty-two different types of ecosystems in different areas (Figure). The ecosystems were divided into five types, i.e., peatlands, wetlands, rivers and lakes, estuaries and offshore, and agriculture fields (Fig. 6). The results indicated that a difference existed for the N-DAMO rates of the different ecosystems. More intense N-DAMO processes were found in rivers and lakes, peatlands, and wetland ecosystems. Together with other results, the N-DAMO process rates in the mangrove soils varied from 25.93 to 704.08 nmol CO2-C g− 1 d− 1 (Fig. 6), with extreme peak rate reaching at 704.08 nmol CO2-C g− 1 d− 1(Zhang et al., 2018). This is relative higher compare to other wetlands (p < 0.01), showing mangrove soils are hotspot of N-DAMO process.
Mangrove soils are high in organic matter contents. Hence, they are a strong reducing environment and have large CH4 emissions (Devol, Anderson, Kuivila, & Murray, 1984; Yin, Cai, Liu, Zhou, Richter-Heitmann, Aromokeye, Kulkarni, Nimzyk, Cullhed, Zhou, Pan, Yang, Gu, Elvert, Li, & Friedrich, 2021). A high substrate concentration is a critical factor for the N-DAMO rate and relevant microorganism activity. The activity and abundance of methanogens are important factors for the differences in CH4 production and release in mangrove soils (Sun, Wang, Sun, Peng, & Deng, 2012). The relative abundance of Methanosarcina was as high as 2.6%. Such phenomenon might be the reason for the relatively high CH4 production, and this was consistent with the increased methanogen activity and abundance as a result of human activity (Zhang, Pan, Liu, Duan, & Li, 2020). In addition, the N-DAMO rate was consistent with the NC10 abundance in the different ecosystems (Fig. 7). This was consistent with a previous finding that the N-DAMO rate is relevant to the microorganism activity in different ecosystems (Hu et al., 2014). Although the NC10 bacteria abundance was small in the mangrove soils studied here, i.e., only 1% of the total bacteria abundance, it still converted 65.76% of the CH4 generated by the soil to CO2. Such a phenomenon indicated the dramatic impact these bacteria have on reducing the warming potential of greenhouse gases. The N-DAMO process has a direct impact on emissions of CO2 and CH4.
The total mangrove area in the Zhangjiangkou Natural Reserve is 1.179 km2. Based on Eq. 5, the average annual CH4 emissions are at least 1.9 t a− 1. Without the N-DAMO process, the emissions would be increased by up to three-folds. Global-scale research has also shown that the CH4 concentration would increase by 10–60% without the AOM process (Conrad, 2009).
The content of another substrate, such as nitrate, can also affect the reaction rates (Ding, Ding, Fu, Zhang, & Zeng, 2014). The influx of a large amount of nitrogen-containing nutrients transported through rivers into adjacent estuaries or bays is another significant factor that should be considered (Kristensen, Bouillon, Dittmar, & Marchand, 2008). This provides sufficient substrates for the N-DAMO process. Hence, the N-DAMO process also play an important role in nitrogen removal by mangrove ecosystems.